Maximum registering thermometers (including autoclave maximum registering thermometers) are thermometers designed to indicate the highest temperature attained during a process. They are typically used in processes in which it is difficult to measure the temperature during the process. Autoclaves and sterilizers are typical applications.

The construction of the thermometer incorporates three distinct differences from normal “indicating” thermometers:

1. The maximum registering thermometer has a constriction, or restriction, in the capillary, normally about 1/2″ or so above the juncture of the bulb to the capillary.

The intent of this restriction is to permit the mercury to pass through under pressure (during heating of the thermometer, or conversely, to return the mercury to the bulb after use by using centrifugal force). The constriction should NOT permit the mercury to retreat by force of gravity.

2. The area above the mercury column is a partial vacuum.

On regular indicating thermometers, the area above the mercury is filled with pressurized nitrogen, which is why the mercury column retreats when cooling, even in a horizontal position.

3. Calibration: the major manufacturers design the thermometer to indicate the temperature to which it has been exposed not while in the process, but after having been removed from the process and permitted to cool.

In production, the way this is accomplished is to subject the thermometer (with no scale markings) to the desired temperature(s) in a precision bath, allow it to come to equilibrium, then carefully remove it from the bath and gently place it in an upright position to cool. After cooling is complete, a mark is made on the stem of the thermometer where the meniscus of the mercury resides. This mark is used for placement of the scale.

In use, the thermometer is reset for the next use by ‘shaking’ it, similar to the motion one uses to ‘shake down’ a fever thermometer, thereby generating centrifugal force that impels the mercury downward, through the constriction, and into the bulb. Note that not all the mercury returns to the bulb, however the top of the mercury column should reside below the scale of the thermometer. The thermometer is now ready for use.

Place the thermometer in your process and permit the process to run its course. When finished, gently remove the thermometer and slowly and carefully place the thermometer in a vertical position, taking care not to jar it. When cooled to room temperature, read the thermometer. It’s reading should indicate the highest temperature attained in the process.

When you read the thermometer is not important. The mercury will remain at this level until it is reset, or until it encounters severe vibration or jarring.

The following information is courtesy of the Cannon Instrument Company (State College, Pennsylvania) catalog.

Capillary Viscometers – How Do You Clean Them?

Clean viscometers are essential if precise and accurate measurements are to be made. Because Cannon receives a significant number of requests for advice about cleaning methods, they offer the following instructions as a guide to cleaning most glass capillary viscometers.

Removing the test sample from the viscometer

The first step in cleaning is to remove the bulk of the test sample. For low viscosity liquids, the viscometer may be turned upside down and allowed to hang while the test sample drains into a trough. For high viscosity liquids, the sample may have to be drawn out under vacuum. The material remaining in the viscometer must then be removed by flushing with a suitable solvent. Distilled water is an obvious choice for aqueous solutions. Petroleum-based lubricants and asphalts can usually be dissolved with light naphtha, heptane, octane, highly aromatic solvents, and many other petroleum-derived solvents. Varsol^® is a commercial solvent that works very well for this purpose. For some types of samples it may be difficult to find a suitable solvent.

Highly viscous samples will not easily pour from the instrument nor do they respond well even under vacuum. The best approach is to lower the viscosity by heating the instrument in an open oven or with a stream of hot air. Simply inverting the instrument and suspending it in an open oven over a receptacle to catch the sample usually works well. Another method is to draw the bulk of the sample out while the instrument is at an elevated temperature in a constant temperature bath. This method works particularly well for certain viscometers (such as the Zeitfuchs^® Cross-Arm viscometer), as the entire cleaning can be performed while the viscometer remains fixed in the constant temperature bath. Cannon often places viscometers in an open aluminum oven {2″ wide x 7′ long x 5″ deep), maintained at an elevated temperature, during the cleaning procedure. Even after the bulk of a viscous sample has been removed from the instrument, dissolving the rest of it may pose a considerable problem. We have found that a mixture of octane isomers is especially effective in removing the last traces of high viscosity standards from viscometers.

Drying the viscometer after cleaning

The viscometer must be completely dry before another sample is loaded. Highly volatile solvents are recommended for cleaning since any remaining solvent will evaporate quickly after the sample has been flushed from the viscometer. Often, however, the best choice of solvent for the material in the viscometer is not especially volatile. In this case, a second highly volatile solvent, which will dissolve the first solvent, can be used for the final step in cleaning. Acetone is commonly used as the second solvent because of its high volatility and its ability to dissolve traces of petroleum solvents and water.

A low velocity stream of clean air will be sufficient to evaporate remaining traces of a volatile solvent, but be aware that rapid evaporation of these solvents can cool the surface of the glass to such an extent that humid air may be brought below the dew point, causing a film of water to form on the inner surfaces of the viscometer. Heating the air being drawn into the instrument or heating the glass itself will usually overcome this problem.

Cleaning insoluble deposits

Capillary viscometers are often used to measure materials which leave strains or significant deposits of material insoluble in normal cleaning solvents. The most common approach for removing this material involves filling the instrument with a chromic acid cleaning solution and allowing the instrument to soak in the acid for up to 24 hours. Chromic acid solutions are strongly oxidizing and will convert many materials to a soluble form. Chromic acid will not attack the borosilicate glass of the viscometer and thus will not alter the calibration constant. Proper procedures must be followed when using and discarding chromic acid since it is a hazardous material. A commercially manufactured oxidizing reagent (Nochromix) is chromium-free and may be substituted for chromic acid solutions. Nochromix is available from Cannon Instrument Company.

Beware of glass cleaners with a high pH. Changes in viscometer calibration as great as 20% have been observed due to the prolonged use of alkaline cleaning solutions. If alkaline cleaning solutions with a pH greater than 10 have been the viscometer calibration should be verified to ensure that there has not been a significant change.

Insoluble particles stuck in the capillary of a viscometer can sometimes be dislodged by using an ultrasonic cleaner.

Glass thermometers and hydrometers are remarkably stable and reliable indicating devices. Nonetheless, changes in the indications of a given instrument do occur, as a result of temperature cycling and day-to-day handling.

When a thermometer is heated, the liquid within the bulb expands and is forced upward into the capillary where its level indicates the temperature value. Each heating and cooling cycle imparts tremendous stress to the bulb. After repeated use, even the highest quality liquid in glass thermometer will undergo a slight change in bulb volume due to this expansion and contraction. When a change of this type does take place, the indication of the thermometer will also change.

Re-calibration of the certified thermometer updates the indications and thus allows the user to maintain accurate, reliable and consistent results when making temperature measurements.

Re-calibration at regular intervals to document traceability to NIST is an important part of most quality programs such as the ISO 9000 series of quality standards, to assure that required levels of accuracy are being met.

What type of changes can I expect?

The amount of change which will occur in a given period of time (say one year) is a factor of how well made the instrument is, the particulars of the testing for which it is used, and the frequency with which the instrument is utilized. For example, from our experience, thermometers used in air or in non-aggressive liquids, at temperatures around room temperature, tend to experience relatively small changes. In contrast, thermometers used at high temperatures (over 150 °C) change much more quickly. Similarly, frequency of usage is a major factor. A thermometer used several times each week at high temperatures will experience a greater change in a given period of time than an identical thermometer used for the same application, but only used once or twice a month.

Hydrometers used in clean, light, non-corrosive liquids, and carefully handled, tend to exhibit small but measurable changes in indication after one year. Hydrometers used in hot liquids, acids, caustics, or in heavy oils or other viscous liquids which necessitate vigorous cleaning with solvents can change appreciably in short periods of time, from effects of chemical action, temperature cycling, and the abrasion and mechanical stress of cleaning.

Significantly, thermometers and hydrometers often develop problems over time.

It is not uncommon to see a thermometer which is several years old begin to exhibit discoloration of the mercury, or begin to leave debris, or fragments of mercury or oxidized mercury along the capillary. Such complications are the result of an imperfect filling, wherein moisture, foreign material, or air (oxygen), or sometimes all three, albeit in miniscule quantities, were sealed inside the instrument. When and if such problems occur, the instrument should be removed from service as its indications will become increasingly unreliable. A good calibration laboratory will catch such problems and bring them to your attention.

Often miniscule separations of the mercury occur, typically on high temperature thermometers, where a portion of the column actually distills from extreme temperatures. The distilled mercury condenses in the upper limits of the thermometer – and accordingly the temperature indicated by the instrument is somewhat lower than the actual temperature. This problem may be undetected by the casual observer – but will be noticed and rectified by a competent calibrator.

It is not unusual for a thermometer to be damaged from accidental or unintentional overheating, or from sudden, unintentional rapid cooling. We have had, on rare occasions, thermometers submitted for a periodic re-calibration which at first examination appeared in excellent condition, but did not function properly; an examination under the microscope revealed a nearly invisible stress crack in the bulb through which a quantity of mercury had escaped – changing the reading of the thermometer in excess of 10 degrees C !

We have caught many hydrometers (actually, in most cases these were thermo-hydrometers, with a thermometer incorporated in the lower portion of the hydrometer), submitted for routine re-calibration, which had suffered stress cracks from rough handling, allowing a quantity of the (test) liquid to infiltrate into the instrument, changing the weight (mass) and thus the readings of the instrument. By how much? 10 scale divisions or so – enough to absolutely invalidate the integrity of the testing, but not necessarily a large enough value to immediately alarm the user.

These are a few of the problems which many laboratory people will miss in the press of day-to-day activities. We, on the other hand, earn our living working with these instruments, and you pay us to spot easily overlooked problems which may affect the correct function of your thermometer or hydrometer, and in turn, the integrity of your data.

Re-calibration at regular intervals permits the user to see the magnitude of the changes taking place, and whether or not those changes affect the level of precision desired. Evaluation of the changes observed throughout a series of recalibrations permits the user to set forecasts based on historical data and thereby determine appropriate calibration intervals for the future.

Consideration should be given to the frequency of use, the parameters of the application (temperatures at which it is used, the severity of the use) and the requirements of the regulatory agencies and/or the quality system you may be using. In general, for most laboratory and industrial applications a re-calibration interval of one year is considered a reasonable and prudent time frame.

Most of our clients who maintain an ISO 9000 or QS 9000 series program are using a one year recalibration interval, but one year may be too long (or too short!) for your particular application. By all means consult your quality department or your quality consultant.

Are there any recommendations for calibration intervals from respected sources?

Yes.

If you perform petroleum testing, consider that the American Petroleum Institute (API) publication ‘Manual of Petroleum Measurement’, Chapter 7, recommends that liquid-in-glass thermometers and electronic digital gauging thermometers be recalibrated annually by a qualified laboratory. This document can be purchased from the American Petroleum Institute.

ICL tries not to recommend calibration intervals, but offers the following logical guidelines:

Start with a prudent calibration interval, following industry norms or recommendations.

When a suitable history of calibrations, and changes in the instrument’s indications, is available, (for example, three years of annual calibrations), review that history and the trends in the instrument’s indications and make a judgment accordingly. Perhaps the device has proven sufficiently stable that the recalibration interval can be extended to two years.

Remember that calibration should take place with a frequency sufficient to prevent out-of-tolerance conditions from occurring.

Try not to make generalizations about types of instruments, but consider all the aspects of each particular application. That little glass thermometer in the plastic bottle in your lab refrigerator may not experience much change in the course of a year, but the glass thermometer used weekly for melting point determinations at high temperatures is entirely a different matter.

Remember too that thermometers and hydrometers are dynamic with use. Shock, contamination, exposure to extremes in temperature, exposure to aggressive fluids or vapors, very rapid cooling or heating, mechanical stress, or any number of factors may cause an instrument to drift out of calibration prior to the expiration of its assigned calibration interval.

If your thermometer has been previously calibrated, the test points have already been established and appear on the test report. Generally, those test points should be repeated in future calibrations, which will allow the user to see the magnitude and the direction of any changes with each new calibration.

If your thermometer has never been calibrated (or you don’t know, or don’t have a test report), you can either let us choose the most suitable (default) test points, or you may wish to specify those test points to us.

To assist you in choosing test points, we present the following considerations, which are drawn from ASTM and NIST recommendations. One should consider all three suggestions:

1. A minimum of three temperatures should be calibrated, generally low, medium and high on the scale of the instrument.

This is the old “10% – 50% – 90%” (of scale) rule. Example: you have a thermometer with a range of -10 to 110 °C in 1° divisions. Calibrating this thermometer at 0 °C (low on the scale), 50 °C (mid scale), and 100 °C (high on the scale) is sufficient, and will allow you to use the thermometer at virtually any temperature it measures by making a straight-line interpolation. See ASTM-E-77 and NBS Monograph 150 for more information on interpolating. Unfortunately, this is not adequate for all thermometers. See suggestion #2.

2. There should be no more than 100 graduations between any two calibrated temperatures; for the ultimate precision, calibrate every 50 divisions.

For example, if your thermometer has a range of -1 to 51 °C in 0.1° divisions, suggestion #1 above, to calibrate three temperatures, does not provide an adequate calibration. You must calibrate every 100 divisions: 0, 10, 20, 30, 40 & 50 °C to have an adequate calibration.

3. If the temperatures used by the manufacturer for scale placement are known (or can be easily determined visually), the temperatures used for calibration should correspond.

This will assure linearity of spacing between the calibrated points, therefore allowing the user to interpolate intermediate values. Example: your thermometer has a scale of 25 to 60 °C in 0.1 degree divisions, and a careful examination of the thermometer reveals that “scale placement marks’ (usually a scratch in the glass, under a major graduation line, visible with a magnifying glass) were made at 25, 30, 40, 50 & 60 °C, then those temperatures should be used for the calibration to afford the best linearity.

Certainly. You are the customer, and the one who best knows your needs. Many times a single point calibration is all that is needed, for example when a thermometer is dedicated to the measurement of a single temperature and will not be used for other work. Under ANSI/NCSL Z-540-1 we are required to identify the test report of a single point calibration as a “limited calibration”, or “not a full scale calibration”. The intent is logical and desirable: if your thermometer is calibrated only at 37 °C, for a dedicated test, you want to know that, and not to use it for a critical application at 50 °C.

I want to use the thermometer only across a defined range within its scale. Do I need to do a full scale calibration?

No, we can choose test temperatures which “bracket” the range within which you are going to work. As above, the test report will specify that this calibration is NOT a full scale calibration, and that the thermometer can be used with full confidence only within the range bracketed.

Some helpful information on ASTM and other glass laboratory thermometers

All ASTM and other glass laboratory thermometers can be classified into 2 general groups – those designed and fabricated for total immersion and those designed and fabricated for partial immersion.   If you use glass thermometers, it is essential that you understand the difference, and how each type of thermometer is used.

Most laboratory errors in temperature measurement result from incorrect usage (immersion) of the thermometer!

Total immersion thermometers are designed with scales which indicate actual temperature when the bulb and the entire liquid column are exposed to the temperature being measured. In practice, a short length of liquid column (usually one-half inch) is permitted to extend above the surface of the liquid being measured to allow reading of the thermometer.

Most total immersion thermometers can also be used in a condition of complete immersion, wherein the entire thermometer is exposed to the temperature being measured, as with a thermometer inside a refrigerator, freezer, incubator or other chamber.

Partial immersion thermometers are designed to indicate the actual temperature when a specified portion of its stem is exposed to the temperature being measured.

How can I know the difference?

Partial immersion thermometers

The immersion line is a quick and easy visual indication to the user. The thermometer should be immersed to this line for correct temperature indication. The reverse of the thermometer should have the inscription “76MM IMM” (or as appropriate). Partial immersion thermometers are usually easy to identify.

Note: some partial immersion thermometers do not have an immersion line inscribed; ie, thermometers with standard taper joints, or thermometers with very short immersions, such as for melting point applications. Pay special heed to the inscription on these thermometers, and immerse them as specified for the most accurate readings.

Total immersion thermometers

Total immersion thermometers are sometimes a little trickier to identify. Some of the better manufacturers are inscribing TOTAL or TOTAL IMMERSION on the reverse of the thermometer, but regrettably this is not an industry-wide practice. The photo below is of an older ASTM 112C thermometer, designed for total immersion. There is no immersion line, and there is no “TOTAL IMMERSION” marking on the reverse.

If there is no inscription on the reverse indicating immersion, you should assume the thermometer is designed for total immersion.

What’s the difference in use?

As explained above, the partial immersion thermometer is immersed in the liquid being measured up to the line, or ring.

The total immersion thermometer must be immersed into the medium being measured to within approximately one-half inch of where the top of the liquid column (the meniscus) resides (ASTM E-77).

So what happens if the total immersion thermometer is not immersed to the depth it should be?

You will have an erroneous temperature reading. The amount of the error depends upon what the temperature is that you are measuring, and how much of the liquid column that should be immersed is outside the medium you are measuring. An extreme example: you have a -1/201 °C thermometer, 24 inches in length, total immersion, and you are testing the liquid in a beaker on a hotplate. Only about 2 inches of the thermometer is in the liquid. The thermometer indicates 190 °C. How much error do we have? Almost 5 degrees C. The liquid in the beaker is 5 degrees hotter than the thermometer indicates.

I have this expensive, calibrated, total immersion thermometer I bought recently, similar to the one in the example above. I need it for applications similar to that described. Is this useless to me?

No, it may not be the best thermometer for your application, but you can use it. However, you’ll need to calculate and apply a correction.

Please refer to the following articles:

• Using a total immersion thermometer only partially immersed
• Using a partial immersion thermometer in condition of total immersion
• Using a partial immersion thermometer incorrectly immersed

Some helpful thermometer suggestions:

GENERAL CONSIDERATIONS FOR MAKING AN ACCURATE READING

The error due to parallax may be eliminated by taking care that the reflection of the scale can be seen in the mercury thread, and by adjusting the line of sight so that the graduation of the scale nearest the meniscus exactly hides its own image: the line of sight will then be normal to the stem at that point. In reading thermometers, account must be taken of the fact that the lines are of appreciable width. The best practice is to consider the position of the lines as defined by their middle parts.

PERFORMING A CALIBRATION AT THE ICE POINT (0 °C or 32 °F) *

Select clear pieces of ice, preferably ice made from distilled water. Rinse the ice with distilled water and shave or crush into small pieces, avoiding direct contact with the hands or any chemically unclean objects. Fill a Dewar or other insulated vessel with the crushed ice and add sufficient distilled and preferably pre-cooled water to form a slush, but not enough to float the ice. Insert the thermometer, packing the ice gently about the stem, to a depth sufficient to cover the 0 °C (32 °F) graduation (total immersion), or to the immersion line (partial immersion). As the ice melts, drain off some of the water and add more crushed ice.

Raise the thermometer a few millimeters after at least 3 minutes have elapsed, tap the stem gently and observe the reading. Successive readings taken at least one minute apart should agree within one tenth of one graduation.

APPLYING THE CORRECTION AT ICE POINT*

Record readings and compare with previous readings. If the readings are found to be higher or lower than the reading corresponding to a previous calibration, readings at all other temperatures will be correspondingly increased or decreased.

*Reproduced in part from ASTM E77

A separation of the mercury in your thermometer is not a defect!

It is a condition, normally caused by shock in transit, which of course must be rectified before using the thermometer, or you will experience significant errors in your readings.

PLEASE RESIST THE IMPULSE TO PUT THE THERMOMETER INTO DRY ICE OR TO HEAT IT! (YET) More often then not you will make the separation more difficult to rejoin, and you may damage the thermometer. PLEASE READ THESE INSTRUCTIONS BEFORE ATTEMPTING TO REJOIN THE SEPARATIONS!

Most well constructed thermometers are filled above the mercury column with pressurized nitrogen gas (there are a few exceptions, which will not be considered here). The nitrogen serves many purposes: it is an inert gas, which minimizes the possibility of oxidation occurring inside the thermometer; the pressure is what makes the column retreat when the thermometer is removed from heat; and the pressurization is what permits the construction of thermometers for use above the boiling point of mercury (approximately 250 °C). The nitrogen gas is of course invisible. When you have a mercury separation in the capillary of the thermometer, the ‘spaces’ between the pieces of mercury are actually quantities of gas. In most cases it is virtually impossible to ‘tap’ the column back together – you cannot force the mercury through the gas in such a confined space, so don’t bother trying – you may well break the thermometer. To be able to ‘tap’ the mercury back together, we must move the ‘separations’ into a larger chamber.

THE TRICK TO REMEMBER IS THAT WHILE THE MERCURY SEPARATIONS ARE IN THE CAPILLARY, THEY CANNOT BE EASILY REJOINED. WHEN WE MOVE THEM INTO A LARGER SPACE, THEY CAN BE EASILY MANIPULATED.

THIS IS EASY!

Firstly, determine the type of separation you have:

1. Separation(s) in a thermometer with a contraction chamber

Fig. 1 Separation of mercury in the contraction chamber

Fig. 1a Mercury is separated in chamber but is also lodged in upper part of chamber and in capillary

Fig. 1b The mercury is separated in the chamber and also in the capillary below the chamber

If your thermometer has a range that starts significantly above room temperature (for example, a range of 98 to 152 °C), the thermometer is constructed with an enlargement in the capillary between the bulb and the main scale. (See figure 1 below). This enlargement, or contraction chamber, is where the mercury normally resides at room temperature. This type of thermometer is extremely prone to mercury separations, especially during shipment. Fortunately, the separations are usually very easy to rejoin.

Firstly, determine how the separation(s) appear. If all the mercury appears to be within the chamber (figure 1), the thermometer may be tapped gently (vertically) onto a padded surface until the separated portion falls and rejoins with the mercury in the lower portion of the chamber.

ONCE AGAIN – THE TRICK TO REMEMBER IS THAT WHILE THE MERCURY SEPARATIONS ARE IN THE CAPILLARY, THEY CANNOT BE EASILY REJOINED. WHEN WE MOVE THEM INTO A LARGER SPACE, THEY CAN BE EASILY MANIPULATED.

If the separated mercury is lodged in the upper portion of the chamber, and/or is located in the column above the chamber (figure 1a), it will be necessary to bring the separation down into the chamber so that it may be tapped as described above. Cool the thermometer bulb a little at a time (dip it into a mixture of ice and water) until the mercury retreats into the chamber. While it is lying in the chamber, tap as described above to rejoin the separation.

If the separation is located in the lower portion of the chamber, or in the capillary below the chamber (figure 1b), we must do the REVERSE of the above. Warm the bulb (try hot tap water first) a bit at a time until the separation is located in the chamber. When the separation is in the chamber, tap as described above to drive the separated mercury down (actually, we are forcing the gas up).

Sometimes with severely separated thermometers it is necessary to rejoin the separation(s) in stages. Just remember that it is necessary to move the separation into the chamber to be able to rejoin it.

2. Separations in the column (thermometers without contraction chambers).

Fig. 2 Separations in the upper part of the mercury column

This type of separation is less common, and a little trickier to rejoin. There are basically two methods:

COOLING METHOD (preferred)
Obtain a small quantity of dry ice or other source of extreme cold. Immerse the thermometer BULB ONLY (TAKE CARE NOT TO IMMERSE THE ENTIRE BULB OR ANY PORTION OF THE STEM) halfway into the dry ice and observe the descending mercury column carefully. The main column will disappear into the bulb, followed by the separated pieces of mercury. Wait a few seconds more, and then withdraw the thermometer from the dry ice and gently and carefully tap it onto a padded surface. The tapping will permit the separated pieces of mercury to fall and rejoin the main mass of mercury now within the bulb. Allow the thermometer to warm naturally (do not heat it) in a vertical position, and observe the mercury column as it ascends into the capillary to be certain it is intact.

HEATING METHOD CAUTION: DO NOT ATTEMPT THIS METHOD WITH THERMOMETERS WHOSE RANGE EXTENDS ABOVE 150 °C OR DAMAGE MAY RESULT. Most well constructed thermometers have a small chamber at the extreme top of the capillary, called an expansion chamber. The purpose of this chamber is to provide over-range protection in case the thermometer is heated beyond its scale range. This chamber may be used to rejoin separations provided 1) the amount of separated mercury is very small (not more than a few scale divisions in length) and 2) the thermometer’s range does not exceed 150 °C. The thermometer may be heated (in hot water, hot oil or other suitable medium compatible with the temperatures to be attained) so that the separation(s) enter the expansion chamber followed by a small portion of the main (intact) column. GREAT CARE MUST BE EXERCISED TO NOT FILL THE EXPANSION CHAMBER MORE THAN HALFWAY, OR BREAKAGE OF THE BULB (AND SPILLAGE OF THE MERCURY) MAY OCCUR) The separations will normally fall to the side of the expansion chamber, and the main column will come into contact with them. Remove the thermometer from the heat, maintain it in a vertical position, and observe the mercury column as it retreats to be sure it is intact.

3. Separations in the expansion chamber

Fig. 3 Separations in the expansion chamber

This type of separation is less common, and a little trickier to rejoin. There are basically two methods:

Some thermometers are designed with very low ranges (for example, ASTM 62C, a common certified reference thermometer, has a range of -38 to +2 °C) such that the mercury at room temperature resides in an oversized expansion chamber at the extreme top of the instrument. (figure 3) Again, often in shipment, this mercury can become separated.

Normally the separation is visually apparent, and can be rejoined quickly and easily by simply tapping, however, if you see a separation in the column, the thermometer must be heated (warm water) until the separation (gas) enters the expansion chamber, where it can be rejoined by tapping. Again – THE TRICK TO REMEMBER IS THAT WHILE THE MERCURY SEPARATIONS ARE IN THE CAPILLARY, THEY CANNOT BE EASILY REJOINED. WHEN WE MOVE THEM INTO A LARGER SPACE, THEY CAN BE EASILY MANIPULATED.

AFTER REJOINING MERCURY SEPARATIONS, IT IS HIGHLY RECOMMENDED THAT THE THERMOMETER BE VERIFIED IN A KNOWN TEMPERATURE PRIOR TO BEING PLACED INTO USE. IF THE THERMOMETER READS CORRECTLY AT THIS KNOWN TEMPERATURE, IT MAY BE SAFELY ASSUMED THAT THE SEPARATION HAS BEEN CORRECTLY REJOINED.

IF THE THERMOMETER’S INDICATION AT A KNOWN TEMPERATURE IS HIGH, THERE IS GAS (A SEPARATION) EITHER IN THE BULB OR THE COLUMN WHICH IS DISPLACING MERCURY AND CAUSING A FALSELY HIGH READING. GO BACK AND FIND THE SEPARATION AND REMOVE IT.

IF THE THERMOMETER’S INDICATION AT A KNOWN TEMPERATURE IS LOW, IT MAY BE ASSUMED THAT THERE IS A MERCURY SEPARATION SOMEWHERE ABOVE THE COLUMN (LOOK FOR IT IN THE UPPER REACHES OF THE COLUMN OR IN THE EXPANSION CHAMBER).

IF ALL ELSE FAILS, CALL ICL CALIBRATION AT 772 286 7710 AND WE WILL WALK YOU THROUGH THE PROCEDURES.

When total immersion thermometers are used in a condition wherein the entire liquid column is not exposed to the temperature being measured, a stem correction must be computed and applied to the observed reading to obtain the actual temperature of the liquid being measured.

Example: You have a total immersion mercury thermometer graduated from -1 to 101 °C in 0.1 divisions. You are measuring the temperature of liquid in a beaker on a hot plate; the thermometer is immersed to the 31 degree mark. The reading of the thermometer is 90.15 °C.

How much error do you have for incorrect immersion of the thermometer?

What is the actual temperature of the liquid being measured?

1. We need to determine 4 variables:

k = the coefficient of expansion of the thermometric liquid and the glass, combined.
For Celsius mercury thermometers, k = 0.00016
For Fahrenheit mercury thermometers, k = 0.00009
For red liquid Celsius thermometers, k = 0.001
For red liquid Fahrenheit thermometers, k = 0.0006

n = the number of scale degrees of the thermometer column between the surface of the liquid being measured and the meniscus of the liquid column.
In this example, the thermometer is immersed to to 31 degree mark, and the reading of the thermometer is 90.15, so the value of N is 90.15 minus 31, or 60.15 (the distance, expressed in scale degrees between the 31 graduation at the surface of the liquid in the beaker and the meniscus at 90.15).

T = the reading of the thermometer in situ
(In this example, 90.15 °C)

t = average temperature of the emergent liquid column.
To obtain this value, suspend alongside the main thermometer a secondary, total immersion thermometer. Position this thermometer so that its bulb is centered halfway between the surface of the liquid and the temperature indicated on the main thermometer. The temperature indicated on the second thermometer will be the average temperature of the emergent liquid column. For this example, we will assume a temperature of 25 °C was observed.

2. Now, find the magnitude of the correction from the following equation:

correction = kn(T-t)

(0.00016 x 60.15) x (90.15-25) = 0.627

Adding this value to the observed reading of the thermometer yields 90.15° + .627 = 90.777 °C which is the actual temperature of the liquid being measured.

If this were a red liquid filled thermometer, you would need to use a different value for k (above). Notice how much greater the correction is:

(0.001 x 60.15) x (90.15-25) = 3.918

Adding this value to the observed reading of the thermometer yields 90.15° + 3.918° = 94.068 °C which is the actual temperature of the liquid being measured.

Caution: although this equation (from ASTM E-77and NBS Monograph 150) is reasonably accurate, the measurement of the temperature of the emergent stem is difficult and often imprecise, and will increase the measurement uncertainty.

Remember that the greater the departure of the test temperature from room temperature, the greater the correction – and the greater the uncertainty of the measurement.

The ideal situation is to use the correct thermometer for your application, and not try to ‘make do’ with what you have at hand.

Suppose you have an ASTM 91C thermometer, with a range of +20 to 50 °C in 0.1 divisions, 76mm immersion, and you want to place the thermometer inside an incubator (in condition of total immersion) and read the temperature. Do you have to make a correction? Yes.

1. We need to determine 4 variables:

k = the coefficient of expansion of the thermometric liquid and the glass, combined.
For Celsius mercury thermometers, k = 0.00016
For Fahrenheit mercury thermometers, k = 0.00009
For red liquid Celsius thermometers, k = 0.001
For red liquid Fahrenheit thermometers, k = 0.0006

n = the number of scale degrees of the thermometer column between the immersion mark on the thermometer and the meniscus of the liquid column.
The ungraduated portion of the thermometer between the immersion line and the start of the scale (if any) must be evaluated and included in the value of n. This concept is a little more difficult. Suppose on this thermometer the scale starts (the first graduation is at 20 °C) 25mm above the immersion line. The thermometer in situ reads 37.12 °C The value of n, therefore is the number of scale degrees between 20° and 37.12 °C (17.12) plus the number of degrees represented by the 25mm of ungraduated capillary. Using a metric ruler, place the 0 on the ruler at 20 °C on the thermometer. What temperature on the thermometer coincides with 25 on the ruler? Let’s say 23.8 °C. So, 25mm equals the span from 20 to 23.8, or 3.8 degrees. Add the 3.8 thus determined to the 17.12 we figured above, and we find that n = 20.92

t_o = the reading of the thermometer in situ
(In this example, 37.12 °C)

t_s = The specified temperature of the emergent liquid column, from ASTM E-1 ‘Specifications for ASTM Thermometers’.
This particular thermometer was manufactured anticipating an emergent stem temperature of 25 °C at all temperatures, so use this value for t_s Thumbnail rule: if your thermometer is NOT to be an ASTM thermometer, use 23 °C as the value of t_s.

2. Now, find the magnitude of the correction from the following equation:

Magnitude of the correction = kn(t_s – t_o)

(0.00016 x 20.92) x (25-37.12) = -0.04 °C

Add this value (algebraically) to the observed temperature to find the actual temperature in the incubator:

37.12° + (-0.04°) = 37.08 °C

Remember that the greater the departure of the test temperature from the specified stem temperature, the greater the correction – and the greater the uncertainty of the measurement.

The ideal situation is to use the correct thermometer for your application, and not try to ‘make do’ with what you have at hand.

Well, this is a new twist but maybe not as unusual as I had thought. A customer called us today with this question: he has an NIST traceable calibrated thermometer, calibrated for partial immersion (76mm). He is going to use it only immersed to 50mm. What kind of an error will he have? The thermometer has a range of -1 to 101 °C in 0.1° divisions, and he wants to perform testing at 40, 70 & 90 °C

My response to that question was, as typical, “The magnitude of the error introduced by the decreased immersion will be modest, but good science demands that we calculate and quantify the error, and then decide if it is important (or not) to the application.”

Here’s the issue: the thermometer is designed to be immersed 76mm (3″) into the liquid being tested, and has been calibrated (certified) in that condition of immersion.

In this application, the thermometer is only immersed 50mm, so 26mm of the thermometer stem which should be exposed to the temperature to be measured is instead exposed to some unknown temperature. How does that affect the indication of the thermometer?

The calculation is almost identical to the determination of a correction for a total immersion thermometer partially immersed.

1. We need to determine 4 variables:

k is given.
k = the coefficient of expansion of the thermometric liquid and the glass, combined.
For Celsius mercury thermometers, k = 0.00016
For Fahrenheit mercury thermometers, k = 0.00009
For red liquid Celsius thermometers, k = 0.001
For red liquid Fahrenheit thermometers, k = 0.0006

n = the number of scale degrees of the thermometer column between the surface of the liquid being measured and the immersion line.
In this case, the thermometer is immersed to to 50 mm degree mark, so 26mm of stem is emergent. If we measure 26mm on the scale of this thermometer, we see that 26mm represents 5.4 degrees C. So, n = 5.4

T = the reading of the thermometer in situ
(Let’s assume that the thermometer reads exactly 40.00 °C)

t = average temperature of the emergent liquid column.
This is the ‘tricky’ variable. To obtain this value, suspend alongside the main thermometer a secondary, total immersion thermometer. Position this thermometer so that its bulb is centered halfway between the surface of the liquid and the immersion line. The temperature indicated on the second thermometer will be our best estimate of the average temperature of the emergent liquid column. For this example, we will assume a temperature of 30 °C was observed.

2. Now, find the magnitude of the correction from the following equation:

Correction = kn(T-t)

(0.00016 x 5.4) x (40.00-30) = 0.0086

Adding this value to the observed reading of the thermometer yields 40.00° + 0.0086 = 40.0086 °C which is the actual temperature of the liquid being measured.

Example 2.
Suppose the thermometer is reading 60.03° (T), n remains unchanged because the immersion is constant, and the measured temperature around the emergent stem is 37 °C

(0.00016 x 5.4) x (60.03-37) = 0.019°

Adding this value to the observed reading of the thermometer yields 60.03° + 0.019 = 60.049 °C which is the temperature of the liquid being measured.

Example 3.
Suppose the thermometer is reading 90.06° (T), n remains unchanged because the immersion is constant, and the measured temperature around the emergent stem is 54 °C

(0.00016 x 5.4) x (90.06-54) = 0.031°

Adding this value to the observed reading of the thermometer yields 90.06° + 0.031 = 90.091 °C which is the temperature of the liquid being measured.

Remember that the greater the departure of the test temperature from room temperature, the greater the correction – and the greater the uncertainty of the measurement.

The customer then asked, “The calibration report has a correction factor at (for example) 90 °C of -0.04 °C. Do I still apply that correction? Yes, absolutely. The correction from the report is for that thermometer, properly immersed. So, the actual temperature of the medium being measured in the hypothetical example above is 90.06 + 0.031 – 0.04 = 90.051 °C

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