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.

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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