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 is an ASTM thermometer (it will have the inscription ‘ASTM 1C’ or similar), the standard test temperatures are specified by ASTM specification E-1 (and appear in our ASTM thermometer listings). These test temperatures have been specified considering the intended application as well as the behaviors of the particular instrument, and should be used in order to assure that the calibration has been performed in accordance with ASTM requirements.

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.

Can I just have one temperature calibrated?

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.