Why is it necessary to have my calibrated thermometer or hydrometer re-calibrated?
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.