Liquid-in-glass thermometers are simple, accurate, economical, and easy to use temperature measurement devices. They operate on a simple set of basic physical principles: the predictable thermal expansion of a known liquid within a capillary of known volume. Unlike their electronic or mechanical counterparts, liquid-in-glass thermometers have no moving parts, need no outside power source and require almost no maintenance.

During manufacture, the glass forming process causes the glass to experience considerable amounts of stress, called strain. The manufacturer will anneal the glass to relieve these strains. Any residual strain can be seen under polarized light. It may take several years for the glass to completely recover from this process. Therefore, newer thermometers will change calibration more quickly than older, more stable instruments.

Also, when the 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 a stress to the bulb. After repeated uses, even the highest quality liquid-in-glass thermometer will undergo a slight change in bulb volume due to this expansion and contraction.

Changes in bulb volume, either due to use or aging will take place, and the indication of the thermometer will also change. Recalibration of the certified thermometer updates the indications and thus allows the user to maintain accurate, reliable and consistent results when making temperature measurements.

This is a complex question. Consideration should be given to the temperature range of the instrument, calibration history and frequency of use. The requirements of regulatory agencies and/or your in house quality system should also be taken into consideration.

Even a thermometer that is infrequently used will continue to change due to the changes in bulb volume discussed above. NIST Good Measurement Practices (11) suggests that thermometers used for measuring temperature critical parameters should be re-calibrated every 6 months and all other thermometers should be recalibrated every 12 months. However, you should choose a recalibration interval which satisfied your quality system and then be sure to adhere to it.
As always, member of the Ed Martin and Associates staff can assist you with this decision and all of your calibration decisions.

If the instrument to be calibrated is an ASTM thermometer and you are doing an ASTM test method, the suggested calibration and verification temperatures found in ASTM E1 should be used.

If the thermometer is not an ASTM, or should you wish to choose temperatures other than those specified, the following guidelines are presented for your consideration:

  1. EMAA suggests a minimum of three temperatures should be calibrated, generally a low, medium and high on the scale of the instrument.
  2. If the instrument has an auxiliary ice point, or the scale includes the ice point (0 °C or 32 °F), it should be one of the calibrated temperatures.
  3. Within the above guideline, there should be no more than 100 graduations (lines) between any two calibrated temperatures. Therefore, more than three calibrated temperatures are often necessary. Interpolation of temperature values between calibrated temperatures may not be valid and will greatly increase your uncertainty of measurement.

Pack the thermometer carefully, insure it for its full value, and send it along with a copy of the calibration certificate and your purchase order to our Houston facility. If you do not have facilities for proper packing and shipping, contact us and advise the number and length of your thermometers to be recalibrated. We will send you, without charge, “loaner” shipping cases via UPS, and we will have UPS pick up the case the day after you receive it back, to return it to us. Only the very nominal UPS charges will be added to your invoice.

Calibration is performed in Miller & Weber’s ISO/IEC 17025 accredited New York calibration laboratory by highly qualified and trained personnel. Your instrument will be compared with primary master standards traceable to NIST and NPL at the temperature(s) you specify, in accordance with ASTM & NIST methods. A new ISO/IEC 17025 accredited Report of Calibration will be issued, valid for the time period you specify. The Report of Calibration will also meet the requirements of ISO9000, QS9000, Z- MIL-STD 46662A and ANSI/NCSL Z-540-1.

In addition to the ISO/IEC 17025 accredited Report of Calibration we will do the following:

  1. Your instrument will be examined under magnification for any chips, scratches, impurities in the bore or other defects which may impair its function. This examination includes bringing all of the thermometric liquid into the bulb.
  2. Your instrument will then be examined under polarized light to check for strains in the glass.
  3. Any separations of the liquid will be repaired.
  4. If the thermometer is of the etched stem type, and the etchings have faded, the etchings will be re-pigmented without charge. Unfortunately, if the thermometer is silk screened or decaled, once the ink fades, it cannot be repaired.
  5. A calibration sticker will be affixed to your thermometer with the date of calibration, recalibration due date and other appropriate information. Hydrometer calibration labels will be affixed to the storage box or case.
  6. Your recalibrated instrument will be shipped promptly upon completion of recertification, via the method that you specify. Please note that mercury-in-glass thermometers cannot be shipped via UPS air service.
  7. The thermometer(s) and hydrometer(s) are our responsibility on return shipment to you. If it is damaged in transit to you, you will receive a prompt, no charge PRIORITY replacement. We only ask that you retain the shipping cartons for inspection by the carrier.
  8. We maintain an accurate Customer Registration program. This enables us to notify you in writing one month before the expiration of your Report of Calibration that recalibration is due.


Normal turn around for thermometer calibration is about one week. Some calibration laboratories will advertise 24 hour turn around times. Although we can commend them for trying to achieve superior customer service, they are actually not following basic principles of liquid-in-glass thermometer calibration. When your thermometer is received into our laboratory it is inspected as described above. The thermometer bulb will be frozen in a mixture of dry ice and alcohol. Separations will be repaired. We look for strains under polarized light which will heat the thermometer. Your thermometer may have spent time in a hot truck or cold airplane hold. You may have used it at its top temperature and then air expressed it to us the next day. We do not know the history of use and storage 72 hours prior to when we receive it.

The first cardinal rule of liquid-in-glass thermometer calibration is a calibration laboratory is not to begin calibration until the thermometer has been at rest at room temperature (25 °C) for a minimum of 72 hours (3 days). After we inspect your thermometer it spends the next 72 hours in our environmental chamber set at 25 °C.

When possible, please allow us to take this time in calibration of your thermometer. If you require expedited service, we will do our best to accommodate your needs. Customer service is of primary importance to us!

MILLER & WEBER, INC. has been manufacturing precision liquid-in-glass thermometers for over 60 years. Our calibration laboratory personnel have served an average of 20 years in our facility and we are well versed in the calibration procedures used at the National Institute of Standards and Technology (NIST) and the National Physical Laboratory in the UK (NPL). MILLER & WEBER, INC. involves itself in continuing education and training programs and calibration proficiency testing, as required by both ISO9001:2000 and ISO/IEC 17025. Deanne Miller Emory, President, Is a third generation thermometer manufacturer, degreed chemist and is currently chairman of ASTM SUBCOMMITTEE E-20.05 on Liquid-in-Glass Thermometers and Hydrometers. She is also active in other ASTM E20 subcommittees and subcommittees of ASTM D02 on Petroleum.

MILLER & WEBER, INC.’s manufacturing facility and calibration laboratory are ISO9001:2000 certified with full design capabilities. MILLER & WEBER. INC. was the first U.S. liquid-in-glass thermometer manufacturer to achieve ISO9001:1994 certification. All ASTM thermometers, including non-mercury PerformaTherm® thermometers and SAMA Laboratory Thermometers are completely manufactured in the U.S.A. The MILLER & WEBER, INC. calibration laboratory is also ISO/IEC 17025 accredited.

Simply send your instrument, fully insured, along with a check, company purchase order, or VISA/MASTERCARD number to:

If you would like additional information regarding MILLER & WEBER’s THERMOMETER AND HYDROMETER certification and calibration services, please call our technical staff at our Houston Facility (713) 895-8188.