Scope: This Guideline provides a method required for calibration and verification of reference glass thermometers.
Frequency: 3 months
REFERENCE GLASS THERMOMETER CALIBRATION PROCEDURE
PREPARATIONS:
Before each thermometer’s initial use and at least once a year after its initial use, each thermometer shall be compared to a thermometer certified by the National Institute of Standards and Technology (NIST) or an equivalent thermometer of traceable accuracy. The comparison shall be made at three or more temperatures to ensure that the thermometer is accurate. Typically, the checkpoints should be at 10 %, 50% and 90% of the temperature range in which the thermometer is expected to be used.
The verification is accomplished by placing the operational temperature device and the reference thermometer into a source of uniform temperature. Using a temperature-controlled source (e.g., temperature bath) may do this. However, it should preferably be performed in the operating pipe or vessel, providing that suitable facilities exist for the simultaneous registration of temperature by the operational device and the calibration instrument. The verification point positions should be adjacent to one another and the temperature must be allowed to stabilize before recording the comparative readings.
CALIBRATION EQUIPMENT:
A) NIST certified reference thermometer
B) Water Bath
PROCEDURE:
Field Inspection
A thermometer is a precision instrument; therefore, it must be handled with care and examined before each use for breakage and separation of the mercury column. Some other items to remember while field inspecting a thermometer are as follows:
a. After use in heavy or high-pour-point oils, all parts of the thermometer assembly shall be cleaned to avoid the formation of insulating film oil.
b. An armored-case thermometer is particularly susceptible to the formation of an insulating film so the glass thermometer should be removed frequently from the metal assembly and cleaned.
c. A thermometer that has lost a substantial amount of pigment from its engraved scale should not be used in light colored products and condensates because it is difficult to read the graduation increments and numbers.
d. A thermometer with a separated mercury column shall never be used. If the mercury has been completely rejoined, the thermometer may be used provided it is found by bench inspection (as described in 8.3.1) to be accurate within 0.1 degree F.
REFERENCE GLASS THERMOMETER VERIFICATION PROCEDURE
The verification procedure shall consist of at least one comparison at a temperature within the operating range, between the temperature device and the reference thermometer. If the deviation limits of 0.1degree F are exceeded, then the temperature device shall be recalibrated or replaced.
a. Precision glass thermometers have scale graduation intervals of 0.1″C to 0.2″C (0.2″F to 0.5″F) and should be supplied with a calibration certificate that details corrections that are to be applied to observed readings. They may be of the partial- or complete-immersion type. Partial-immersion thermometers should be immersed to the proper level as marked on the thermometer. Usage at different immersion depths or at significantly different ambient temperatures from which the thermometer was certified may require that stem corrections be made.
Complete-immersion thermometers are typically used partially immersed into a thermowell. The portion of the stem that is outside the thermowell will respond to the ambient temperature, and may cause a significant error in the reading, depending on the difference between the temperature being measured and the ambient temperature and the amount of active stem that is exposed to the ambient condition.
b. When a verification test is performed, the “as found” reading should be recorded and compared to the reference thermometer.
c. Should the “as found” reading do not agree with the reference thermometer, within the required control limit, a calibration of that device will be required. The reference thermometer should have a minimum resolution of 1 ºC (0.2ºF).