The NIST Alternative Thermometer tutorials are designed for educating various industrial user groups about the upcoming and current changes that ban the use of mercury products. The slides convey the choices available to industry on the different types of alternative thermometers, how to select an alternative thermometer and some results of an in the field test comparing mercury and alternative thermometers. Dawn Cross is leading the NIST efforts to assist U.S. industry in their transition from mercury thermometers to safer alternative thermometers.
Selecting and Using Alternative Thermometers
Sept. 19-20, NIST, Gaithersburg, MD
Can a NIST–traceable thermometer be calibrated by using another NIST‐traceable thermometer? And what distinguishes reference thermometers from traceable instruments?
a. Any calibrated thermometer that maintains metrological traceability can be used to calibrate any other thermometer, thus continuing the maintenance of metrological traceability.
b. A "traceable" thermometer can be used to calibrate any thermometer as long as the definition of metrological traceability is met (i.e., result plus uncertainty).
c. A NIST reference thermometer is a special thermometer that is used only at NIST to calibrate other thermometers for a customer. Thermometers that are designated as reference thermometers can be of different types (e.g., platinum resistance, thermocouple, thermistor).
d. A calibrated customer thermometer is used to either calibrate other thermometers or determine temperature for an application. A thermometer calibrated for a customer becomes the customer reference thermometer.
e. A NIST "traceable" thermometer is any thermometer that maintains metrological traceability to the SI through NIST. The NIST "traceable" thermometer can be used to calibrate other thermometer (e.g., customer reference, NIST "traceable", off the shelf) regardless of the type (e.g., platinum resistance, thermocouple, thermistor, analog) as long as the results are associated with uncertainties.
f. The type of thermometer does not define the status (e.g., reference versus traceable); the traceability is critical.
g. Any thermometer that is not a defining instrument (Standard Platinum Resistance Thermometer) of the International Temperature Scale of 1990 (ITS‐90) is an industrial thermometer (e.g., Industrial Platinum Resistance Thermometer, Thermocouple Thermometer, Analog Thermometer, Thermistor Thermometer).
h. A digital thermometer is a display unit plus thermometer of any type.
Can a NIST‐traceable thermometer incremented in 0.2 °C be used to calibrate a NIST‐traceable thermometer incremented in 0.1 °C?
a. Resolution (or increment) and accuracy are not the same thing.
b. Resolution is the number of digits displayed (e.g., digital thermometer) or read (e.g., analog thermometer increment)
c. Accuracy refers to the uncertainty of the measurement made with the thermometer relative to the unit (e.g., °C) through the maintenance of traceability. In most cases, the resolution is better than the accuracy (uncertainty). This will depend on whether the thermometer relies on a tolerance band (e.g., ASTM E1137) to determine an accuracy value or whether the thermometer was calibrated with a stated uncertainty.
d. Calibration and Performance Band (Tolerance) Testing are not the same thing.
e. A calibrated thermometer means that unique coefficients are derived that define how the thermometer interpolates between the calibration points to within a specified uncertainty.
f. A Performance Band (Tolerance) Tested thermometer is measured against a calibrated thermometer. The reading difference divided by the root sum square of the tolerance band and measurement uncertainty should be less than 1 for a pass condition.