Temperature reading devices suitable for Pharmacopeial tests conform to specifications that are traceable to a NIST standard. Temperature reading devices may be of the liquid-in-glass type or an analog or digital temperature indicator type, such as a resistance temperature device, thermistor, or thermocouple.
An analog or digital temperature indicator consists of a temperature probe, which houses a sensor. The probe is attached to a meter capable of translating a signal in ohms or millivolts into a temperature reading. The temperature probe portion of the analog or digital temperature indicator that is submerged in the medium whose temperature is being measured must be made of inert material. Standardization of analog and digital temperature indicator devices is performed on an established testing frequency with a temperature standard traceable to NIST. In the selection of a temperature reading device, careful consideration of the condition under which it is to be used is essential.
Liquid-in-glass thermometers may be standardized for total immersion, partial immersion, or full immersion. Insofar as practicable, each thermometer should be employed according to the condition of immersion under which it was standardized. Standardization of thermometers is performed on an established testing frequency with a temperature standard traceable to NIST. Refer to the current issue of ASTM standards E1. Standardization of liquid-in-glass thermometers for total immersion involves immersion of the thermometer to the top of the liquid column, with the remainder of the stem and the upper expansion chamber exposed to ambient temperature. Standardization for partial immersion involves immersion of the thermometer to the indicated immersion line etched on the front of the thermometer, with the remainder of the stem exposed to ambient temperature. Standardization for full immersion involves immersion of the entire thermometer, with no portion of the stem exposed to ambient temperature. For use under other conditions of immersion, an emergent stem correction is necessary to obtain correct temperature readings.