4.1 Digital thermometers are used for measuring temperature in many laboratories and industrial applications.

4.2 For many applications, digital thermometers using external probes are considered environmentally-safe alternatives to mercury-in-glass thermometers. (1)3

4.3 Some digital thermometers are also used as reference or working temperature standards in verification and calibration of thermometers and also in determining the conditions necessary for evaluating the performance of other measuring instruments used in legal metrology and industry.

1.1 This Guide describes general-purpose, digital contact thermometers (hereafter simply called “digital thermometers”) that provide temperature readings in units of degrees Celsius or degrees Fahrenheit, or both. The different types of temperature sensors for these thermometers are described, and their relative merits are discussed. Nine accuracy classes are introduced for digital thermometers; these classes consider the accuracy of the sensor/measuring-instrument unit.

1.2 The proposed accuracy classes for digital thermometers pertain to the temperature interval of –200 °C to 500 °C, an interval of special interest for many applications in thermometry. All of the temperature sensor types for the digital thermometers discussed are able to measure temperature over at least some range within this interval. Some types are also able to measure beyond this interval. To qualify for an accuracy class, the thermometer must measure correctly to within a specified value (in units of °C) over this interval or over the subinterval in which it is capable of making measurements. Those thermometers that can measure temperature in ranges beyond this interval generally have larger measurement uncertainty in these ranges.

1.3 The digital thermometer sensors discussed are platinum resistance sensors, thermistors, and thermocouples. The range of use for these types of sensors is provided. The measurement uncertainty of a sensor is determined by its tolerance class or grade and whether the sensor has been calibrated.

1.4 This Guide provides a number of recommendations for the manufacture and selection of a digital thermometer. First, it recommends that the thermometer’s sensor conform to applicable ASTM specifications. Also, it recommends minimum standards for documentation on the thermometer and informational markings on the probe and measuring instrument.

1.5 The derived SI units (degrees Celsius) found in this Guide are to be considered standard. However, thermometers displaying degrees Fahrenheit are compliant with this guide as long as all other guidance is followed.

1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.