Standard Guide for Use and Testing of Dry-Block Temperature Calibrators
5.1 This guide applies to temperature sources with controlled temperature solid blocks. They are known under various names such as dry-well calibrators, dry-block calibrators, and temperature block calibrators. They are typically comprised of solid block materials such as metal or ceramic, a temperature-regulating device, a control sensor, and some built-in indicator of temperature in a portable package. Dry-block calibrators are commonly used for calibration of industrial thermometers. These calibrators are commonly used in either two modes: (1) the direct mode in which the calibrator is used as the calibrated reference, or (2) comparison mode in which the calibrator is an isothermal temperature source for comparing thermometers under test to a separate calibrated reference thermometer. The uncertainty of these calibrations is dependent on which of these two modes is used and a variety of thermal properties of the specific dry-block designs.
5.2 A thermally uniform, stable, and accurate temperature zone for calibration may be achieved with given measurement uncertainty. Various thermal properties of dry-block calibrator blocks have been identified that shall be characterized and/or quantified to determine uncertainty of measurements and care taken during the calibration process to optimize results appropriately. Temperature stability has been long recognized as a variable to be characterized. Others include axial temperature uniformity, radial temperature uniformity, stem conduction, block loading, hysteresis, and controller accuracy. External factors that influence results include ambient temperature, drafts, and power fluctuations. Recognizing and testing these properties will greatly improve calibration results.
1.1 The values stated in SI units are to be regarded as the standard. No other units of measurement are included in this standard.
1.2 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.
1.3 This guide is intended for use with dry-block temperature calibrators without the use of fluids or thermal contact-enhancing media over a range of -100 °C to 1700 °C.
1.4 In this guide, the essential features of dry-block calibrators used for the purpose of thermometer calibration in either the direct or comparison mode are described. The direct mode is defined as using the dry-block calibrator as a standalone instrument with the control sensor and the calibrator display serving as the reference while the comparison mode uses an external sensor and ancillary measurement system as the reference.
1.5 Measurement practices to optimize the accuracy of a dry-block calibrator to obtain optimum results are proposed in this guide.
1.6 Tests that can be performed to define uncertainty limits and how they may be used in creating uncertainty budgets are proposed in this guide.
1.7 Dry-block calibrator accessories such as built-in reference thermometers, switch testing circuitry, computer communications, or current loops will not be discussed.
1.8 It is advised that liquid-in-glass thermometers not be used in dry-block calibrators, as using liquid-in-glass thermometers with a metal block may cause damage to the readout of the thermometer.
1.9 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.
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