Appendix J: Design criteria for digital thermometers for use in critical processes
The following criteria can be used to evaluate resistance temperature devices (RTDs) for use as alternatives to mercury actuated direct reading indicating thermometers on pipelines in High temperature short time (HTST); Batch; Higher heat shorter time (HHST) or Extended shelf life (ESL); and Ultra high temperature (UHT) systems:
- Ensure there is no more than 0.25°C (0.5°F) drift over 3 months use when compared to a certified source.
- Display the readout in units of temperature with a graduation of at least 0.1°C.
- Display changes at a rate that can be noted by the operator or responsible regulatory authority during the thermometric lag test.
- Have self-diagnostic circuitry which provides constant monitoring of all sensing, input and conditioning circuits.
- The diagnostic circuitry should be capable of detecting "open circuits", "short circuits", poor connections and faulty components.
- Upon detection of failure of any component, ensure the device blanks or becomes unreadable.
- Document the effect of electrical noise and have it available to responsible regulatory authority. Protocols for these tests are developed by vendors.
- Document the effect of high temperature and high humidity environment. The device should show no effect after exposure to 38°C and 80% relative humidity for 7 days.
- Ensure both the probe and display case are constructed so that they may be sealed.
- Protect the calibration of the device against unauthorized changes.
- Protect the device against unauthorized component or sensing element replacement.
- Regard the replacement of any component or sensing element as a replacement of the indicating thermometer and subject to all applicable tests identified under the Critical process test procedures.
- Encase the sensing element in appropriate material and construct in such a way that the final assembly meets the criteria in Evaluation of dairy processing equipment.
Refer to the High temperature short time (HTST) pasteurization, Batch pasteurization, Higher heat shorter time (HHST) and extended shelf life (ESL), and Aseptic processing and packaging for additional guidance for indicating thermometers.
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