Dairy Establishment Inspection Manual – Chapter 17 - Higher Heat Shorter Time (H.H.S.T.) Processing and Extended Shelf-Life (ESL) Dairy Products
1.17.10 Indicating Thermometer

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The indicating thermometer provides the official processing temperature of the product, which is a critical factor in the scheduled process. This is to prevent situations where H.H.S.T. processing may be operated with a defective or damaged unit while waiting for a replacement thermometer. General Conditions

This thermometer is required for all H.H.S.T. processing systems. It must be clean and in good operating condition. The thermometer shall be mercury actuated or an accepted equivalent, or an approved resistance thermal device (RTD).

Mercury actuated or accepted equivalent thermometers shall be direct reading, and contained in a corrosion resistant case which permits easy observation of column and scale. The filling above the mercury is to be nitrogen or equally suitable gas. The bulb shall be Corning normal or equivalent.

The RTD type must be fail-safe, utilizing two separate RTDs. It must meet the scale and thermometric response specifications. The criteria in Appendix 19-13 - Design Requirements for Digital Thermometers shall be used to evaluate RTDs when used as alternatives to mercury actuated direct reading thermometers. Location/Accessibility

The official indicating thermometer shall be located in the sensing chamber, along with the probe for the S.T.L.R. The indicating thermometer probe should be located after the probe for the S.T.L.R. The distance between the 2 probes should not be more than 30 cm (12 inches). The indicating thermometer must be easily and safely accessible by the operator, to allow accurate reading of the processing temperature. Specifications

The scale shall be graduated in 0.5°C (1°F) divisions with not more than 9.4°C (17°F) per 25 mm (1 inch) of graduated scale.

The stem fitting shall be pressure-tight against the inside wall of the fitting, with no threads exposed to product. Calibration/Records

Records of tests performed to determine the thermometer's calibration shall be maintained in the plant's files. Tests must include temperature accuracy and thermometric response, upon installation and at an interval of at least every 6 months. The frequency of testing should be increased if the calibration is consistently found to be out of adjustment. If the calibration is consistently found to be out of adjustment, the reason for the calibration problems should be immediately identified and rectified

Testing methods shall comply with the required standards, and must show satisfactory follow-up on out of specification findings. Plant management must investigate the safety of the product produced with out of calibration equipment (e.g. if the indicating thermometer at the outlet of the holding tube is reading higher than the calibration standard, the product may have been under processed). Sealed

The access to calibration adjustments must be sealed once the thermometer has been calibrated. The cover or scale plate on mercury in glass (MIG) thermometers should have a seal attached to indicate tampering. The thermometer panel and the RTD sensor housing should be sealed on resistance thermal devices.

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