Guidance on Resistance Temperature Detectors used as Indicating Thermometers for Retort Vessels

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Traditionally, Mercury-In-Glass (MIG) thermometers have been the only internationally recognized devices permitted for use as temperature indicating thermometers for food thermal processing systems. However, empirical evidence has established that resistance temperature detectors (RTD) can be used in place of MIGs without compromising safety.

The Canadian Food Inspection Agency (CFIA) has received a number of requests from the food industry for guidance on the use of Resistance Temperature Detectors (RTDs) in food thermal processing systems. In response, the CFIA formed a working group to develop this guidance document for the safe use of RTDs as indicating thermometers in retort systems.


The use of RTDs as indicating thermometers on food thermal processing systems allows Canadian producers to use progressive technology in monitoring their thermal food processes. RTDs and their displays have faster response times, are easier to read, are more precise, have higher frequency of recording intervals and are more sensitive to temperature changes.

If companies choose to replace MIGs with RTDs, potential environmental and food safety concerns about broken glass and mercury contamination due from MIGs would be addressed.


This guidance document is intended for retort operators wishing to replace a mercury-in-glass indicating thermometer with a resistance temperature detector, and does not necessarily apply to recording RTDs or any other gauges on the retort.

This document provides operators with guidance for ensuring that the switch to RTDs does not in any way compromise the thermal processing equipment in achieving the intended food safety outcome, such as commercial sterility.

Guidance for the use of resistance temperature detectors

Consider the following criteria to assess whether you are using RTDs in a manner that supports food safety outcomes.

1. The indicating RTD thermometer is the official instrument for all temperature readings, including vent temperatures, come-up temperatures and process temperatures during the processing of low acid canned foods (LACF). It is not used to directly control the retort or thermal processing operation.

2. The indicating RTD is calibrated to an official recognized standard reference thermometerFootnote 1 or equivalent upon installation and at least annually or more frequently if necessary, to ensure accuracy during processing.

If adjustment is required, the operator should evaluate the calibration frequency and determine whether food processed before the adjustment of the device may be under processed.

The RTD is protected against unauthorized changes by the use of seals, tags, electronic prevention or other adequate means.

3. The RTD is accurate to within 0.5°C (1.0°F) of the actual temperature.

4. The RTD has an easily readable display, in units of temperature degrees in graduation at least 0.2°C.

5. The operator ensures that adequate power is provided to the RTD while in operation.

6. Operators ensure that indicating RTDs are accurate during processing by using devices with suitable features, and having procedures in place to verify the system.
For example:

  • the RTD must possess self-diagnostic circuitry which provides constant monitoring of all sensing, input and conditioning circuits.
    • the diagnostic circuitry should be capable of detecting open/short circuits, poor connections, and faulty components.
  • upon detection of a failure of any component, the device must be capable of providing a warning to the operator of the thermal processing equipment.
  • the device is calibrated (see requirements 2 and 10).
  • processors may also compare multiple thermometers on the retort to determine whether failure of any of these thermometers has occurred.

7. The operator ensures that the accuracy of the device is not affected by electromagnetic interference (eg. from hand-held communication devices) or environmental conditions (eg. temperature, humidity, moisture) within the processing plant. The effects of electrostatic discharge, power fluctuation, conductive emission and susceptibility, and radioactive emission and susceptibility is tested and accounted for. The device complies with the requirements for performance level characteristics of industrial devices. Proof that the device meets these requirements is documented and available to the CFIA upon request.

8. Replacement of any component is considered a new installation and is subject to all applicable calibration testing before reactivating the unit.

9. The RTD probe and display case are constructed of suitable, corrosion resistant materials.

10. The installation, maintenance and calibration are carried out according to the manufacturer's recommendations, by qualified and trained professionals with working knowledge of the thermal processing equipment and the indicating thermometer.

Each RTD is installed where it can be accurately and easily read and the temperature sensing area of the probe is fully exposed to a uniform heating medium, with a continuous heating medium flow past the probe.

Calibration and other tests are documented and made available to the CFIA upon request. For each specific indicating RTD outline the following information:

  • Identity of the unit, unit serial number, model number, seal or tag
  • Identity of the standardized reference device used to calibrate, along with a description of the calibration method
  • All test records are signed and dated by the person(s) who carried out the testing


Acidified Foods
Low-acid food to which an acid or an acid food is added to produce a food with a final equilibrium pH of 4.6 or less and a water activity greater than 0.85
The standardization of a measuring instrument, by testing a measuring instrument against an accurate standard to determine any deviation and correct for errors
Commercially Sterile
The condition obtained in a food that has been processed by the application of heat, alone or in combination with other treatments, to render the food free from viable forms of microorganisms, including spores, capable of growing in the food at temperatures at which the food is designed normally to be held during distribution and storage
Hermetic Seal
A container designed and intended to be secure against the entry of microorganisms, including spores
HTST Pasteurizer
Thermal processing equipment used for heating liquid food
Indicating Thermometer
Thermometer used to indicate the temperature environment of the food thermal processing equipment. It is used in the recording of the processing temperature to determine that an adequate thermal process temperature has been reached and maintained as set out in the processing schedule to achieve commercial sterility
Low Acid Foods
A food, other than an alcoholic beverage, where any component of the food has a pH greater than 4.6 and a water activity greater than 0.85
Mercury-In-Glass (Indicating Thermometer)
Reference Thermometer
Standardized thermometer used to calibrate the Indicating Thermometer
Closed vessel used for thermally processed hermetically sealed food containers under pressure, to achieve commercially sterile food
Resistance Temperature Detector which utilizes changes in thermal resistance to measure temperature
Thermal Process
The application of heat to food either before or after sealing in a hermetically sealed container, for period of time and at a temperature scientifically determined to achieve a condition of commercial sterility

For Inquiries contact Grant Campbell at 613-773-5477 or at

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