VAL-235 Calibration Procedure (Ice Point) for Themometers and Thermocouples

A. APPLICATION 

To provide an established procedure for use when performing temperature calibration on equipment to controlled standards at the GMP site.

This document covers preparation of the apparatus used and the method to be followed when performing an ice point calibration. It also covers the procedures to be followed when completing associated records.

This procedure is applicable to the calibration of the following:

 – Thermometers

 – Thermocouples

B. RESPONSIBILITY AND AUTHORITY

Person performing calibration:

 – To calibrate appropriate equipment in compliance with this procedure.

 – To complete and sign all forms needed as stated in the Procedure section of this document.

 – To properly report/record any unusual conditions found during the calibration/certification procedure. 

C. SAFETY AND PROCESS SPECIFIC INFORMATION

All safety requirements for relevant areas must be followed at all times.

EQUIPMENT

The following equipment is required to perform an ice point calibration:

 – Insulated container – such as a wide-mouthed flask or small Esky for storage of ice slurry bath

 – Purified water (WFI or deionized) – for making the ice slurry bath 

 – Ice – made from purified water (WFI or deionized)

 – Mallet/hammer – to break ice into small pieces if required

 – Stirrer – to ensure an even mixture

 – Suitable clamp and stand – for supporting the thermometers or thermocouples

D. PROCEDURE

 1. Background

 1.1. Temperature reading devices can be calibrated using sophisticated calibration equipment such as a dry block calibrator, available from instrument suppliers. However, in most cases calibration in melting ice (0°C) for cold measurements is an effective method. The ice point is the equilibrium between a mixture of melting ice and air saturated water, and represents an easily reproduced reference point to check accuracy. An ice point is always included as part of the full calibration for all thermometers that have a 0 °C mark, and for thermocouples where a low reference point calibration is required. 

 2. Ice Point Calibration

 2.1. Crush the ice using a hard implement such as a mallet or hammer by placing of ice cubes into some large plastic bags. Use double bagging to prevent leakage. Crush the ice using a rubber mallet.

 2.2. Fill the insulated container with the crushed ice. The ice should be wet, i.e. starting to melt (it will not be opaque but appear “glassy”). If necessary add some water to wet the ice then add clean freshwater to a depth of at least 10 cm (4 inches) – 50% ice, 50% water. If the ice floats on a pool of water there is too much water and the excess should be drained off.  

 2.3. Stir the ice and water then wait a minimum of 5 minutes to be certain the water is completely cooled and good mixing has occurred.

 2.4. Suspend the stem of the items being calibrated in the stirred ice slurry. Clamp in place if necessary. Ensure they are immersed to the required depth and to hold the stem of the instrument away from the bottom and sides of the container (preferably 2 cm) to avoid error. They can touch each other.

Note: The required depth of immersion should be checked from the instructions or certification that accompanies the thermometers. The required depth may be marked on the thermometer. Secure the thermometers in place with a clamp. The thermometers must be immersed to the required depth but liquid-in-glass thermometers must also be readable with the top of the liquid column at eye level to avoid parallax errors.

 2.5. Wait 5 minutes until the indicator stops changing. Do not lift the thermometers out of the ice mixture to read them. Allow the thermometers to reach a steady reading. A steady reading is reached when there is no difference between two readings taken 1 minute apart.

 2.6. Note the temperature of the reference. Record this temperature on the calibration record sheet or log book and apply any correction required. Record the identification of each working thermometer or thermocouple (if required) and the stable temperature it reached on your calibration record sheet. Record the difference between the corrected reading of the reference and the working thermometer reading as the offset or correction. 

Note: The temperature of the working thermometer must be subtracted from the reading of the certified thermometer or reference source to obtain the correction. This will produce a negative number if the temperature reading of the working thermometer is greater than the temperature reading of the certified thermometer.

 2.7. For digital thermometers if the ability exists adjust the thermometer to 0ºC (32ºF) while immersed in the ice water bath. These thermometers are generally adjusted with a zeroing screw. 

2.8. Write the correction for each thermometer on a tag attached to the thermometer.

 2.9. If the thermometer ‘offset’ is more than 1°C, replace the thermometer.

Note: Thermometers and temperature loggers should be calibrated near the temperatures they are intended to monitor. Thus, thermometers used should be calibrated at a cold temperature e.g. 0°C and a hot temperature e.g. about 37 to 70°C. 

E. REFERENCED DOCUMENTS

VAL-255 Verification of Dataloggers