TEST METHOD: Thermometer calibration at 0oC and 100oC

Version: 30 May 2007 (additional detail on icepoint method)

OVERVIEW

Laboratory glass thermometers may become inaccurate over time. The 50oC thermometers may be calibrated against a single point (0oC) while the 100oC thermometers should be calibrated against two points: 0oC and 100oC. As the St Kilda laboratory is at sea-level, these temperatures may be found at the melting point of ice (accurate to 0.05oC) and the boiling point of water (accurate to 0.5oC).

TASK SAFETY REQUIREMENTS

Lab coat and safety shoes are mandatory. Use appropriate PPE.

SPECIFIC JOB STEPS HAZARDS WITHIN THIS STEP HAZARD CONTROL
Ice Point Method
Fill a tall plastic container with a mixture of fine crushed rainwater ice and rainwater. Sit the tall container in a 5L plastic beaker or container and surround it with ice cubes (this ice does not need to be crushed)

How to I get a fine crushed ice?

 

How can I ensure the temperature remains stable?

Put several icetrays of ice cubes into some large zippy bags. Use double bagging to prevent leakage. Crush the ice using a rubber mallet. Any excess can be stored for a short period in the freezer. Alternatively, use a blender to crush the ice.

The tall container must have crushed ice throughout the water column so that the melting ice can provide an environment of 0°C. You may have to pack more crushed ice into the container during the process.

When the mixture of the water has stabilized after four or five minutes, insert the thermometer to be calibrated to the appropriate immersion depth. How can I be sure the thermometer is not being affected by the container? Be sure to hold the stem of the instrument away from the bottom and sides of the container (preferably one inch) to avoid error.
If your thermometer does not record 0oC, record the correction 'offset' on it with an indelible pen, or replace the thermometer.
How do I decide on an 'offset' or replacement? If the thermometer 'offset' is more than 1oC, replace the thermometer.
Boiling Point Method
Use a large beaker of distilled or rainwater on a hotplate. After the water in the container has reached a complete “rolling” boil, insert the instrument to the appropriate immersion depth.
How can I be sure the thermometer is not being affected by the container? Be sure to hold the stem of the instrument away from the bottom and sides of the container (at least 2 inches from the hotplate) to avoid error.
If your thermometer does not record 100oC, record the correction 'offset' on it with an indelible pen, or replace the thermometer.
How do I decide on an 'offset' or replacement? If the thermometer 'offset' is more than 1.5oC, replace the thermometer.

 

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