Errors due to wrong Immersion
Most laboratory errors in temperature measurement result from incorrect usage (Immersion) of the Thermometer.
Total Immersion thermometers are designed with scales, which indicate actual temperature when the bulb and the entire column are exposed to the temperature being measured. In practice, a short length of column usually one inch or less is permitted to extend above the surface of the liquid being measured to permit reading of the thermometer.
Total Immersion thermometers are inscribed ‘ Total Immersion’ on the reverse of the thermometer.
Partial Immersion thermometers are designed to indicate the actual temperature when specified portion of its stem is exposed to the temperature being measured.
Partial immersion thermometer will have 1) an inscription on the reverse of the thermometer stating the required immersion depth ie. 76 mm Imm. Or similar marking, and 2) In some cases a ‘Ring’ is inscribed on the stem of the thermometer indicating the depth to which the stem should be submerged.
What is the difference in use?
As explained above the partial immersion thermometer is immersed in the liquid being measured up to the line or ring. The total immersion thermometer must be immersed up to within approximately one inch of where the mercury column resides at the temperature being measured.
So what happens if the total Immersion thermometer is not immersed up to the depth it should be ?
We will have an incorrect temperature reading. The amount of the error depends upon what the temperature is that we are measuring, and how much of the thermometer stem that should be immersed is out side the liquid which is being measured.
An example – say we have a –10 to 250 ºC range thermometer, 24inch in length. Having 12inch immersion, and we are testing the liquid in a beaker on a hotplate. Only about 2inches of the thermometer is immersed in the liquid. The temperature indicates 190 ºC. How much error do we have? An error of almost 5 ºC, or even more. The liquid in the beaker is much more hotter than the thermometer indicates. This is because the mercury, which is in the stem, is not getting the proper temperature of the beaker. This has happened due to the reason the thermometer has not been properly immersed.