What are the factors that affect the use of glass thermometers?

 

1. The impact of personnel readings

 

When reading the temperature value, if the line of sight of the eyes is not perpendicular to the thermometer scale, the reading will be too high or too low. For a first-class mercury thermometer, the reading deviation can be eliminated or reduced by taking the average value of the positive and negative sides.

 

2. The influence of scale displacement on indication

 

When the glass of the thermometer is heated, it will expand, which will cause a slight change in the relative position of the scale and capillary of the internal standard thermometer, thus affecting the temperature indication and becoming a measurement uncertainty component. Generally speaking, this component caused by thermal expansion can be ignored.

 

3. Impact of capillary non-uniformity on indication

 

When setting the scale point, grading and calibrating the glass thermometer, it is done at several specified points. This grading and calibration method is based on the assumption that the capillary is uniform, which is not the case in reality. Since the capillary pore size is not uniform, there will be errors in the small interval scale. For thermometers with low accuracy, this error can be ignored, but it must be corrected for first and second standard mercury thermometers.

 

4. Influence of exposed liquid column on indication

 

Theoretically, the conditions for using full immersion thermometers and partial immersion thermometers should be consistent with the conditions for graduation. However, sometimes due to limited conditions, the full immersion thermometer must be used as a partial immersion thermometer, and the influence of the exposed liquid column will cause the thermometer to indicate a low value. The partial immersion thermometer will also affect the indication because the ambient temperature of the exposed liquid column is different when it is divided and when it is used. The influence of the above two situations on temperature measurement must be eliminated by correcting the temperature of the exposed liquid column.

 

5. The influence of time lag on measurement

 

The time lag error of a thermometer is expressed as a time constant, which is the duration required for the temperature indication to rise or fall to 62.3% of the difference between the final value and the initial value.

 

The time constant is related to the type, length, shape of the temperature sensor bulb and the thickness of the glass. It is also related to the surrounding conditions of the measured medium, the type of liquid or gas and whether it is uniform. Since the thermometer has a time lag error, when using or calibrating the thermometer, the thermometer and the measured medium must be in thermal equilibrium before reading.

 

6. Impact of zero position change on indication

 

The zero point change is caused by the thermal aftereffect of the glass. When the temperature felt by the glass temperature-sensing bulb gradually increases, the glass molecules also rearrange accordingly. The temperature increase makes the volume of the glass temperature-sensing bulb increase. At this time, if the thermometer is taken out of the high-temperature medium, the sudden cooling will cause the rearrangement of the glass molecules and cannot keep up with the temperature change, so that the volume of the temperature-sensing bulb cannot be restored to its original state. This is the thermal aftereffect of the glass. Due to the thermal aftereffect, the volume of the temperature-sensing bulb is slightly larger than before use, so the zero point at this time will be lower than before use. Although this reduction in zero point is temporary, as the glass molecular structure slowly recovers, the volume of the temperature-sensing bulb will gradually recover, but it will take a considerable amount of time. This is the reason for the zero point change of the thermometer, especially the standard thermometer. The change in zero point will directly affect the measurement uncertainty of temperature.