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[China Glass Net] The influence glass thermometer is a commonly used temperature measuring instrument, which has the advantages of low price, accurate measurement, convenient use and wide application range. Glass thermometers also produce data inaccuracies during the measurement process. What is the cause? The following is a brief introduction to the reasons that affect the measurement of glass thermometers. I hope to help everyone.
First, the impact of personnel reading
When reading the thermometer value, if the line of sight of the eye is not perpendicular to the line of the thermometer, the reading will be higher or lower. For first-class mercury thermometers, the positive and negative sides can be averaged to eliminate or reduce the reading deviation.
Second, the impact of the scale displacement on the indication
Since the glass of the thermometer is thermally expanded after being heated, a slight change occurs in the relative position of the scale and the capillary of the internal standard thermometer, thereby affecting the temperature indication and becoming a measurement uncertainty component. In general, this component of thermal expansion is negligible.
Third, the impact of capillary unevenness on the indication
The glass thermometer is fixed at several points in the fixed point, scale and timing of the scale. This indexing and verification method is based on treating the capillary as uniform, which is not the case. Since the capillary aperture is not uniform, there will be errors in the small interval scale. For thermometers with low accuracy, the error is negligible, but it must be corrected for the first and second standard mercury thermometers.
Fourth, the effect of the exposed liquid column on the indication value
In theory, the conditions used for the full immersion thermometer and the immersion thermometer should be consistent with the conditions of the indexing. However, due to limited conditions, the full immersion type should be used in the immersion mode, and the effect of the liquid column will cause the thermometer to be low. The immersion type also affects the indication value because the exposed liquid column is different from the ambient temperature at the time of indexing. The effect of the above two conditions on temperature measurement must be eliminated by correcting the exposed liquid column temperature.
Fifth, the impact of time lag on measurement
The time lag error of the thermometer is expressed as a time constant. The time constant 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, the length, the shape of the temperature sensitive bubble, and the thickness of the glass, and also to the condition around the medium to be measured, the type of liquid or gas, and whether it is uniform. Since the thermometer has a time lag error, it is necessary to read the thermometer when the thermometer and the measured medium are actually in thermal equilibrium when using or calibrating the thermometer.
Sixth, the impact of zero change on the indication
The zero point change is caused by the thermal aftereffect of the glass. When the temperature felt by the glass temperature-sensing bubble gradually increases, the glass molecules are also re-arranged, and the temperature rises to increase the volume of the glass-sensing bubble. If the thermometer is taken out of the high-temperature medium, the temperature will suddenly drop. The rearrangement of the glass molecules can not keep up with the change of temperature, so that the volume of the temperature-sensing bubble can not be restored to the original state, which is the thermal aftereffect of the glass. Due to the heat aftereffect, the volume of the temperature sensitive bubble is slightly larger than before use, so the zero position at this time is lower than before use. Although the reduction of this zero position is temporary, the volume of the thermosensitive bubble will gradually recover as the molecular structure of the glass slowly recovers, but it takes a long time. This is why the thermometer, especially the standard thermometer, changes in zero. The change in zero will directly affect the uncertainty of temperature measurement.
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