Thread correction

In liquid glass thermometers, the thread correction is the necessary correction of the temperature displayed when the thermometer is operated under conditions other than those under which it was adjusted. It is a computational correction of the measurement deviation that then occurs .

Unless otherwise stated on the scale , thermometers are adjusted “fully immersed” so that they must be completely immersed in the object to be measured for correct temperature measurement. This is not a problem with thermometers that measure air temperature . However, thermometers that are completely immersed in a liquid often cannot be read. This is especially true if the vessel wall is opaque or if the liquid is cloudy. A reading in solids can be ruled out completely immersed.

The thermometer, which protrudes for reading, shows too little compared to the one completely immersed around the thread correction

For the purpose of continuous temperature measurement (temperature observation) with a liquid glass thermometer, in most applications the thread (the liquid column in the thermometer tube) protrudes from the material to be measured. If the ambient temperature is lower than the measuring point temperature, the thermometer indicates that the temperature is too low; because the thread is cooler than the object to be measured and the liquid in the thread does not expand enough for a correct measurement.

The thread correction K results from:

${\displaystyle K={\frac {n}{M}}\,(t_{a}-t_{f})}$

n = length of the protruding thread, expressed in ° C on the temperature scale

t _a = displayed temperature

t _f = mean temperature of the protruding thread

M = material constant

M = 6000 ° C for mercury

M = 10000 ° C for gallium

M = 1100 ° C for petroleum

M = 1000 ° C for ethanol, toluene, pentane

The source cited below contains the value that contradicts its previous edition and supplement 1 of DIN 12770

M = 10000 ° C for ethanol, toluene, pentane

The mean thread temperature t _f can be provisionally determined with a second thermometer held halfway up the protruding thread or, in the case of higher accuracy requirements, with a special thread thermometer.

example 1

The picture shows a completely immersed mercury thermometer on the right and one with a protruding thread on the left. Despite the identical thermometers and ideally mixed liquid, the one on the left shows a lower temperature. Here, M = 6000 ° C and an average filament temperature t _f = 35 ° C was used.

${\displaystyle K={\frac {(193-155)\,^{\circ }\mathrm {C} }{\displaystyle 6000\,^{\circ }\mathrm {C} }}\cdot (193-35)\,^{\circ }\mathrm {C} =1,0\,^{\circ }\mathrm {C} }$

Example 2

An oven temperature is measured with a mercury glass thermometer at t _a = 279 ° C. The thermometer protrudes from the oven above the scale value of around 50 ° C. A protective tube around the thermometer has a temperature of around 40 ° C, which is then also the thread temperature. Then, with the appropriate calculation, K = 9 ° C.

This means that the temperature displayed is 9 ° C too low. The corrected oven temperature is 288 ° C.

Tabulated values ​​for temperature correction can be found in books of tables, for example in Küster-Thiel .

source

VDI / VDE guideline 3511, sheet 2 (1996) "Technical temperature measurements"

Individual evidence

1. ^ Küster-Thiel, calculation tables for chemical analysis , Walter de Gruyter. Berlin New York, 1982, p. 114f ISBN 3-11-006653-X .