Sensor Installation Handbook Preface 3rd edition - Siemens ...

Principles of operation
62
Time constant of measuring sensors in liquids
For measuring or acquiring medium temperatures in piping systems,
sensors are usually installed with protection pockets. These pockets
represent the first delay element in the measuring process, the air gap
between pocket and sensing element the second. The third delay
element is the sensor’s time constant. Of these three delays in series,
that of the air gap between pocket and sensing element is the greatest
since the heat conductivity of air is poor. This poor transmission of
heat from the medium to the sensing element can be considerably
improved by filling the air gap with oil or glycerin.
If glycerin is used, the welded protection pocket must be inclined.
Transfer with time constant = T
No sensors transmit the change in a measured variable instantaneously.
The delay in transmission time (the time constant, or “lag”, T) can be shown
in diagrammatic form.
Change in measured variable
Time constant T
The time taken to transmit 63% of the total change in the measured
variable is referred to as the time constant, T.
It takes a period equivalent to five times the time constant to transmit
approximately 99% of the change in measured variable.