WIKA SC15 - Handbuch (Seite 8)

GB
5. Installation, commissioning, operation ... 7. Indicator check
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009 GB/D
WIKA Operating Instructions Combistat Model SC 8
5.2 Ambient conditions
Unless the ingress protection of the thermometer's case is specifically identified
in the order confirmation, the instrument must be protected from humid air and
other aggressive atmospheres.
The ambient temperature at the indicator case should be between 0 °C ... + 40 °C
in order to ensure the best possible measuring accuracy. Higher or lower ambient
temperatures can lead to indication errors.
6. Troubleshooting
Expansion thermometers with micro switch sare maintenance-free instruments
according to their basic design. As measuring instruments, their measuring
accuracy should be checked at application-specific intervals.
The wear condition of probes exposed to a permanent thermal stress (even
if this stress is very low), of electro-mechanical contacts and of capillaries
exposed to vibratory stress must be checked from time to time.
If any visible damage is found, the instrument must be replaced.
7. Indicator check
Indicator checks should only be carried out in comparison with a more accurate
instrument or, if possible, with a calibrated instrument. The temperature during
the check must remain constant. Fluctuating temperatures can lead to reading
errors caused by the different response times of the probes.
Before checking the indicators of thermometers without thermowells, a waiting
time of at least  minutes with the stem‘s full length properly inserted is
required to allow temperature equalisation.
Temperature checks using thermometers with probes assembled with thermo-
wells can, in many cases, only result in a reference temperature at the
instrument under test, due to permanent heat dissipation caused by the
thermowell. In temperature applications fitted with static checking thermo-
meters (e.g. pipeline systems), permanent offsets in reference temperatures can
also be caused by the heat transfer path.
Ambient temperatures around the indicator housing which differ substantially
from room temperature can lead to steady indication errors with constant
ambient temperatures and to varying indication errors with fluctuating ambient
temperatures.
Permanent indication errors caused by the ageing of the measuring system can
be ignored, as they only account for a fraction of the indication accuracy.