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8 Handling
8.4 Maintenance
To ensure the thermometers work reliably, their functionality must be checked at regular intervals. This
check should look at the functionality of the measuring circuits and whether there is any damage to the
protection tubes.
Individual items from the functional check (chapter 8.3 "Functional test", Page 36) can be repeated if re-
quired. We recommend measuring the insulation resistance of the measuring circuit against the protec-
tion fitting every 12 months (except for welded mineral-insulated thermocouples). If there are several
measuring circuits, also check the insulation between them.
The minimum insulation resistance should generally be 100 MΩ at DC 100 V (measured at room tem-
perature). DIN EN 61515 provides a more detailed description of how to measure the insulation resis-
tance for thermocouples. The specifications in the standard are decisive.
Drift effects in temperature probes (caused by moisture) are often not detected but nevertheless cause
significant measurement deviations. Checking the insulation resistance enables this error to be detected
and subsequently remedied.
Also check the following points:
• Corrosion and correct positioning of the contacts and terminals of cable connections
• The seals of terminal heads and cable runs
• Damage and corrosion on protection tubes
• Interruptions, testing through "knocking" on the thermometer/measuring insert
• Acquisition of a reference value (check the process value using a reference thermometer)
The check intervals can be based on the following guideline values, but will need to be tailored to the
operating location and conditions in individual cases:
Maximum operating tempera-
ture
Pt RTD temperature probe Thermocouple
200 °C 5 years 5 years
550 °C 2 years 5 years
700 °C 1 year 2 years
1000 °C 1 year
(2 years for precious metals)
1500 °C 1 year
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