Returning inspections
Thermometers and their measuring circuits shall be controlled regularly (1 to 12 months) with regard to:
- mechanical, thermal, chemical damage of the protecting tubes
- corrosion and bad contacts at cable connections
- tightness of connection heads
- function of the measuring circuits (drifts, insulation resistance)
Control of measuring circuits
Loosen cable connections in connection head and check instruments with regard to the operating temperature for
- thermocouples with mV signal
- for resistance thermometers with test resistance
Thus one can find out if the thermometer or the instruments are the cause of trouble.
During operation the following values of a gauge slide can be checked:
- the transmission
- the loop resistance of all conductors
- insulation resistance
- electromagnetic force of thermocouples
- resistance of Pt sensors
- wrong polarity of thermocouples
The insulation resistance of the complete ungrounded measuring circuit, cables and thermometer shall be bigger than 1 MΩ measured with 10 V DC. Process temperature and material of the cable insulation have to be considered!
Control of removed gauge slides
Rapid control of removed thermocouples and resistance thermometers and of the corresponding measuring circuits.
Necessary instruments:
- mV - meter
- Ohm - meter or resistance balance
- insulation measuring device with 10 - 100 V DC test voltage
All measurements are executed at room temperature. By „tapping” wire breaks and interruptions can be found out. The transmission and insulation control is done at room temperature. A thermocouple can be considered to be okay, if R < 20 Ω (value depends on wire section and length). For insulated thermocouples the insulation resistance shall be 100 MΩ. The warming of the measuring point to a temperature from + 200°C up to + 400°C (without temperature control) allow further conclusions with regard to wrong polarity, breaks or too low insulation resistances, etc.
Control of protecting tubes
Protecting tubes are wearing parts! At measuring points with particularly strong mechanic, abrasive or chemical attacks, every machine stop shall be used to check the condition of the protecting tubes. If necessary, they have to be exchanged in order to prevent unplanned operational interruptions. Within this brochure only some of the most frequent failures and their elimination can be described. The enumeration does not claim to be complete. The indications only refer to thermometers. The subject of the topped instrumentation is not treated here.