How to use a multimeter to measure a valve position switch and a valve position transmitter that are in operation?
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For normal valve position switches, it’s possible to take measurements directly when the power is turned off. But how can a multimeter be used to measure valve position switches that are in operation and cannot have their power turned off?
The best way is to check the transmitter’s user manual.
Valve position switches are generally normally open, meaning they close when in contact. If it is a valve position switch with interlock, in accordance with the interlock priority principle and fail-safe concept, a normally closed type is generally used, meaning it opens when in contact. Valve position switches generally come in contact-type and proximity-type versions. Taking a general case as an example, for contact-type limit switches, dry contacts can be used to measure resistance – the resistance is approximately 0 when they are closed – while wet contacts can be used to measure voltage; there is no voltage when they are closed. Proximity limit switches require current signal measurement depending on their different types and the corresponding type of safety barrier. The valve position transmitter can measure the current signal when the control valve is not in automatic mode.