Issues with connecting grounded thermocouples and ungrounded thermocouples to DCS
Thread Content
I once encountered a situation where the DCS readings were inaccurate when the armored thermocouple core was grounded, but they returned to normal once insulation was restored. DCS is direct thermocouple signal input (without an external isolator). Has anyone else encountered this? How to deal with it? After all, there are non-insulated thermocouple products. Thank you!
The measuring end comes in three basic forms: exposed type, encased type, and insulated type. While meeting the temperature measurement response time requirement, we used insulated types, and no ground interference issues arose.
The insulation of armored thermocouples is poor for the following reasons: 1. Moisture at the cold junction terminal box leads to water ingress, which results in poor insulation. The solution is to dry it out and clean the terminals and leads. 2. If the measurement temperature is too high, or if there is a corrosive atmosphere, it can cause the magnesium oxide powder in the insulation material of the armored thermocouple to oxidize, resulting in a decrease in its insulating properties; this is a normal phenomenon. Solution: Add a temperature isolator, or an isolated temperature transmitter.
I’m sorry; what I said earlier was that the insulation type has problems, which results in inaccurate DCS measurement values. The ground-type configuration isn’t working properly; it’s mainly due to interference. You can try adding 0.1uF ceramic capacitors between the positive and negative terminals of the thermocouple, as well as between those terminals and ground, to see if that helps. It is best to use an oscilloscope to check the type of signal from the interference source in order to determine the appropriate solution.
It has nothing to do with interference; it is the DCS hardware that does not allow the measurement terminal of the thermocouple to be grounded. It remains low as long as it is grounded; it improves immediately when not grounded
Is it a direct access to the card slot or is there a barrier? It’s not easy to make changes if you go straight to the sensor module; either you need to use heating to make the changes, or you should opt for an insulated type of thermocouple. . . Regarding the isolation barrier, you need to consult the manufacturer.
Moderator: Hello! The digital display units I’ve used with grounded thermocouples gave inaccurate readings; it turned out that grounding caused interference in the millivolt signals, leading to incorrect displays. Adding a capacitor for filtering resolved this issue. The DCS hardware may differ from those of the digital display units, so you can take measures during installation by insulating the thermocouple’s casing from the equipment’s casing. For example, in the case of threaded connections, using PTFE tape around the threads prevents the thermocouple from making contact with those threads, thereby insulating the thermocouple.