S800 I/O with 4-wire PT100
We are using S800 I/O product family. Our PT100 sensor has 4 wire. So what is suitable RTD-module for the 4-wire PT100 sensor and right connection for that purpose.
I think the only input module for direct connection of PT100 is AI830, RTD, 8 channels.
It could be combined with TU810, TU812, TU830.
It is for direct connection of PT100 sensors, 3-wire, but according to measuring principle it could be used for 4-wire (if connect together measuring and comp. connectors together at the one end of the sensor).
The other one is to use external signal convertors from 4-wire RTD to 4-20 mA for example
I try to understand how the Ai830 module measure a resistance. Here is connection according to the document "S800 I/O Modules and Termination units". Is that correct diagram? If I would like to use 4-wire sensor, the it should be connected to C1 terminal, Am I correct? Then two wires are in series and the resistance is half.
When I'm using 4-wire sensor, then it is easy to twist current wires together (Twisted pair cable). How about now if there are two return conductors. How to use twisted pair cable correct way?
Thanks for answers. I try to understand how the AI830 compensate the wire resistance. Here comes new connection with 4 wire PT100.
Current 0,5 A is example. I think that correct value is about 1mA?
The compensation is between terminals B1 and A1. But in 4-wire connection the other side resistance is half due to the resistances are in parallel. So the compensation is too high if I understood correctly?
How does the module calculate resistance value of PT100 module? I don't unrestand. Could you clarify? First it measures V1 and we can calculate what is total resistance of circuit V1. Then it is possible to measure V2 and calculate what is total resistance of circuit V2. Then we can calculate vhat is difference between these two resistance values and it is PT100 resistance? Am I correct?
If I would like to use twisted pair cables, I need to twist positive and negative current wires together. Current value in both wires must be same if I would like to get full benefits from the twisting. Now I thinkt that I can twist wire from C1 to PT100 and B1 to PT100. Then wire from C1 to PT100 and A1 to PT100. Is that correct solution?
Thanks for help!
Can somebody help me and tell me how the AI830 measure and calculate exact resistance of the PT100-element.
Here comes my guess for the 4-wire sensors
If I would like to use 4-wire sensor, do I need to set some parameter in software so that the system knows that the wire compensation resistance should be half.
AI830 accept 3 wire only , hence leave 1 wire not connected.
do not parallel them as this will create more error than a 3 wire solution.
How does four wire works:
two wires are used to supply a current to the pt100
the other two wires are used to measure the voltage over the pt100
this will compensate for the voltage drop over the field cable
now AI830 has compromised and used a 3 wires system , assuming that the voltage drop are equal over the two wires and hense measure one and software compensate the other.
In this case we can support both 3 & 4 wire transmitters with same configuration
and you get spare wire at the same time.