I have two, 3-wire RTD sensors but the cable back to the converter has only 5-wires available. Can I combine wires somehow? Thank you. [email protected]

 

In a three wire RTD one of the wires is connected to one side of the temperature element and the remaining two wires are connected to the other side of the element. The purpose of the third wire is to allow the temperature transducer to compensate for the resistance that the field cable adds to the total resistance in the RTD circuit. A typical transducer uses a Wheatstone bridge arrangement for the measurement circuitry.

Without lead length compensation, the temperature measurement would be off by the amount of resistance added to the RTD's resistance, and given that a typical industrial RTD is only 100 ohms at 0 deg C, the error can be significant depending on the length of the field cable.

I wouldn't share field wires between the two temperature elements as this will set up a loop between the two transducers that will most likely cause measurement errors. The best solution is to pull in a new cable but if that's not practical, and all wires are the same gauge, you can measure the resistance of a pair of leads, divide that value by two and install an equivalent resistor across the transducer inputs. (They should be clearly marked.)

As an alternative, you can just install a jumper, check the temperature reading in a calibrator or ice bath, and offset your reading in the transducer or your DCS, etc.

 

I have a solution but it is rather weird. Use two wires to drive the coil of a 3PDT relay located near your RTDs. You should prefer a DC coil. Connect the common point of each pole to one of the 3 wires. Connect a RTD to the NO side of each pole. Connect the other RTD to the NC side of each pole.

Drive the coil with a square ware, let's say at 1 Hz or less. In your PLC (or possibly DCS), you will "freeze" a RTD value while you are reading the other.

The relay must be a mechanical one and it's life will be rather limited. One million operations is typical for a mechanical relay. Mercury wetted contacts have a better life.

Hope this help.

 

Since most RTD modules are multiplexed, it's possible to share the wire going to the end of RTD which has only 1 wire on. (sketch sent)
Roy

 

Can You please send me same sketch at [email protected] ???
Thanks a lot

 

If the two RTDs are mounted in the same physical location or relatively close to each other, then you can use one wire of the 5-conductor cable as the line (wire) length compensation lead for both RTDs, since the line (wire) length should be roughly equal.

One way to do this is to take the line lead compensation wire of each RTD, connect them together at one terminal, and then use one wire to get the signal back into the "monitor" or control system, and split that signal again to the two inputs of the "monitor" or control system.

So, take one of the two common leads of each RTD (there should be a white and two blue leads, or a white and two red leads, for example; the common leads are the ones of the same color) and connect them to the same terminal of a terminal board. Use one conductor of the 5-conductor cable to get that signal back into the control room, and then on the other side of that wire connect two wires and run them to the compensation input of the "monitor" or control system. That will leave you with two wires for each RTD in the 5 conductor cable.

Just be aware that if you're not using shielded cable, you will likely have electrical noise problems! And, if the cable isn't run in an appropriate conduit or cable tray to provide signal level separation, it will be pretty "noisy". And, it would be best if the 5-conductor cable was composed of two twisted, shielded pairs (one for each RTD), and a single, insulated wire to use as the compensation lead wire.

But this only works if the RTDs are relatively close to each other. And only if the right kind of wire/cable is used, and only if the wire/cable is run in an appropriate conduit or cable tray.

 

One of the three wires is a common. You should be able to tie the two
commons together and run them back as a single wire.