RTD Out

I've never encountered a signal converter that *outputs* an RTD (resistance) signal. The cost of this kind of signal converter may likely exceed the cost of a better thermostat (i.e. one that can accept a 4-20 mA signal directly).

If you terminate the RTD wires as close as possible to the RTD then the lead resistance becomes relatively immaterial. From there run 3 wires in order to provide compensation for the line resistance. Might cost you 1% in accuracy but usually thermostats are not that accurate in any case.

The RTD amplifier injects a small current and actually reads the voltage drop across the resistance. There is (usually) some compensation for the voltage drop in the wiring.

As long as your 4-20mA is isolated (or the thermostat), feed it to a low ohm resistor to generate millivolt signal and connect it to the signal and zero wire. Leave the current source open. The thermostat will read that. Note that some RTD sensor also detect an open wire when the current flow is too low. In this case, you may want to connect the source wire to the return through a 100ohms resistor to form a dummy load.

But, the scaling for the RTD scale will not work. You will need to do some work on your 4-20mA source to read the correct temperature. This may be very difficult.

>a supplier that can offer this? (google fails miserably)

The only devices that simulate resistance that I am aware of are commercial instrumentation calibrator/simulators.

The Beamex MC5 calibrator (from Finland) makes this overview statement about how it accomplishes that:

It's not that it can't be done, but apparently, there's little commercial market for such.

If your thermostat is only 2 wire input then use whatever it offers for offset/bias compensation because wire resistance is 'seen' as a constant 'high error' offset.

Thanks for commenting.

My thermostats are typically 3W RTD, and I know the implications of going to 2 wire very well. (reason to choose for 4-20mA on the wires)

It looks as if there is no commercial product available to translate 4-20mA to RTD.

You might find it strange but there are sometimes good reasons to work with a specific type or family of thermostats.

• To start is the integration in the management system.
• Current clamping and supervision is another good reason.
• Local standardization: process operators and maintenance is trained on that specific type and adding another is just not accepted.

And this type of tricks can make the difference in winning a project (I could use the twisted pair available, the competition has to add a cable of sufficient size to sense a temperature 1000m from the control room)

If you might find/know of or manufacture a device that might do the trick, don't hesitate to inform me, others might also listen in and use it for their specific problem.

Regards,

Gwen

It depends how flexible you, your 4-20 mA transmitters and "RTD input" are.

Many RTD instruments apply a current source to the RTD to get a voltage proportional to resistance e.g. 1 mA gives 100 mV at 100 ohm and 200 mV at 200 ohm. If you feed 10 to 20 mA into a 10 ohm resistor you will get 100 mV to 200 mV to apply as your "RTD input". The 1 mA bias current can be nulled out by adjusting for 9 to 19 mA from your remote RTD to 4-20 mA temperature transmitter.

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