Lesman Tech Tips

RTDs are great temperature sensors – accurate and easy to install.  But they are not friendly when it comes to trying to get a single RTD to go to two places, like when an RTD temperature measurement has to go to both a controller and a recorder.  People call and ask, “How do I split an RTD signal?”  The short answer is, “You can’t.”

An RTD cannot be wired in parallel or in series to a second device.  Any RTD input supplies a known, regulated ‘excitation’ current to the RTD.  Mixing RTD inputs would mix currents and that’s a Big No-No.

There’s also a lead wire compensation circuit for 3- or 4-wire RTDs that would create problems if a single RTD were connected to two different RTD inputs.  There’s just no feasible means of making two RTD analog inputs play nice together.

But all is not lost. There are several ways to achieve your goal.

You just need some additional hardware to replicate the RTD signal. Here’s a breakdown of your available choices:

Dual element RTD

Use a dual element RTD. Replace the single RTD element with a dual element, then wire RTD Element 1 to the first device, and wire RTD Element 2 to the second device. Use copper wire for the connections. Of course, each RTD requires its own RTD input, but that’s the assumption to start with.

Retransmit analog output

Retransmit a 4-20mA current or 0-5VDC signal from the first device to the second device, assuming the first device (digital indicator, recorder, controller) has an analog output with retransmit capability.

Many times it’s available as an option: Check your specs. Honeywell calls it auxiliary output.  Precision Digital calls it retransmit output.  Sometimes the 4-20mA is active, powered internally, like the Honeywell.  Other times the 4-20mA is passive, powered by an external loop power supply.  Precision Digital provides a DC power supply to power its passive analog output, if needed.

Temperature transmitter

A temperature transmitter’s 4-20mA output can sometimes be connected in series to two different devices.  I say ‘sometimes’ because there can be issues with ground loops when three different devices (the transmitter and two separate receivers) are involved.  It usually works with high-end analog inputs with good isolation.

If you’re using a low-end PLC or the like, be wary: This method is fraught with peril when used with low-end single-ended, non-isolated analog inputs because of the possibility of ground loops.

Splitter or Signal multiplier

A splitter, aka signal multiplier, is a stand-alone signal conditioner that is really a dual output temperature transmitter.  The advantage is that it provides two isolated outputs for a single input. The isolation ensures there won’t be a problem with ground loops when connecting to two different devices.  The photo shows API’s model APD 1393 RTD splitter with two isolated outputs.

Digital protocol

A device that can handle an RTD input, like a controller or PLC, might speak a digital communications protocol, like Modbus.  If your device has Modbus, a Fieldbus protocol, or OPC, you can use digital communications to distribute the temperature data.  This option requires a higher level of implementation skill than the preceding analog solutions.

Depending on your needs, budget, and implementation resources, you can now decide which method works best to get your RTD temperature signal to more than one device.

Now, it’s your turn

I’m curious: Have you tried any of these options? Which one works best for you? Which have you tried without success?