RTD and Thermistors Interchangeability

Where have you looked, so far?

Text books?

Google?

RTD manufacturers?

Thermistor manufacturers?

Anywhere?

I have checked with different vendors but I have not found any.

Both RTD and thermistor are resistance based sensors. As far as I know the difference lies in the resistance vs temperature curves. There are transmitters available for thermistors in which we can program different thermistor curves. But is it possible that we program for RTD instead of thermistor? Any way for catering lead resistance in such a case?

Lead resistance can be subtracted out by means of 3-conductor cable, or simply computed for 2-wire cable.

I don't know about transmitters, but there are for sure temperature controllers with digital display and relay switch contacts that have inputs for the sensors you mentioned.

Some have also thermocouple inputs. Check for controllers, they might have transmitters too.

The query doesn't make sense.

• An RTD has a characteristic curve that produces a change in resistance with temperature.
• The transmitter re-ranges the resistance as a standardised signal, 4-20mA in the simplest instance, for communication with other equipment at the control panel and so-on. The transmitter has a zero and span adjustment on it, which is normally factory-set to the details on the Instrument Specification Sheet submitted with the purchase order.
• At intervals, it is the task of the site Instrument Technician to verify the correct setting of the transmitter, adjusting the zero and span should the need arise and recording the change, on a periodic Planned Preventative Maintenance basis.

So why is there any need for anythin in the device to be <...programmable...>, please?

For temperature transmitters please see

http://www.acromag.com/page/temperature-transmitters

We know that different type of thermistors have different characteristic curves. The function of transmitter is to change the resistance into temperature. But we cannot make a unique transmitter for every thermistor. The solution is a programmable transmitter which can be used for many different types of thermistors. It is also popularly known as "configurable transmitter".

http://www.acromag.com/catalog/1005

Now my question is, are there any transmitters which can be coupled with thermistor "or" RTD? Which has options for both RTD and thermistor?

Hope now its clear!

Why would anyone need such a thing? Thermistors and RTDs are such different animals. It is easier to match the transmitter to the sensor, and be done with it.

Yes Wal you have got right.

Why I am looking for such a transmitter? Because I want to use them for thermistors in a project and in large quantity. As a contingency I want versatile transmitters so that they can be used later even though they may be little expensive.

I am not in dire need of such a transmitter. However, this is a risk averting effort.

These tranmitters will give 4-20mA signal to DCS.

I believe if there is such a thing it will be helpful for many!

I can dig that requirement.

You probably need to be having this conversation with some vendors.

Versatility costs money. If they exist I don't think they'll be cheap, but if you do get lucky let us know.

I have done this on a project where the client wanted to test RTD and thermistors. I used some temperature transmitters with a resistance input range large enough for the range of RTDs and termistors, trasmitter 4/20mA output was linear over the range. The 4/20mA signals went into a PLC where the characteristic curves were programmed in and selected according to the RTD or thermistor connected.

Do you need more details?

That would do the Original Poster's [OP] task, should programming within the receiving equipment and not the transmitter itself, be acceptable. Of course, the OP needs to select thermistors that operate over a similar resistance range to the RTDs in question.

The OP needs also to understand that connecting a millivolt source such as a thermocouple to a resistance input on a transmitter, is only going to produce disappointment.

Planning large quantity... I wouldn't add RTD option.

Transmitter for thermistor is different than for RTD:

1. Supplying current for thermistor is usually smaller (to avoid selfheating), than for RTD

2. Voltage change on RTD (100 Ohms standard) is much smaller than on thermistor and requires much better input amplifier (more stable zero), more stable bridge and reference resistor(s), and more stable supply voltage.

So, to get good quality, You would have to make two circuits, one for thermistor, another for RTD.

http://www.omega.com/search/esearch.asp?start=0&perPage=10&summary=yes&sort=rank&search=tx1500&submit=Search&ori=tx1500

This transmitter accepts multiple types of inputs and might work for you.

Omega has a very knowledgeable engineering staff that will help solve any challanges you may have or ever incurr.

Precision Digital, Red Lion. Most any of the manufacturers who make a panel mounted display meter that is also a temperature transmitter have this capability. (I have been caught by the PD units when I didn't switch the dip switch to RTD - Red Lion units are done with prograqmming)

CORRECTION - I just caught the fact that the option other than RTD is thermistor.

Thermistor? Thermistor? Don't talk about a thermistor.

Are you sure this isn't a thermocouple and RTD?