Reading multiple RTD's with myDAQ

[Sean1234]

I'm working on a project with myDAQ that requires reading three RTD's constantly.

Two of the thermistors are 3-wire, the other is 4-wire. I followed the tutorial on the website for RTD's with one difference - I am connecting the RTD to the analog input. The excitation current is provided by the 5V power source on the myDAQ. I measured the current to be 37.3mA. When I open the DAQ assistant and pick all of the settings, the resistance reading is a constant 124 ohms. I've held the RTD in my hand, put it in ice water, and this reading hasn't changed. However, when I provided power to the RTD, and checked the resistance with a multi-meter, it read 115, and this resistance *did* change when placed in ice water.

Can anyone help me out here?

[Matt_L]

Hi Sean,

To clarify, do you mean that you're trying to connect the 4-wire RTD to an analog input pin on the myDAQ?  I believe that only the DMM terminals on the myDAQ can be used to take this type of measurement, be sure you have the thermistor wired according to the guide here: http://zone.ni.com/devzone/cda/epd/p/id/6412.

[Henrik_Volkers]

What sensors are you using exactly? (link to datasheet?)

Temperature range?

Needed resolution?

Desired uncertainty?

3 or 4 wire makes no difference here since the device can only measure 2 wire 😉 

If you run a 124 ohm sensor with an excitation current of 37,3mA, it will self heat (burn) the sensor with about 0.17W !!  Read the spec of the sensors about useful exitation current ranges!

Usually you run 100 Ohm RTDs with 1mA to 0.1mA !!

If you still have the choice to choose sensors I would recommend 10k NTCs and 10k resistors in a voltage bridge, but that depends on your answers to my questions.

[Sean1234]

I connected the RTD's using this as a guide: http://forums.ni.com/t5/Academic-Hardware-Products-NI/Reading-multiple-RTD-s-with-myDAQ/td-p/2385932

Are you saying that if I use the diagrams under section 2 as a guide, with v- and v+ being the analog pos and neg imputs, this will just not work? 

Also, I understand that 37mA might cause self-heating, but that's not the issue yet, if it was self-heating, I wouldn't be getting a static reading. The problem is that I get a normal reading using a handheld multimeter, but when I use the analog inputs on the myDAQ, the reading never changes.

Also, I ordered the thermistors from omega, and they came with no datasheet except the website says they are .00385 coefficient 100ohm platinum RTD's. I have been using a seperate datasheet I found here: http://instrumentation.com/RTDTables/385_c.pdf. 

[Matt_L · Accepted solution]

I couldn't see the guide you linked to, it looks like it redirected back to this forum?  If you were referring to the myDAQ, then you could use a 3 or 4 wire thermocouple, just in 2 wire mode as the as the article I linked described.  And yes, this measurement would appear to only be possible between HI and COM, not between analog input +/-.

[charlesy]

Hello Sean,

RTDs and thermistors are thermally-sensitive resistive devices -- you want to measure its resistance and back-calculate the temperature. myDAQ has the ability to measure resistance directly on its DMM terminals (the banana jacks). In this case, you don't apply any external excitation. Just connect the 2-, 3-, or 4-wires from your device to the HI and COM banana jacks. Obviously, this defeats the wiring resistance compensation feature of 3- and 4-wire RTDs.

If you want to use the two AI channels on the screw terminals, you have to add excitation. Remember, you need to measure the resistance of the RTD, but the AI channels only measure voltage. At the simplest, you could create a voltage divider from the +5V rail with a resistor (R1) and your RTD. Connect the AI+ and AI- across the RTD. The voltage you measure is equal to 5V*[R.rtd / (R.rtd + R.R1)]. You'd pick the size of R1 to balance self-heating in the RTD against the necessary measurement resolution. The AI inputs on myDAQ have +/- 10V and +/- 2V ranges with 16-bits of resolution (you'll want to use the 2V range). This will give you rough measurement capability on the two AI lines, but it's not very accurate because the +5V rail is not tightly regulated. A better solution would be to hang a reference or linear regulator on the +5V rail (maybe creating +2.5V) and use that for your RTD excitation.

Hope this helps,

Charles Y.

National Instruments

[Sean1234]

Thanks everyone for your responses.  We learned a decent amount just from those.

Special thanks to Matt Lee; we didn't realize the thermistors had to be read through the DMM.  It added a decent amount of work, but at least it allowed it to work.  We managed to switch between the 3 thermistors using relays and time delays.  Also, the fact that they can be converted to 2 wire simplifed the thermistors themselves significantly.

Thanks again,

Sean