measure temperature with 3 wire RTD and ni 6361

[NR_leo]

Hi,

I am trying to measure temperature by  NI 6361 and 3 wire RTD (Resistance thermometers, NI 745691-02).

according to the NI support (http://digital.ni.com/public.nsf/allkb/5BFB36CFCC32C662862570D5006DC0C0) the connection of 3 wire RTD should be as shown in picture 1 (attached to this question).  

 In general, I understand that I need to drive current between red wire to black wire and measure voltage between black and red wire, which can be done by 6361, but I don't understand what current do I need to drive and what is the transfer function of measured voltage to temperature.

does someone know where I can find those parameters?

Thanks Harel 

[Blokk]

As you wrote, you need to supply a constant current to the excitation inputs of the RTD, and then measure the voltage drop on the proper pins. After this, you can calculate the R using Ohm's Law: R = U/ I

For RTDs, you can read here how to calculate for different types the actual temperature (for example, a PT100 has a nominal resistivity of 100 Ohms at the T_0 temperature): http://www.ni.com/tutorial/7115/en/

The problem in your case is that, I think this NI6361 cannot supply constant current, but voltage. So you cannot use this device for excitation, only for the voltage measurement task. NI sells dedicated RTD DAQ hardware, which can internally supply a constant current for the RTD sensor...

So either you buy one of these from NI, or look for a constant current source (you cannot use a voltage source since the current drown by the RTD will vary by the R, so the temperature).

Edit: about your question regarding the current level for excitation. The higher is better since you can easier measure the created voltage. However, if the current is too high, there is the undesired self-heating effect. So usually a value is chosen which is a trade-off between these effects. Recently I used a NI-9217 RTD module in a cDAQ hardware. I used a PT100 RTD connected in a 4-wire method. I think the default value for I_ex was 1 mA.

[Minions]

As these Platinum RTD's are made to be nearly linear, a simplistic equation for calculating temperature in degrees Celsius for a 100-ohm RTD is:

Tc = (Rrtd - 100 ohms) / (0.385 ohms/degrees Celsius) 

This equation is not for thermistors or other temperature sensing devices.

[Blokk]

Actually you have another option to measure the temperature only using that NI 6361 unit. The key is to use a voltage output channel to drive your circuit, plus a shunt resistor which you use to measure the actual current in the circuit.

So you will need to use two analogue voltage inputs, to measure the voltage drop across your PT100 RTD sensor: U_sensor, and to measure the voltage drop across your shunt resistor: U_shunt. Note that, your NI 6361 can only supply up to 5 mA current when used as voltage supply (the AO channel). So when you size your circuit, you need to calculate the maximum driving voltage for your AO. Example, if you use a 250 Ohm shunt resistor, your total R = 100 (this changes with temp!) + 250 Ohm = about 350 Ohm. So this circuit will already suck 5 mA current from your DAQ unit if you use a constant 1.75 Volts supply at the AO channel. Therefore, you should use a somewhat lower voltage drive.

I refer to this website: http://www.ni.com/tutorial/7114/en/

Figure 3. shows how to setup your circuit. The "Load" is your PT100 sensor. The +- power supply in your case is one Analogue output of your DAQ board (U_AO). When the temperature changes, the PT100 sensor resistance will change, so at constant voltage source, your circuit will suck varying current from the board. Do the calculations as I explained above to properly size the U_AO and the shunt resistor's size.

You need to use two analogue inputs to measure two voltages: U_sensor (voltage drop across the sensor) and U_shunt (the voltage drop across the shunt resistor).

Calculations:

1. you can calculate the actual current in your serial circuit by: I = U_shunt / R_shunt
2. The PT100 sensor resistance then: R_sensor = U_sensor / I

[NR_leo]

ok, thank you very much for the tips.

I think that shunt resistor will be the most simple solution.

[vic_grc]

Hi,

I am working on a similar problem to this one and read the solution you posted. At first, I am measuring impedance with the current test that it is set by default in DAQ assistant when measuring resistance, so I will create an external current excitation since my USB 6128 does not create this excitation current itself.

But, I also need to measure this impedance also by creating a voltage excitation (need to compare between both methods). Your solution will work, but my question is if this work only for 2-wire connection or how I should connect the R load so I can choose between measuring my load resistance with 2,3 or 4 wire method. Thank you¡

[Blokk]

Either you use a real current source for excitation, or a voltage one, there is no difference. The only difference it that, if your excitation source is a constant voltage output, then you also need a shunt resistor (connected close to the GND pin of the voltage AO of your voltage source DAQ). By measuring voltage drop on the shunt resistor, you can calculate the unknown actual current going through the shunt resistor AND the PT100 sensor (the load).

So there is no difference in how to connect the excitation current (or voltage) inputs to the sensor. Just follow the white paper how to make the 2/3/4 wire connections.

By the way, why you wanna do extra work by also using voltage excitation and a shunt resistor? If you have a stable enough current source, just go with it. Simplifies the whole circuit...

[vic_grc]

Good question. Actually I need to measure with both systems to compare them. My teacher ask me to do with voltage excitation also since its is commonly used for measuring impedance in textiles and not with current excitation. 

[vic_grc]

Hi,

I have another issue now concerning the circuit design and calculating maximum driving voltage. I will use it for 2 applications: bioimpedance and textile impedance and the values for impedances depending on which we use vary a lot.

Bioimpedance: 1 KOhm and electrode impedance around 100 Kohm 

Impedance in textiles: 200 or 300 Kohm and contact impedance in textiles 1 Mohm

I think that to get more accurate results the shunt resistor should be similar to our load value right? Which option will work the best? As you can see form those values, I will have to use 3 or 4 wire connection in measuring impedance due to the high values of electrode resistance and contact impedance in textiles.