temperature measurement pt100 daq

[sergymax]

The inputs for the Convert RTD function for the sensors Pt100 are very similar as the default values for R0, A, B. But it shouldn´t be the problem because it doesnt read anything anyway. Do I have to wire the Convert RTD function directly to the thermometer or how can I do it? Is that te way to get the temperature?

I have been looking at the Help file but I dont find the way to make it work.

[smercurio_fc]

Well, "very similar" isn't necessarily the same. You have the DAQmx Create Channel function set up to read 2 channels, but your DAQmx Read is set up to read 1 channel. If you're going to read 2 channels, then you should read 2 channels. As I noted earlier, this will give you an array of waveforms, and you need to use Index Array to pull off the individual waveforms for the thermometer indicators (2 of them since you have 2 sensors). The wavefororm graph can be wired directly to the array output. Remember, reading the DAQ card returns a voltage. You need to convert this to a temperature. The Convert RTD Reading function doesn't "read" anything. It simply converts a voltage to a temperature. See attached mod.

Message Edited by smercurio_fc on 02-19-2009 01:21 PM

[sergymax]

Thank you smercurio. I really appreciate your help. Now both channels are plot in the graph. But the thermometer do not seem to work. There is no temperature measured and the digital display say that is not a number (NaN). What do you think could be the problem?

[smercurio_fc]

Either

(a) the default values for A, B, R0, and Iex are not correct for your sensor -or-

(b) the voltages you are reading are not correct.

Can't tell you whether it's (a) because I don't know what the specs are for your sensor - you're the one who has the documentation. As for (b), what does your graph look like? What is your excitation current? What's the nominal resistance of your sensor?

The Convert RTD Reading function can be run stand-alone. Open it up and look at its diagram. It's just a formula. It also has a great big comment explaining how it works. The underlying assumption here is that you are measuring temperatures above 0 deg C.

[sergymax]

The graph plot both of channels. It seems to be ok. the voltage from the sensors are around 1.2. I add a picture.

I have set default values of Ro, A, B according to the expecifications I found on Internet.

Nominal Resistance Ro= 100 ohms at 0ºC, 138.5 ohms at 100ºC. So I set Ro=100. A=0,003908. B=-5.775E-7. The temperature is above 0º.

I have tried RTD Converter with Scaler and Waveform Type, but it doesnt work neither. I am trying to wire the DAQmx Read to the RTD Converter VI directly but It is not possible. I am trying everything but I only get NaN as a result. Do you have any idea what should I do?

Thanks

[smercurio_fc]

The values that you "got from the internet" may not be applicable for your sensor. I've said this before, and I'll say it again: did you look at the documentation for your sensor?

You are getting NaN because you are not feeding in the correct values for the constants. This isn't calculus. It's simple algebra. If you are getting a value of around 1.2 Volts this would imply that you are using an excitation current of about 12 milliAmps. Yet, you have a constant of 170 microAmps wired for Iex. They both can't be right. This assumes you have a sensor that's 100 Ohms at 0 deg C. There are some Pt100 sensors that are not 100 Ohms at 0 deg C. Again, did you look at your documentation? 

[cebailey]

> If you are getting a value of around 1.2 Volts this would imply that you are using an excitation current of about 12 milliAmps.

By the way this would be a pretty high current to run through an RTD. It would probably make the RTD give an erroneously high reading because it is heating itself up.

>Yet, you have a constant of 170 microAmps wired for Iex. They both can't be right.

I agree. Also, 170 uA is a pretty low current. You probably want around 1 mA as your current and then you will read 100 or 200 or so mV depending on the temperature.

>This assumes you have a sensor that's 100 Ohms at 0 deg C. There are some Pt100 sensors that are not 100 Ohms at 0 deg C.I think the "100" in "Pt100" means that the resistance at 0 C is 100 ohms. However, this still doesn't pin down exactly what the sensor response is. I think the alpha = 0.00385 calibration curve, which Omega calls the European curve, is the most common in the US. There is also an American curve that is not as common in the US as the European curve. I think there are other curves that are more common in Japan, but am not sure. Elsewhere, I don't know.

>Again, did you look at your documentation?

This is the biggest missing element to this whole conversation, isn't it? What does it say? You should probably harvest everything you can out of that before doing anything further.

[smercurio_fc]

---

cebailey wrote:

I think the "100" in "Pt100" means that the resistance at 0 C is 100 ohms.

---

Quite right. Thanks for the correction on this part.

[sergymax]

Yes, this is the problem. I change the Iex into upper values and now it reads the temperature, although the values are not correct.

As I started working here when the "mechanical work" was done, I do not have the documentation of the sensors (pt100 samey 66RYS05). I called the company who made this but they don´t have it and I didn´t find it on the company website. I am trying to find it, but do you know where I could do?

Thank you guys for the help

[smercurio_fc]

The results are quite sensitive to Iex. I calculated 12 mA based on a ball-park value from the graph you showed. You should know what the excitation current is since you are providing that to the sensor. For instance, using the default values of A, B, and R0, a voltage of 1.2 Volts corresponds to a temperature of -2 degC with Iex=12mA, but with Iex=11mA, that same voltage corresponds to a temperature of 23.34 deg C. Don't guess at Iex. Put in what you are actually providing to the sensor.

By the way, here is the page for the sensor: http://www.samey.is/hitanemarv.html. The PDF is in Swedish. Given that, it's probably using the European curve, which are the default values for A and B in the Convert RTD Reading VI.