USB-6008 + RTD Pt100 + Ext.source + PROBLEMS

[s7op]

Hi

First of all, I know 6008 doesn't provide an internal exctitation source for Pt100 and is not recommended for temperature measurements. Anyway, if I have an external excitation source is it possible to read temp values with USB-6008?

I have a working setup with PID-controller that controls temperature in a measurement box depending on a current temperature measured by RTD Pt100 (3-wire). So as far as I understand the PID-controller provides excitation source for Pt100? Okay, so this setup works but I need to manually write down the temperatures from controllers LCD-screen. Instead of writing I would like to read temp values to Labview via USB-6008. At the moment I've connected three extra wires to PID-controllers RTD connections so that I could use those three wires with 6008. But the problem is that when I use DAQ Assistant to read temp values I get values jumping between -130 and -240C! If I try to read voltages it jumps between +-8V and with resistances it goes between -60 and 100 ohm. I am not sure about the connections. With all the measurements I had a white wire and one of the red wires connected to ports 2 and 3. What about the third wire (other red one)? Is the measurement even possible with this setup? Ordering a new temperature element or DAQ-card is not the first thing in mind. A big thanks for all!

PID: http://www.farnell.com/datasheets/311043.pdf (as you maybe noticed controller has also RS485 output but I have no idea how to read that with Labview)

[s7op]

No one?

The thing is that RTD works correctly with PID, so should I be able to read those values with USB-6008? How do I need to connect those three wires (2xred,1xwhite) to USB-6008 to read temp values? Please help...

[s7op]

Any help?

[Henrik_Volkers]

Hi,

only a guess: the PID controller uses a type of ac excitation, nice to cancel thermoelectric effects, and maybe even turn off the excitation in between measurements to reduce sensor self heating.

Additionally the 6008 might disturb the sensor reading of the controller and is not recommended.

Best would be to use the RS485 connection. You would need a RS232 to RS485 adaptor (or a USB to RS485) to read the PID controller. Price vary, simple one are about 10$ but can go up to >300$ for high speed isolated industrial temperature/emc robust types.

Maybe there is already a driver vi to read and set the controller. Search the driver network at NI or ask the vendor.

The communication protocol should be (is usually)  well described in the controller manual.

Of course you can use the 6008 to read another sensor. Here I would recommend a 10k NTC together with a 10k resistor as voltage divider driven with 5V. But values and sensors depend on the application,  temperature and the resolution you need.

[s7op]

Thanks, so I can't just connect those RS485 A and B ports from PID to 6008 digital inputs? Why so?

[Henrik_Volkers]

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@s7op wrote:

Thanks, so I can't just connect those RS485 A and B ports from PID to 6008 digital inputs? Why so?

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Short: NO

Long:Well, physically it is possible 😉 but it is very hard to impossible to establish a communication in that way.

Why do want to connect the telephone line to your 6008, write a software for speech recognition, display/read the text, use speech output to get a binary representation that you output via the 6008 (If it had a analog output??)  ... JUST TO MAKE A PHONE CALL ( to order a pizza, make a date) ?? 😉

The PID communicate via the RS485 to other devices (PC). The RS232 and the RS485 are both serial protocols with some (minor) differences (mostly covered by an adaptor) and your PC (usually) has already hard and software to handle this.

[cpalka]

Hi

If I understand, you want to read the temperature at the same time of the temperature controller. The better way is using a thermistor or the LM35 (I use it) to use with 6008 to measure the temperature, just put the sensor near the pt100.

If you want to read from the controller, the controller is a A/D converter like the ni6008. It seems the controller has a communication port and you can use it without ni6008, only connect it to the rs232 (or use a usb/rs232 converter) and in labview is visa communication.

To read it with ni6008 you need to find inside the controller the output signal that goes to the display of the controller (you can damage the controller)

King regards from Portugal

CPalha