RTD and thermistor temperature measurement possible with NI PCI-6259 without additional signal conditioning hardware?

[fizzy]

Hello people!

I'm new to NI hardware and software - so please be patient. I will learn though.

Straight to the point: I'm not sure whether I can use the PCI-6259 DAQ device as a "stand alone" solution for measuring with RTDs or thermistors respectively. They definitely need signal conditioning, but do I need further hardware such as SCC-68 with approriate modules or can the PCI-6259 handle signal conditioning itself?

I guess it's either YES or NO - so could someone please point me into the right direction.

Thanks!

[johnsold]

To measure resistance you need to know both current and voltage. The PCI-6259 does not generate or measure current directly. You can use a fixed resistor in a voltage divider configuration and measure the voltages across both the fixed resistor and the sensor. Then you can calculate the sensor resistance. The accuracy will be no greater than the accuracy of the value of the known resistance.

The AO lines can be used to provide excitation but are limited to 5 mA.

So, technically, the answer is NO, but a resistor is not a very complicated signal conditioner.

Lynn

[fizzy]

Thanks for your reply.

I do have a constant current source available. What I meant with required signal conditioning is if it is necessary to pre-amplify and low-pass the voltage signal before feeding it to the AI of the PCI-6259 or if the PCI-6259 comes with amplification and filtering itself.

[johnsold]

The answer to your question about amplification and filtering depends on your system.The PCI-6259 has input ranges from +/-0.1 V to +/-10 V, so it already has an amplifier. If one of those ranges fits the signal you get (Current source X maximum resistance of sensor), then no other amplifier would be needed. Whether filtering is needed depends on how much noise might be on the signal, what kinds of interfering signals (such as power line frequency signal) might be present, and how fast you will be sampling.

Typically temperature measurements are slow. If you do not expect much noise or itnerference, you can probably get by without any other equipment.  If you are trying to make rapid measurements in the middle of an induction furnace with a variable speed drive on a large blower and have hundreds of meters of cable between the sensor and the DAQ device, you will certainly need a sophisticated signal conditioning system.

Lynn

[fizzy]

Thank you very much - now I understand. I'm pleased to hear that it's not either YES or NO. Makes things a whole lot more interesting.

[Buechsenschuetz]

In most cases, thermistors or RTDs are used in a voltage divider mode, i.e. a resistor is placed in series with the sensor and a (constant) voltage is connected to one end of the resistor and one end of the thermistor. The voltage across the thermistor is proportional to the temperature.

You should keep the current through the thermistor as low as possible to avoid self-heating. On the other hand, a large series resistor will yield a low voltage across the thermistor. You can find lots of applications for circuits using thermistors.

Also, most if not all thermistors are not linear, i.e. their resistance does not vary directly proportional to the temperature. You can use analog lineariziation (with a resistor network providing linear output over a certain range) or do this correction by software.