Measuing temperature in the range of -25C to 285C

[cklabs]

Hi,

Which sensor could be used to measure temperature in the above range? We are planning to use CompactDAQ, is there any module available for it that can be used for this purpose?

Best,
Chandra

[Neil_S.]

Chandra,

Thanks for posting to the NI forums.  There are a number of different available options.  The option that will best suit your application will depend on the accuracy, price, and stability of the different options.  There are three popular general types of temperature transducers:  RTDs, Thermistors and Thermocouples.  RTDs are generally the most expensive type of transducer, followed by the thermistor and then the thermocouple.  Thermocouples can be very inexpensive, however, they also are typically the most inaccurate.  Thermocouples are typically accurate to 1-3 degree C, while RTDs are often as accurate as 0.03 degrees celsius.

For more information refer to the following resources:

Thermocouples:

Developer Zone Tutorial: Fundamentals of Thermocouples
Developer Zone Tutorial: Taking Thermocouple Temperature Measurements

The 9211 is an ideal module for taking thermocouple measurements.  It has a 60 Hz noise rejection and 24 bits of resolution.  Its product page is located at:

NI 9211 4-Channel, 14 S/s, 24-Bit, ±80 mV Thermocouple Input Module

RTDs or Thermistors:

Developer Zone Tutorial: Temperature Measurement with a Thermocouple or RTD
Developer Zone Turorial: Measuring Temperature with an RTD or Thermistor

Both thermistors and RTDs work on essentially the same principle.  Both vary their resistance as the temperature changes.  To be able to measure this change in resistance you must apply some excitation current. 

RTDs can be measured with either of the following modules:
NI 9217 4-Channel, 100 Ω RTD, 24-Bit Analog Input Module
NI 9219 24-Bit Universal Analog Input

There are no out of the box solutions for thermistors with the cDAQ chassis available at the current time.  You can measure the thermistor with a 9215 but using this module would require external excitation current.  You could also potentially use the 9219, but thermistor tasks are not supported on the module in NI-DAQmx so you would have to implement your own scaling.  I would recommend using either thermocouples or RTDs with the cDAQ.

Let me know if you have any questions about pursuing any of the options I have listed above.  Good luck with your application.

Regards,

Neil S.
Applications Engineer
National Instruments

[cklabs]

Hi Neil,

Thanks for your detailed reply. Our application doesn't need the accuracy of RTDs and I think we can do with thermocouples. I am now looking at 9211 datasheet but it only has -40 to 70 degree Celsius operating range. The temperature ranges we need to measure are from -40 to 190 degree celsius. Therefore, I am not sure if 9211 will fit our needs. In fact, I could not find any sensor from NI that is able to measure the temperature range we want.

Can you let us know if there's any sensor that's suitable to measure our temperature range and if we could use that sensor with any NI product like 9211?

Best regards,
Chandra

[Neil_S.]

Chandra,

The 9211 can actually measure a much greater range.  The operating range specified on the product page is the operating range for the device not the measurement range.  In other words you cannot operate the 9211 in an environment greater than 70 degrees, but you can measure higher temperatures.  You can't place the cDAQ chassis in an oven and expect it to work, but you can place the thermocouple inside the oven and measure it using the 9211 that is outside the oven.   The range over which you can use a particular thermocouple depends on its type.  For example J type have a range of 0 to 750 degrees C, and K has a range of -200 to 1250 degrees C.  I suspect the best choice for your application will be a T type thermocouple which has a range of -200 to 350 degrees C.

Let me know if you have any additional questions.

Regards,

Neil S.
Applications Engineer
National Instruments

[cklabs]

Hi Neil,

Thanks a lot for the detailed reply. It helps a lot.

Best,
Chandra