rtd's with SCB-68 and PXI

[Call Breaker]

I need to hook up some type-T thermal couples and some very old 10 ohm copper, three conductor RTDs to our SCB-68.  Is the SCB-68 capable of measuring the 10 ohm copper RTD's?  Can it measure the type-T thermal couples and the RTD's at the same time? 

We use the SCB-68 with a PXI-1031.  It has one PXI-8174 Embedded Controller and two PXI-6229 M Series Multifunction DAQ cards.

[mallorim]

Hello Call Breaker.

The SCB-68 is a Shielded Connector Block for 68-pin Data Acquisition (DAQ) devices like your PXI-6229 M Series DAQ card. This means that when the SCB-68 is cabled to the PXI-6229, it provides screw terminals where signals of interest can be securely connected to the pins of the PXI DAQ Card. So, the question then becomes, not can the SCB-68 measure the RTDs, but instead, is the DAQ card that the connector block is connected to capable of measuring an RTD?

In your case, No the PXI-6229 will most likely not be able to measure your RTDs. This is because RTDs typically require signal conditioning in the form of an excitation current, which this M Series card is incapable of providing. We do have some hardware meant specifically for the measurement of RTDs. I have included link below to our online product search based on measurement type.

RTD Measurement Devices

With regards to the measurement of your T-type thermocouples, you Will be able to measure these because the 6229 is capable of differential voltage measurements on its analog input channels. The SCB-68 connector block actually has a built in CJC sensor which means that you will be able to perform cold-junction compensation for your measurements, as long as you appropriately configure the SCB-68 as an accesory of the 6229 in Measurement and Automation Explorer (MAX).

Something to be aware of in your thermocouple measurements is that the voltage created by a T-type thermocouple is 52 microVolts per degree Celsius. The resolution of the 6229 in its smallest range of 0.2 V has a 16 bit analog to Digital Converter (ADC). This means that it will have a resolution of 6.1microVolts (400 mV/2^16). This is the smallest detectable change but this does not represent that absolute accuracy of the card, which can be calculated using the formula on page 4 of the 6229 Specifications. 

For highly accurate thermocouple measurements, we typically recommend more signal conditioning, in the form of things like amplification. This is built into our SCC and SCXI signal conditioning product lines. The link below discusses your options and the benefits of each. 
 
Recommended NI Hardware for Thermocouple Measurements: 
 
Cold Junction Compensation (CJC) references:
What is CJC?
How Do I Configure the Switches for CJC Sensor Input on the SCB-68?
Taking Thermocouple Temperature Measurements
How to Determine the Thermocouple Type and the Correct Polarity of Signal Connections 

I hope this answers your questions.  
Regards,
Mallori 

Message Edited by mallorim on 01-15-2010 04:23 PM