How to set scan rate with NI Switch scan voltages

Question

Hi
I have SCXI 1130 switch and NI 4070 DMM . I have connected 3 voltage channels on the SCXI .
When I read just one channel at one time, I get correct voltage reading. here I gave scan input as ch0->com0.
Later , i placed a For loop in the block diagram and programmatically wired the scan channel input,
for the switch and read the voltage output from the DMM, i do not get the correct outputs.
That is , for my 3 channels, i gave For loop iteration count as 3 and ,'i ' is taken and appended for ch i ->com0. the DMM measurement is not proper. But if I highlight execution in the block diagram ( if i put the bulb and the probe), i can see the correct output voltages coming out. The moment, i turn off the execute high light, the program gives incorrect output. So  do I have to give a scan dealy or what time has to be set to get correct values. I am using software trigger in the bloack diagram.

Answer 1

Hi Hema,
CJC is an acronym for Cold-Junction Compensation, and this value adjusts for the change in voltage caused by the thermocouple wire to copper wire junction. 
For example, a J-type thermocouple will have thermocouple wire consisting of iron and constantan metals.  When these iron and constantan metals meet the copper at the switch connection, a difference in voltage results.  This difference in voltage is the "cold-junction".  The difference in voltage resulting from the iron and constantan connection in the thermocouple is the "hot-junction".  When you measure temperature using a thermocouple, what you desire is the "hot-junction" change in voltage.  Unfortunately, the DMM is going to measure the sum of both the "cold" and "hot" junctions, and a CJC measurement is needed so we can adjust the measurement to remove the undesired offset.
Once Cold-Junction Compensation is performed, converting from voltage to temperature is fairly simple.  Each thermocouple type has its own temperature to voltage conversion equation and associated coefficients.  Here's a great resource for the equations, coefficients, and specific voltage to temperature tables:
NIST ITS-90 Thermocouple Database
http://srdata.nist.gov/its90/main/
Hope this helps!
Chad Erickson
Switch Product Support Engineer
NI - USA