how to set scan rate with NI Switch scan voltages

[hema]

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.

[Hani R]

Hi hema,

When sending signals to the DMM using a switch, it is important to remember the settling time and synchronization required to make an accurate reading.  Because you are using a For Loop, without taking timing into consideration, the DMM is not measuring the voltage at the correct time.  For the most precise measurements, the DMM must receive a trigger from the switch telling it to begin measuring.  Communication between the DMM and the switch is known as "handshaking."  There is a lot of information about handshaking in the NI Digital Multimeter Help (you should already have the file installed). 

There is a shipping example in LabVIEW which demonstrates handshaking between the DMM and the switch.  You should be able to use the example for your application.  You can find the example by going to the NI Example Finder then under Hardware Input and Output » Modular Instruments » NI - Switch (Switches) » niSwitch DMM Switch Handshaking

Keep me posted on your results. 

[hema]

Hi Hani R

Thanks for your help. The links u sent were useful.

i am stuck at one more point. I have connected one thermocouple to the  SCXI 1130 switch and reading using NI DMM 4070. I am getting the thermocouple output voltage correctly. I do not know how to convert it to temperature. During testing, I am holding the free end of the thermocouple, so i have to get the room temperature as the output. There  is NI Switch Convert thermocuple voltage to temperature.Vi built in, but i do not know what to give for the cjc voltage input. please can u help me on this.

The thermocouple volatge reading, when i touch the free end is around 0.00052 volts.

thanks a lot

hema

[chericks1]

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