Delay between real signal and acquired signal

[RavensFan]

Now I'm confused again.  You say you are missing data.  How do you figure?  You say you only got 800 data points, but you say there were 1000?  How do you figure there were 1000 datapoints?  There is no data acquisition rate to set.  The thermocouple VI only returns 1 data point at a time. That means your VI is software timed.  You get as much data as how ever many times the loop has run.

If you need a higher sample rate, then you will need to use a DAQ device that has an internal hardware clock and is able to return multple data points per call.  Perhaps this device does have that capability and you haven't configured it to the right settings and need to use a different subVI from its LabVIEW library.

What data acquisition rate are you looking for?  Usually thermocouple measurements are pretty slow since the temperature of most things can't change that fast.  200 hundred data points in 5 seconds. is 40 points/second.  That is rather fast for for a temperature reading.

Message Edited by Ravens Fan on 10-17-2009 10:50 PM

[Dennis_Knutson]

I agree that the comments are confusing. There is no possibility that there is 'missing' data. There does not seem to be any basis for the statement.

[JCNT]

The DAS TC board has connected a thermocuple in one channel and a radiant energy sensor in another one (both are millivolts signals). The signal which is changing so fast and LabVIEW can not detect its change, is the radiant energy signal. I figured it out  because I connected a multimeter in the radiant energy sensor (which is connected to the DAS TC channel too) to measure the milivots and the multimeter got all variation signal but LabVIEW didn´t

I hope not confuse you again and clarify a little bit

[JCNT]

I do not know if this could help but in one of my tests the multimeter detected a maximum value of 825 mV, but when I got the maximum from the array  in LabVIEW it was 710 mV so the question is where is the 825mV value.

[RavensFan]

That just means either the multimeter is sampling at a higher data acquisition rate.  Or it could even be sampling at a lower rate, but it just so happens that when the rapidly changing signal hit 825mV the multimeter happened to be sampling at that instant while the DAS-TC board was between samples.

It isn't missing data, you just didn't sample the signal at that instant.  Technically, for an analog signal, you would have potentially infinite number of data points, and if you had an infinite bandwidth, that means you could sample them all.  That doesn't mean that you have infinity minus 40 missing data points.

In the end, you are either not setting up your data acquisition board for the data acquisition rate you need, or you are using the wrong hardware altogether.  I suspect it is the latter.  Thermocouples generally don't go to 800+ mV, so if you are using a T/C channel to read some other sensor at that level, it sounds like you are using the wrong hardware.

Message Edited by Ravens Fan on 10-18-2009 12:34 AM

[JCNT]

I agree when you say that I am using the wrong hardware, so which NI hardware could I use in order to get a higher data adquisition rate? The idea is to get all the data that the multimeter gets in LabVIEW because every single change of the signal is very important.

JC

[Dennis_Knutson]

All of the NI DAQ products have hardware timed analog inputs. For that matter, the board you have right now may have hardware time acquisition. You just might be using the wrong function.

Decide what kind of sample rate you really need, the number of channels, the signal conditioning, etc. Then you can browse the product page for something that meets your specs. A call to your local sales engineer would seem appropriate.

[JCNT]

I´ll figure it out what is really going on with the board and if there is an extra function although I have used all of them.

Thank you very much for your help and time.

Regards,

JC