channel scale effect over saturation

[usuario]

Hello,

I have a .vi to acquire 8 channels, with a 6062E card and a BNC-2115 board.

For each channel, I have created a scale, so that I get physical units. Up to today, I used to read Volts, and then multiply the signal by the sensitivity and add an offset, but I want to make the conversion properly (and so, for example, I will save to disk data with channel names, units, and so on).

But, here is the part I don't understand.

If I set the slope to 1, that is, I can read the Volts, the measurement is fine. I compare it with a multimeter reading and both agree. Readings range from 1 to 5V.

So, I then set the gain to its actual value: 22,5 (physical units/volt), and no offset (y-intercept = 0). And here's the weird stuff: the reading will get saturated at a very much lower value than it should (5*22.5 = 100), it saturates when reaching 11.87.

If I set the slope to 1 again, readings match again the multimeter reading, ranging from 1 to 5V.

So, one last check: I set again the slope to 22.5, and measure also with the multimeter: the multimeter makes the same reading as always, read value will saturate at 11.87.

This value of 11.87 depends on the y-intercept I set, can't tell you right now the tries I've made but didn't just sum to the y-intercept I entered, it changed it with apparently no sense.

Please, anyone has an idea of is happening / what am I doing wrong and why?

[aeastet]

You need to open the min and max on the channel to the max for your sensor. When you update the slope that causes DaqMX to reset its gains. After you set the scale you need to adjust the min and max for the device.

[usuario]

Thanks a lot for your answer aeastet.

I don't have global channels defined at MAX. I create the task each time I run the program and clear it at the end.

Also, I set the max and min for the channel in Volts. I've tried setting them in physical units, but I got an error back (valid values -10...10 V).

Any other way of solving this?

[aeastet]

Do you have code that I can look at?

[usuario · Accepted solution]

Hello,

well, I finally solved it. Actually, I finally found what was I doing wrong.

My main error was not to carefuly read the error I mentioned above, just see the error, the code, & roughly read it.

Actually, since the Scale is done before the channel definition, MIN and MAX values correspond to Scaled Units, and not to volts. I got the error because the value entered, even in scaled units, was overrange. And as I was entering the volt corresponding values (1 to 5), these values in scaled units where really low, so it would saturate at 11.87 that corresponds to the maximum readable level adjusted by the MIN and MAX values entered.

So, finally the issue is solved. Again, thanks a lot for your time and effort, it is really interesting to have a forum like this with active people helping us newies.

Thanks.