Scaled measurment coerced to lower value on Multifunction DAQ

[SeanJ]

I am using a NI 6008 to take pressure sensor readings from a honeywell prerssure sensor.  The sensor is a 1 PSI sensor and so should operate to 68.9476mbar.

Using the DAQ assistant, I cannot gett he scalled measurement to esceed 56.1mbar as shown below:

This coercion is not happening at all on the voltage input, e.g. for the same presssure:

The values below were used for the pressure sensor table:

2.5271, .01
2.561, 1.12
2.588, 1.96
2.645, 3.9
3.94, 47.9
4.289, 60
4.55, 68.7
4.65, 72.4
4.83, 78.6

I'm assuming there is something small somewhere causing this coercion but after hours looking for it I have come up with nothing.

EDIT:  I just noticed, if I change the settting to RSE, the coercion is removed but the resoltuion is severely diminished.  Is it possible to have the higher resolution of differential and no coercion?

Does anyone know what is going on here?

Rgds,

Seán

[SeanJ]

I tried a polynomial scale instead and g0t exactly the same result, coercion at approx 56.1

My workaround is table --> 3rd order SVD coeeficients --> measure differential voltage (no scaling) --> evaluate using coeficicents.

This appears to be a bug in the NI-DAQmx Scale feature.  It's very annoying, perhaps someone can explain why it didn't work?

[MihneaTr]

Hello Sean,

I noticed by looking at your screenshots that your signal reaches a maximum value between 4.23 V - 4.24 V. Also looking at the lookup table that you generated this value should match a value of around 59.29 mbar. I noticed that the maximum value when you are using the custom scale is 56.1 mbar. Is this information correct?

Are you concerned by the fact that you are having a difference of approximately 3mbar? By increasing the range of your signal the resolution will decrease.

Also it will be good if you can reiterate where exactly do you think the bug is?

Thank you.

Best regards,

Mihnea

[SeanJ]

Hi Mihnea,

If I increase the pressure above 56.1mbar the scale stays the same at 56.1mbar, 

e.g.

Scale Value      Manometer Value

54                       54

56                       56

58                       56.1

The table may need to be tweaked but yes it is concerning that the measurement is off.  

To reiterate, the voltage changes but the scaled value will not go above 56.1.  It seems to be coerced to a maxium of approx 56.1, maybe related to some legacy voltage concern?