waveform graph Y axis "Amplitude" is using what units ???

[lightlight]

i am using a waveform graph to display my data on the y axis it is written "amplitude" but i don't know what units is used is it "Volt" or "Amper" or what i am measuring a pressure with my DAQ card ,my analog input is pressure so how can i get the reading in unit of pressure "bar"???
do the pressure sensor give readings in Volts or Bars ?

[CoastalMaineBird]

The graph is labeled "Amplitude" only by default.

If you're using the DAQ Wizard, you can assign units such as PSI or bar to your channels, otherwise you'll have to do the scaling yourself.

Usually, you read from the DAQ routines in either binary (you'll have to scale that to get voltage), or voltage. Your transducer has a certain PSI/Volt (or bar/volt, whatever) rating. You'll have to multiply your voltage data by PSI/Volt to get PSI.

You can change the "Amplitude" label to "bar" or whatever you want - it's independent of the data itself.

[chutla]

Ultimately, if you're using a DAQ card, everything is converted to a voltage...since the A-D converter only works with voltages, regardless of the original source. Your actual pressure sensor should have some published specs as to volts (or millivolts) per bar. Once you know that, you can convert to displayed amplitude.

It is always good practice to use an independent means of calibrating your instrument, so you KNOW how many volts/per bar you are actually getting from your sensor. It's usually a simple matter to wire in a calibration "pot" into your VI, usually using a MULTIPLY node, so that the display reflects reality.

Be sure to read the manual of your particular DAQ card, as the NI hardward documentation is pretty thorough.

Eric

[lightlight]

thank you for your reply Eric so i want to tell what i understand now from what u said Eric:
that the amplitude on the y axis on the waveform graph in labview is in volt unit..and i have to multiply the reading i get by a factor depending on my transeducer to taransform this volts to bars (for pressure) well is that ok Eric?
and also i can do that by adding from the numeric functions in the labview the Multiply node to the reading results so i woud have the readings in bars directly ok Eric?
thank you again

[chutla]

I believe you have got it. You might want to also check the DAQ examples, especially with regard to offset and scaling. You can achieve offset by merely adding an ADD node to the data stream. Also, if there is a non-linearity in your transducer, you can correct for that by adding an exponential function to the data stream. I believe most of your pressure transducers are designed to be very linear.

Eric