MAX Virtual Channels and Scales (Max and Min)

[pete_dunham]

Setting up scales with virtual channels in Measurement and Automation Explorer(MAX) could be more intuitive. I would like a change in the hierarchy of these settings.

IDEALLY, I would want to set up a virtual channel with the Min and Max value in RAW voltage, rather than the min and max values that are dependent on the scaled value.

I usually know the voltage range/gain settings I am expecting on each channel, but sometimes add the custom scale later. This is a pain to go back and change the min and max values accordingly.

For example, say I have 3 voltage channels with an expected range of -1V to 1V. (I set these up in virtual channels). Now, assuming each channel has a unique custom scale, I have to go and select the scale and THEN I have to go back and modify each channel's min and max value. Before I knew better, I would forget this step and some of my scaled measurements would appear to be saturated at the raw voltage measurement. Also, a change in this custom scale doesn't propagate these min and max values through any channels that is using this scale!

Edited July 13, 2009 by pete_dunham

[David Boyd]

Setting up scales with virtual channels in Measurement and Automation Explorer(MAX) could be more intuitive. I would like a change in the hierarchy of these settings.

I've been bitten by my misunderstanding of custom scaling and min/max definitions too.

Classic example: I have a 0-15psi pressure transmitter that's 4-20mA current loop. I create a custom scale thusly:

• prescaled units in Amps
• scaled units in psi
• slope 937.5
• intercept (-3.75)

I specify this custom scale while defining a virtual channel that uses an NI current input, and am tempted to specify the channel min and max as 0-15 psi. Bad choice on my part! While trying to perform a software cal on a sensor, if the raw current reading is 3.7ma, the scaled value is pinned to zero psi even before I apply software correction factors. Perhaps this isn't really particular to the custom scaling, but I would still somehow expect the scaled channel to return all values within the chosen range of the underlying hardware. Having the max and min values specified in terms of the scaled value is just confusing here - what I SHOULD have done was to specify (-3.75psi) as the min value and (+16.41psi) as the max since the hardware will choose an input range that can actually report (0ma) to (+21.5ma). If I DON'T do this, and go with my original impulse, my scaled readings will never appear outside their valid ranges even if the hardware disconnects (goes to 0ma) or otherwise "goes south".

Dave