What am I doing wrong with my scaling?

[David99999]

I'm converting 2 - 10VDC to a range of 0 - 200. I have a USB-6001 device and am using DAQmx VI's. I have tried making both the linear and the map custom scales but for some reason it's always about double what I should be getting. When it's receiving around 2.24 Volts I should be showing 3.5. But it's giving me 6.5.

Maybe I'm just too hungry but I'm really drawing a blank on this one. See attached files

[paul_cardinale]

I can't open your block diagram.  It seems to be corrupt.

[Kevin_Price · Accepted solution]

The scaling is right, your expectations are wrong.

Just estimating in my head I see that 2.24V moves you about 1/4 V into your 8V input range (2-10V), so a little less than 1 part in 32.  Mapping to 1/32 of a 0-200 range says that 6.5 seems about right.

-Kevin P

[GerdW]

Hi David,

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@David99999 wrote:

When it's receiving around 2.24 Volts I should be showing 3.5. But it's giving me 6.5.

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The "linear scale" image also shows the resulting formula!

Did you apply that very simple math on your voltage reading?

formula: Y= 25*X -50
X=2.24 -> Y=6

Why do you expect a value of 3.5?

[David99999 · Accepted solution]

Well, I'm expecting a value of 3.5 because I'm converting 4-20mA to workable voltage for the DAQ. I'm using a 500Ω resistor to do so. The power supply is outputting 3.5 amps, and I had about 2.24V across the resistor.

Anddddddddd I just realized after typing that out what the issue is. I'm trying to scale it 0-200 Amps, but the jumper is removed so the range is actually 0-50 Amps. Woops. Thanks guys!

[billko]

Don't forget to give yourself the credit for the solution, and kudos to those who helped you get there!  🙂

(You can have more than one solution.)

[Kevin_Price]

Sorry to nitpick, but something still seems off.   Scaling 2.24 V where a 2-10 V range maps to 0-50 A should give you 1/4 of the ~6.5 I referred to earlier, so more like 1.6.   Still not 3.5.

I understand that (as far as you know) the real world *actual* current might be 3.5 and that's why you're anchored to it, but the ranges and voltage measurements you've presented don't get you there.  Just looking to emphasize clarity.  The deifference between a 0-200 A range and a 0-50 A  range doesn't *actually* explain what you've observed so far.

-Kevin P

[David99999]

2.24V probably doesn't represent 3.5A anymore after putting the jumper on the correct spot. I didn't measure the voltage across the resistor, but after doing that I get a current reading in labview of 2.7 - 2.9 amps (it fluctuates a little) while my BK 9201 power supply shows 2.9999 amps being drawn from it, when I have the current output set to 3 amps. (In the example I just gave, forget about the 3.5A, I changed it to 3A earlier today).