calculating scale factor from miliamps input to labview output

[mrbob2]

I am setting up a testing fixture with Fieldpoint and Labview. I am bringing in a 4-20MA single to the fieldpoint. I need to scale it so I can have a pressure and vacuum reading in my labview screen. Do I use a multiply function or make a sub VI.
Thanks, Bob

[AnujD]

If all you need to do is multiple your acquired at by a constant -- the multiply funciton will do.

Regards,
Anuj D.

[Aaron_G]

Bob,

It is fairly easy to use the multiply function (or any function) in many places on a block diagram. The question to ask is how many different channels and types of scaling factors do you have. If you need to scale channels by the same amount in multiple places on your block diagram, then it is typically wiser to make a subVI out of the code. This also allows you to adjust a scale once (if you need to) rather than in multiple locations on a block diagram.

Regards,
Aaron

[mrbob2]

I built a sub VI for my amp meter. I have a current transducer that is set to read 100 amps. So we used y=mx+b and got the scaling right on. I copied the Sub VI and used it for a pressure reading. This time the scale was based on a 250 PSI Transducer. This transducer puts out a 4 to 20ma signal. Again this one works perfect. It is dead on compared to the precision analog gauge. The problem is with my Vacuum Transducer. It is a 0-200 psi or 0-30" in Hg. I calculated the y=mx+b for this sub VI and I can not get a valid vacuum reading. I checked the port to make sure I was pulling vacuum there.
In the sub VI if you click on it, it shows the original calculation for the amps. I tried to save the sub VI as a different name but it will not allow two sub VI's in the same state. I was able to get the pressure to work by changing the value in the constant that is connected to the Y multiplier.
So how do I get the vacuum meter to show a 0-30" value.
Thanks for the reply, Bob

[AnujD]

As I understand it, you have 1 subVI that you can use to scale either pressure or current.

You should be able to have two different subVIs that preform a simple scaling operation -- 1 for pressure and 1 for current. Can you explain a little further about the problems you having in doing this?

Regards,

[mrbob2]

Anuj, I should mention that this was my very first project with Lab view so I apologize if I do get my point across well.When we set up the meter for the amp gauge I was shown how to make a sub vi for the calculation part. I thought I could copy this for the pressure and vacuum meters too. I copied the amp VI and renamed it pressure. When you double click on the sub view you get a small window with four boxes that have the calculation values in them. When I changed the box that had 6250 in it to 15625 it changed the same field in the amp Vi. I later found out that I could only change the constant box that is connected to the mutilply function.
Like I said before the only meter that I can get to work is the vacuum. I have the vacuum scale set .004 /15625. When we did the calculation the formula looked right. It just does not give me a vacuum reading on the meter. When it should show 10"Hg it shows 1.3768 or something like that.
Thanks, again for your help.
Bob

[AnujD]

I think the problem you are seeing lies in how you are copying them. Since I am not there with you, it is difficult to say exactly what steps are causing the problem. I would suggest to just re-write the scaling VI instead of copying it.

Regards,