Need help doing code for enclosed tank

[cwyn]

Hi Guys,

I need some help constructing a code using rslogix 5000 ver 7.02 to read the level of an enclosed tank.
I am trying to use two absolute differential pressure transmitters as the inputs.

The transmitters are mounted opposite each other. one on top and one at bottom.

I understand that i must subtract the the value of the top transmitter from the value of the bottom transmitter.

I also have to take into condideration that the tank is under vacuum.(read by top transmitter) but the vacuum is kept constant.
The pan is used for boiling syrup. so as time goes on the density of the liquid will increase.

I have very little experience with rslogix and I am having trouble getting this to work.

I used a subtract funtion and subtracted
Source A: Local:3:I.Ch1Data (lower transmitter)
from
Source B: Local:3:I.Ch1Data (upper transmitter)
to
Destination: A pre-defined tag in the controller tag section

I am getting a Zero (0) for the answer and this is incorrect.

I could use some help.
Thanks in advanced

CWYN

 

[Ashereng]

This may be too simplistic, but aren't you subtracting the low from the upper when you should be subtracting the upper from the lower?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[a70duster]

Source A and Source B are the same value. Source B needs to come from another location. Go online with RSLogix and open the Controller Tag page and look at the "monitor" screen. Look at the values of the analog input card to determine the location of the data.

And the lower sensor should have a higher vaule than the upper sensor. What is the density of the liquid and the distance between the sensors?

 

[controlsdude]

Source A: Local:3:I.Ch1Data (lower transmitter)
from
Source B: Local:3:I.Ch1Data (upper transmitter)

If you notice your sourceA and sourceB are the same data ch1. Of course your getting zero. Take a deep breath and try again.

Also be careful with taking raw data and doing a calculation. Always buffer the data, test the buffer data for correct range of value. Then perform your calculations. That way you will not perform a bad correction to bad data received.