Torr to micron scaling in contrologix?

dginbuffalo

Member
Join Date
Dec 2010
Location
Buffalo,NY
Posts
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Hello everybody-I'm stumped! I'm trying to bring in an analog input to a Contrologix PLC and i cant get it to scale properly. the range is:

min .00000001
max 800

the analog card rounds the min to zero and it throws all my conversions off. Any ideas or am do i just have a mental block???
 
Yeah, that is a problem that I encounter with vacuum all the time. The problem is not with the ControlLogix, but with how floating point numbers are represented in any computer. The IEEE-754 float provides for a 23 bit mantissa, which gives you precision of 1 out to 1.7E7 counts. But when you mix big numbers and little numbers, the little numbers get lost in the precision. As your number gets larger, the precision of digits after the decimal place shrinks.

First question I need to ask though is what vacuum transducer are you using? Most get around this resolution problem by using a log linear output, which requires a little more math than just simple scaling.

Second, lets talk about units. One torr is 1mm of mercury, and sea level atmospheric pressure is 760 torr. A micron is 1 micron or mercury, or one millitorr, (1E-3 torr). So, if you want to scale 800 torr to micons, then that is 800000 mirons, not 800. If all you want is microns and your aren't going below 1E-3 torr, then use a DINT for scaling and scale 0 to 800000 (but only if your transmitter is linear).
 
Tim,

Using an MKS 972B dual mag vacuum gauge. We used this to get rid of two sensors- a micron and an in hg.

The MKS manual shows the following range:

1.5vdc = .00000001 torr
6.9515vdc = 800 torr

trying to convert it to Microns and then scale part of it to in Hg. Not having any luck but i see your point. I need to find out the lowest we go and possible scale from there.
 
Also...
The MKS 972B does not have a linear out to pressure, it is linear per decade (0.5V per decade), so you have to adjust for that error also.
I always love when a gauge vendor shows a nice linear line across a LOG scale.
You can talk to it over RS232 or RS485 to get true pressure using RS232 to Ethernet IP gateway.

This is an issue with most combo gauges (using 2 gauges to make 1 signal).
It is very hard to find 1 gauge that will give you a full linear signal across the vacuum range when crossing to less than 1 micron (.001 Torr). Now you in the ionization gauge range.

That is why I use Televac gauges, which offer a 3 or 4 decade linear out on most of their active gauges and the MM200.

If you only need a 1 micron ultimate on the gauge, I would use a gauge that reads to at least .1 micron (1x10-4 Torr).
 
Last edited:
Also...
The MKS 972B does not have a linear out to pressure, it is linear per decade (0.5V per decade), so you have to adjust for that error also.
I always love when a gauge vendor shows a nice linear line across a LOG scale.

Thanks Nathan. I got this project dropped into my lap a day before startup and I'm a little weak on the vacuum side to begin with! Will regroup Monday and figure out the next move...
 
OK Everyone, I am the one that stuck this on dginbuffalo! It was unfortunate, but outside of my control. So what it comes down to is how in Control Logix can we perform the math of 10 to the (2 x VOut-11) power. I am looking for a little help as to how I could do that math in the processor. Thanks!
 
First question I need to ask though is what vacuum transducer are you using? Most get around this resolution problem by using a log linear output, which requires a little more math than just simple scaling.

Hey Tim,

would you be able to provide any insight on the math to accomplish this as djcorby suggested?
 
Look at the math in the manual for Granville-Philips 275 Mini Convectron.
I used these equations to create code in a GE PLC. Wasn't too bad.
You might need to register with MKS first to get this link.
 
Look at the math in the manual for Granville-Philips 275 Mini Convectron.
I used these equations to create code in a GE PLC. Wasn't too bad.
You might need to register with MKS first to get this link.

Thanks. Might use that if needed. I believe the calc in the picture will get us where we want to be, but just not sure how to pull off the multiplication to a power as shown.

pressure calc.JPG
 

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