Vacuum Transducer 0 To 10v Dc Output

[panel1]

I'm working on a project using a Micrologix 1100. Im having trouble locating a transducer that will measure vacuum and has a 0 to 10V DC output. I have found a couple of sites that offer this transducer but they have an 8 week lead time. Anyone know of an online site that may have this item instock? Thanks

 

[burnerman]

What range? These are good for low range (mbars) applications

 

[Tom Jenkins]

Omega Engineering tends to have these in stock. http://www.omega.com/

You may find 4-20 mA output more readily available, and the 1100 has 4-20 mA analog inputs. If you already have some 0-10 VDC transmitters and want to keep the same input card, a 500 Ohm resistor across the input will turn 4-20 mA into 2 to 10 VDC and you can scale as required in your program.

 

[SMOKE]

I Like Keyence.
http://world.keyence.com/products/press/ap_40/ap_40.html

 

[panel1]

Are you sure the Micrologix 1100 will accept a 4 to 20 ma signal without adding another analog module? The range I'm needing is 30" to 0 PSI. I was thinking the two input channels supplied on the 1100 only accept 0 to 10vdc inputs. Omega was the online site I checked and they are the ones with an 8 week lead time. Thanks anyway Tom.

 

[TConnolly]

Is this a high vacuum application?

If so then I use convectron gages from Granville Phillips for ATM down to 10^-3 Torr and ION gages for 10^-4 down thru 10^-7 Torr.

These gages have a logarithmic 1/decade ouput. If you need it I have a linearization routine for them.

If this is a lower vacuum applicaton such as you typically find in material handling, where pressure is measured in inHg rather than milliTorr then an absolute pressure transducer is more what you are looking for.


---edit
Nevermind, I see you posted while I was typing and you are measuring in inHg.

The 4-20mA input can be converted to 2-10V by placing a 500 ohm resistor across the input (ohms law: 10V = .020amps * 500ohms). Use an SCP instruction to scale the value to account for the 2V "live zero." The ML1100 built in analog input is a 10 bit input. That means that you will be able to resolve one part of 1024. Probably good enough for vacumm that is measured in inHg,

 

[panel1]

I think Im going to use the 500 ohm resistor to change 4 to 20ma to 2 to 10 volt signal. I did not know this was possible. Thanks to Tom and Alaric for the Ohms law lesson. When I use the SCP instruction will I be OK to make my scale adjustments using an analog gauge? I dont think my vacuum settings are going to be that critical. Also will a 1/4 watt resistor be OK? Do I connect the resistor across the(com- and + points on the controller? Thanks for the advice and help.

 

[mgvol]

Endress + Hauser used to make a low-cost vacuum xducer that I used several years ago. I believe they were fairly readily available. I used 4-20mA, but I'm sure they had a voltage model.

 

[TConnolly]

A 1/4 watt resistor is fine. Here is how it is done:

To figure your resistor wattage we will use Power = V*A. However, accordong ot Ohms law, V=A*R, so substituting A*R for V, we get Power = A*R*A or A²R, or as it is more commonly written I²R.

So .020²*500 = .2 Watts.


The SCP instruction is definitely the easiest way to scale this. If you are not too concerned about accuracy then using a vacuum gage to check your scaling is probably fine. Chances are any transducer you buy will be already calibrated so I would trust it more than a gage and use the gage only for a "ballpark" check to make sure you got the scaling set up right.

A word on vacuum transducers. Some models are sold as vacuum transducers and increase output with higher vacuum levels, however in my experience most are really absolute pressure transducers and have a higher output (towards 20mA) at near atmospheric pressure and a lower output (towerds 4mA) at vacuum. Check your transducer specifications.

 

[Ken Moore]

I agree.
Whenever I have need for a vacuum transmitter, I actually buy a pressure transmitter scaled 0-15psia.

 

[panel1]

Thanks to everyone for helping out. I think I have it... I'm gonna get setup later today and see how it works out.

 

[danw]

30"Hg to atmosphere, 4-20mA. web page says it's in stock

http://www.lesman.com/acatalog/Online_Catalog_Vacuum_and_Compound_Ranges_629.html

Dan