# Differential Pressure Help!



## wildleg (Apr 12, 2009)

I must be missing something. If you don't understand how it works, how are you planning on "trouble shooting" it ? Sounds more like you just want someone to do your homework for you.


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## dhuggins (Dec 14, 2011)

Wildleg you are incorrect. I genuinely need some help on this...


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## dhuggins (Dec 14, 2011)

I have done all the calculations I can figure out but i cannot get my current to come out to 4mA at no load "no pressure".. The numbers (currents) I am getting make absolutely no sense..I don't understand how the differential pressure can be measured from this setup...


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## wildleg (Apr 12, 2009)

is the the controllers you are using ? (ETR 9200)

https://www.iprocessmart.com/images/Ogden/etr9200.pdf


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## dhuggins (Dec 14, 2011)

Yes sir that is the correct controlller..I got 156 ohms off those two resistors in parallel but that number comes out to nonsense answers...if he used that as a voltage divider I got 12v using the 24vdc source from the controller...that also is nonsense voltage because that controller doesn't use anything relative to 12v...I also calculated in so me resistance from the transducers (24/.004=6000 ohms) and got nothing close to 4ma no matter how I configured those transducers or resistors...I worked all the load to one resistance in that circuit and that was also bs numbers...I am missing something I guess


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## dhuggins (Dec 14, 2011)

I don't see how it's possible with that config to measure differential pressure because somewhere there has to be some subtraction ....


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## wildleg (Apr 12, 2009)

you have yet to describe the issues they are having at the plant, and whether or not it has anything to do with the resistance values. haven't you taken any readings on the actual panel ? many of the ckts are similar, it would lead one to believe you could probably figure it out by deduction and still not know anything about the circuit solutions.


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## dronai (Apr 11, 2011)

Sounds like this plant is in good hands


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## Wirenuting (Sep 12, 2010)

wildleg said:


> haven't you taken any readings on the actual panel ? .


He's a plant EE and would have his Minion take readings.


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## JRaef (Mar 23, 2009)

Something that jumps out at me right away is that "TC" means Thermocouple, as does the symbol used in that diagram. That means he is using (most likely) a "Pirani Gauge" system of pressure measurement. The circuit looks right however I have never seen that done with oil (or any liquid), they are usually used in vacuum / gas systems. You put power through the TC to use it like a little filament heater, then the rate at which the gasses passing over it cool it and change it's resistance determines the gas pressure inside, because the lower the gas pressure, the fewer molecular collisions there are against the filament. I have no idea if or how that would work on something that is not a gas though, because the molecular density of the oil would not change, i.e. liquids don't compress. Seems really complicated if you ask me.


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## RIVETER (Sep 26, 2009)

dhuggins said:


> Hello all, I have an interesting one. We have this panel (see attached) that is installed at a plant. They are having some issues and I am trying to troubleshoot it. The guy that drew these up left the company some time ago and he designed this circuit in such a way that it is reading a pressure differential with some resistors and two separate pressure transducers. I have sit here and tried to figure out how the circuit works and why particular resistor values were chosen and also figure out the math. I am stuck and need some help on this circuit. Some particular info that will help...this is a 110VAC panel run into a 24VDC output power supply. All of these modules are Ogden 9200 controllers. The pressure transducers used are 0-600 PSI and the voltage across R1 and S1 is designed for a 0-5v input. The pressure differential is on the oil pressure transducer because that is "inside the vessel" Therefore it is reading the pressure difference between the suction pressure and oil pressure.
> 
> I know I am asking quite a bit on this but I'm sure someone will probably find this fun. Any help on this will be greatly appreciated!!!


Looks to me like you know how to post pictures. However, differential pressure switches are very simple devices. Their output going to the proper input is all that is important.


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## dhuggins (Dec 14, 2011)

Wildleg..I probably used a poor choice of the word "troubleshoot". This panel apparently got wet so it no longer works..and that particular ogden controller is obsolete..now I have been trying to find a replacement with the current setup..Therefore if I don't understand how that circuit works I obviously can't and won't commission another one using the existing setup.

JRaef, I am absolutely positive that there were 0-600psi pressure transducers used not thermocouples..I would almost bet that was the first symbol he found in the CAD library that had the terminal config that was appropriate. There is no doubt about it, it's complicated to me.

Riveted, these are not differential pressure transducers. They are 0-600 pressure transducers They are cascaded in such a way that one is acting like differential pressure transducer. That is the part I don't understand.

I brought this question forward because I needed some help. I don't understand why there has to be such comments. I can't figure it out so I put my problem on here hoping that I can get some input on this circuit from someone else that has more experience in the industry. I don't expect solutions I look for breadcrumbs.


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## dronai (Apr 11, 2011)

Because we get a lot of guys just trolling a fake scenario to mess with everyone. Giving more info to get your answer ,like you just did will get you results.


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## wildleg (Apr 12, 2009)

dhuggins said:


> .....
> 
> I brought this question forward because I needed some help. I don't understand why there has to be such comments. I can't figure it out so I put my problem on here hoping that I can get some input on this circuit from someone else that has more experience in the industry. I don't expect solutions I look for breadcrumbs.


now don't you feel better giving us the real scoop ? somebody who has the time will probably give you a lot more help now that you told us the situation. like Dronai said, we get a lot of kids who post threads here trying to pose homework problems as real situations, and noone here wants to waste their time. If you had simply posted that information to begin with, there likely would have been no questions.

So you don't waste further time - what direction are you going in ? Are you going to try using etr 9300s and build a similar panel, or are you trying to design a new panel from scratch ? It would probably be helpful to provide as much information as possible, like the exact pressure sensors that are actually connected (is it a standard 4-20ma ?)


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## hardworkingstiff (Jan 22, 2007)

dhuggins said:


> I brought this question forward because I needed some help. I don't understand why there has to be such comments. I can't figure it out so I put my problem on here hoping that I can get some input on this circuit from someone else that has more experience in the industry. I don't expect solutions I look for breadcrumbs.


You might get more civil and helpful responses here. 

Good luck.


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## Semi-Ret Electrician (Nov 10, 2011)

dhuggins said:


> Wildleg..I probably used a poor choice of the word "troubleshoot". This panel apparently got wet so it no longer works..and that particular ogden controller is obsolete..now I have been trying to find a replacement with the current setup..Therefore if I don't understand how that circuit works I obviously can't and won't commission another one using the existing setup.
> 
> JRaef, I am absolutely positive that there were 0-600psi pressure transducers used not thermocouples..I would almost bet that was the first symbol he found in the CAD library that had the terminal config that was appropriate. There is no doubt about it, it's complicated to me.
> 
> ...


IMO the pressure sensors being labeled "TC" set everybody off (me included). That would put this device into a science fiction setting, which had little chance of working.

If the discrete sensors are actually pressure sensors being trimmed with resistors that too is doomed to failure.

The system may work for a brief period of time after calibration but will drift all over the place due to temperature differences, aging, etc.

If you really want to measure DP..buy a DP transmiiter with two sensing legs or sensors with a 4-20 madc output or go digital. The marketplace is loaded with choices.

Maybe you can reuse a controller if it accepts 4-20 ma or 1-5 v (with a 250 ohm resistor across the input). Again it's all plain vanilla.


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## dhuggins (Dec 14, 2011)

Wildleg, I am honestly sure not which way I want to go. I can easily and quickly achieve this using a PLC. I just don't think that would be as cost effective as using the 9300 ogden. As Semi said I could easily use a DP transducer using a 4-20mA (which should have been the way to do it in the first place). However if the current application was reliable I would stick with it and lean towards the 9300 ogden or maybe even a Watlow. I have to figure out the numbers and how that transmitter config even works first before I rebuild it.

I am not asking for alternative solutions i am just seeking help on the circuit and how it works and if it is even practical.

Thank you guys for the answers and I apologize if I didn't explain the situation well enough. I honestly thought i posted enough information trying to show that I was putting in effort and not just posting expecting a solution.


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## dhuggins (Dec 14, 2011)

With Semi's post about the inaccuracy of this circuit that scares me. Monday I am going to set this circuit up on my desk and try to dig further into the numbers part and why those value resistors were used.


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## hardworkingstiff (Jan 22, 2007)

dhuggins said:


> With Semi's post about the inaccuracy of this circuit that scares me. Monday I am going to set this circuit up on my desk and try to dig further into the numbers part and why those value resistors were used.


I hope you share your discoveries with us. :thumbsup:


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## wildleg (Apr 12, 2009)

I redrew your diagram so it made sense to me. The circuit appears fairly simple. the voltage across Oil Pressure is V+, which is created the current through TC13 through the resistor to ground. The current through TC16 raises the V- up from ground through the other resistor, giving you the "pressure differential". As to the values, you'll have to figure them out, since I believe you can scale the inputs with these controllers.

The diagram seems to be missing some items between suction and capacity though. I'm surpised that you would not have verified and documented both the wiring and the sensors of the original equipment.

What retired electrician had to say stands, however, but of course, you didn't tell us much about the process or the sizes of the equipment either, so who knows.










BTW, I don't think you are missing anything with the values - either the inputs are scaled or perhaps the "transducers" characteristics aren't what you think they are.


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## dhuggins (Dec 14, 2011)

Wildleg I sent you a PM.


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## dhuggins (Dec 14, 2011)

Ok I have an answer. (312*.02=6.24V) so that is the basis for a 0-5V signal (assuming you never get to full scale) on the pressure transducer. The other circuit is the exact same. Then those two voltages are pumped into the Oil Pressure (0-5V input) which in turn if there are pressure differences then there will also be a voltage difference across those two terminals which is where the 0-5V input comes in.

Thank you all for your help!


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## MHElectric (Oct 14, 2011)

Threads like this are a good reminder of why I like being a dumb electrician. 

My answer would've been "Just wire it up!". Sounds like that would've been the wrong answer. :laughing:


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## dhuggins (Dec 14, 2011)

Well to be honest I was looking at it wrong. Wildleg redrew the diagram in a better way (the correct way) and then I could figure it out. So I appreciate that Wildleg!!


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## pjholguin (May 16, 2014)

Wildleg, 
Would you, please, post or PM me the drawing you redrew?

Patrick




wildleg said:


> I redrew your diagram so it made sense to me. The circuit appears fairly simple. the voltage across Oil Pressure is V+, which is created the current through TC13 through the resistor to ground. The current through TC16 raises the V- up from ground through the other resistor, giving you the "pressure differential". As to the values, you'll have to figure them out, since I believe you can scale the inputs with these controllers.
> 
> The diagram seems to be missing some items between suction and capacity though. I'm surpised that you would not have verified and documented both the wiring and the sensors of the original equipment.
> 
> ...


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