# Mercoid pressure switch



## Ajohnson9025 (Sep 20, 2019)

I have a Dwyer Da-31-2-3 mercoid switch at a small water treatment plant....I think it's being used as a low pressure alarm / shut off switch for the main tank.
The tank was emptied a week ago and new media filter agents put back in. Since then the switch has not been working reliably.
The line from the tank to the switch is 3/8" I believe...we took the switch off and the fittings off to check for obstruction but did not find much.
Put it back together and seem to be ok for a couple days but now is back to bring unreliable.

The plant operator says if he raps the side of the switch that the mercoid will break over.
Is this a sticky switch situation maybe?
Is water supposed to be up inside the bourdon tube or should it only be air?


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## Peewee0413 (Oct 18, 2012)

Is it just out of adjustment? Maybe someone moved the screws. Did you remove it and bench test it?

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## Ajohnson9025 (Sep 20, 2019)

Peewee0413 said:


> Is it just out of adjustment? Maybe someone moved the screws. Did you remove it and bench test it?
> 
> Sent from my SM-G998U using Tapatalk


I have not tested it. I don't have the set up to do that really.
I don't believe anyone has changed the settings on it though.
I'm just the electrician trying to help the plant operator .
Just curious but should there be liquid up in the tube? Or would trapped air inside cause an issue with proper pressure operation of the bourdon tube?
Short of lubing mechanical moving parts inside the switch there's not much I can do besides get him a new one .
He may need to contact a qualified person for this issue.


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## gpop (May 14, 2018)

Air in the tube will not make any difference. Pressure simply makes the curved tube try to straighten which trips the switch.
Cost to call in someone to calibrate (which may fail) may be higher than replacing the switch.


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## Bourbon County (Aug 19, 2020)

Those mercoid pressure switches have been around since the dawn of civilization and are normally extremely durable and accurate, but everything mechanical will fail at some point in time. It should be a pretty easy and simple test with a squeeze bulb (blood pressure cuff) some tubing and a manometer to see if it's working properly. If you need to replace it, Dwyer is located in your state, give them a call. In my experience with them you will get an application engineer on the phone and they are extremely versed on their products and helpful.


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## micromind (Aug 11, 2007)

Did it freeze with water in the tube?


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## Peewee0413 (Oct 18, 2012)

Not sure anyone in here can help you diagnose something you can't perform a test on. My half a cent of knowledge, outside moving the screws, broken tube/mechanics, or clogged source pipe, they pretty much work until they don't. For the cost of a new one, I'd buy a 5 dollar pressure regulator from Harbor Freight and perform a test. 

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## Ajohnson9025 (Sep 20, 2019)

Thank you guys for taking the time to reply.
Gonna take it apart check for debris inside


Peewee0413 said:


> Not sure anyone in here can help you diagnose something you can't perform a test on. My half a cent of knowledge, outside moving the screws, broken tube/mechanics, or clogged source pipe, they pretty much work until they don't. For the cost of a new one, I'd buy a 5 dollar pressure regulator from Harbor Freight and perform a test.
> 
> Sent from my SM-G998U using Tapatalk


no sir.


micromind said:


> Did it freeze with water in the tube?


No sir


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## gpop (May 14, 2018)

One thing that you will need to take into account is the system just got rebuilt. 

Last week i got called to re-calibrate a radar level sensor. Operator said that the high level sensor was shutting down the pumps when he set the fill level to 17.5' so the radar must be out of cal. I turn up and look at the scale and at 15' the radar read 15' so i ask the operator when this started and he responded after they serviced the inside of the tank. Pressure washing the inside of the tank had un-clipped the high level float.
So maybe you need to consider the switch is working correctly and they have a plug atmospheric vent or something on the tank. The reason it works when hit with a screw-driver may be due to it being in the dead-band. (E.G it dropped below its set point now it needs to go set-point plus dead-band to reset). Its always a good idea to understand why the switch was installed and to see if that safety is being exceeded.


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## Wirenuting (Sep 12, 2010)

We used them on our water tanks at a hospital I worked in. We had an air over water main tanks @120#, then sent it upstairs to service tanks. 
Over time the tube gets a memory and briging the tank to zero would throw one out of calibration. 
I just replaced them when it happened. For me it was easy as everything was redundant.


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## SteveBayshore (Apr 7, 2013)

It's the kind with the glass mercury tube in it, right? If so, is it level? Sometimes the mercury "climbs" up the walls of the tube, especially if it was moved from a level position. You should be able to see the tube. Tapping the tube will reset the mercury to the bottom, once it's level.


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## Ajohnson9025 (Sep 20, 2019)

SteveBayshore said:


> It's the kind with the glass mercury tube in it, right? If so, is it level? Sometimes the mercury "climbs" up the walls of the tube, especially if it was moved from a level position. You should be able to see the tube. Tapping the tube will reset the mercury to the bottom, once it's level.


So after a couple weeks now.... The system and mercoid work fine most of the time. But it still won't break over sometimes....the operator will backflush the tank and this fixes the problem for a few days.
I do know now that the pressure the mercoid is working with is 1-4lbs....very small margin imo.

But I think we're just hoping with the new filter media in the tank it's just gonna be what it is...which is fine for a few days until it needs back flushed.
Just wanted to provide some kind of closure. 
Thanks again


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## SteveBayshore (Apr 7, 2013)

If the mercury tube is banging around during backwash you might think of installing a dampener on the process line. I'll attach a cut on one in a day or so. I'm late getting out of here now.


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## gpop (May 14, 2018)

Ajohnson9025 said:


> So after a couple weeks now.... The system and mercoid work fine most of the time. But it still won't break over sometimes....the operator will backflush the tank and this fixes the problem for a few days.
> I do know now that the pressure the mercoid is working with is 1-4lbs....very small margin imo.
> 
> But I think we're just hoping with the new filter media in the tank it's just gonna be what it is...which is fine for a few days until it needs back flushed.
> ...



Time to check the calibration as it may be doing what it was installed to do.


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## Peewee0413 (Oct 18, 2012)

gpop said:


> Time to check the calibration as it may be doing what it was installed to do.


That's what I said 3 weeks ago..

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## CMP (Oct 30, 2019)

If it were me, I would install a *Magnehelic* indicating gauge on the same pressure sensing line, right near the Mercoid switch. In that way the operator and the service technician could visually see what the actual system operation pressure is, in real time. 

This would allow you to see what is happening in the system, and determine what in the process is affecting the pressure or know if the Mercoid switch is operating correctly. They are available in many pressure ranges

Better method than removing the switch and tubing to bench test and calibrate it. Check the system pressure operation under in use conditions.


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## SteveBayshore (Apr 7, 2013)

CMP said:


> If it were me, I would install a *Magnehelic* indicating gauge on the same pressure sensing line, right near the Mercoid switch. In that way the operator and the service technician could visually see what the actual system operation pressure is, in real time.
> 
> This would allow you to see what is happening in the system, and determine what in the process is affecting the pressure or know if the Mercoid switch is operating correctly. They are available in many pressure ranges
> 
> Better method than removing the switch and tubing to bench test and calibrate it. Check the system pressure operation under in use conditions.


Or use a Photohelic and it has the switches in it.😉


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## CMP (Oct 30, 2019)

The drawback to the photohelic is that the setpoint adjustments are right up front on the face and they are much more expensive. A screwdriver adjustment is a bit less prone to inadvertent monkeying with passers by.


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