Verify your Pressure Gauge - build or buy one


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Old 01-23-09, 09:15 AM
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Verify your Pressure Gauge - build or buy one

A while back, NJ Trooper showed a pix of his pressure gauge that he can temporarily attach to his hot-water system to verify the accuracy of the installed gauge. I decided to make one up.

The hose is no problem - just a couple of feet of garden hose or washing-machine hose. Ideally, it will already have a standard female hose connector on one end, but if not, just add one (readily available in the garden hose section at your hardware store). In all liklihood, the are one or more drain cocks on your boiler with standard male garden-hose threads.

For the gauge, I used a 0-30 psi gauge from Omega Engineering: Commercial Grade Gauges, Type C (Cheaper ones may be available elsewhere.) For maximum accuracy, you don't want the gauge's pressure range to be higher than necessary.

The gauge will come with a brass, 1/4" NPT threaded, male connector. To connect the gauge to the hose, my Ace hardware had an assortment of 1/4" female NPT hose barbs (1/4" female connection on one end of the brass fitting, and the hose barb on the other). Take your hose with you, and find a hose barb that fits snugly. Use a small hose clamp to connect the barb to the hose.

The hose-barb fitting must have standard NPT pipe threads (you don't want the brass flare fittings used for gas).

You can leave your hose connected to the boiler, but it must be valved out! The hose isn't rated for that temperature. Probably best to detach the hose when not it use.

I determined that my permanently installed gauge is reading about 1-psi high at 12 psi. Not bad.

When comparing the pressure of the two gauges, you need to hold the temporary one at the same elevation as the permanent gauge. Moving the hose up and down will readily show the 0.5-psi/ft of static head.
Doug
 

Last edited by NJT; 01-05-11 at 04:40 PM.
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Old 01-23-09, 03:33 PM
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Another option is to pick up one of the gauges that the lawn sprinkler dudes use... Watts markets one... HD and Lowes usually carry them... about $10 or so...


image courtesy plumbersurplus.com

The gauge is useless for boiler work, 0-300 PSI... no resolution on the 'down low' ...

So, pick up a 0-30, or 0-50 PSI gauge with 1/4" FNPT bottom mount threads. Try a regular plumbing supply house, or a swimming pool supply.

Remove the gauge from the fitting and install the lower pressure one.

If you build the one that I use, and that Doug built, you do have the option of positioning the gauge at the same 'altitude' as the boiler gauge to get a closer reading, but I only use mine to verify GROSS errors... if it's a few PSI off, I don't worry about it.

Here's the pic again, for reference. You may wish to use a longer hose, and you could fashion some kind of hook for hanging...

 

Last edited by NJT; 01-05-11 at 04:38 PM.
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Old 01-23-09, 04:56 PM
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Actually, I bought two gauges from Omega: the 0-30 psi for boiler use, and a 0-100 psi for my city water supply. Don't ask me why - I might want to check the city pressure some day?

But if you go the lawn-sprinkler route, don't throw away the 0-200 psi, or whatever, gauge. You can use it for checking the city water pressure.

But, back to my boiler. Maybe I should deliberately over-pressurize the system to make sure the relief valve lifts at 30 psi? It would be easy to lift the fill lever on the pressure reducing valve.*

*Note:
That was a joke. I wouldn't ever recommend over-pressurizing the system just to check the relief valve. But, once a year or so, it's a good idea to manually lift the relief valve to make sure that it is operating freely. And, I think it's a good idea to replace the relief valve every five years or so, ideally when the system is drained for some other reason.
Doug
 
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Old 01-23-09, 06:18 PM
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Can use the 200 psi gauge for oil burner pump pressure.



Al.
 
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Old 01-24-09, 08:03 AM
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UPDATE: I previously reported that when my old, permanently installed, pressure gauge read 13 psi, my new gauge read 12 psi. (That was with the boiler warm, but at the low end of its aquastat control band.)

But, I've noticed that there is a much greater disparity at higher pressure, when the boiler is fully hot: The old gauge reads 19 psi, the new gauge reads 14.5 psi. I wonder if bourdon tubes somehow lose their resistance over time?

By the way, my new 0-30 psi gauge is divided into 1/2-psi increments - for that extra precision that I want and need
Doug
 
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Old 01-24-09, 09:00 AM
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I believe that I have seen a hose pressure gage on the Patriot Supply site. It is a gage with a hose connection on it.
 
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Old 01-24-09, 09:03 AM
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Doug, that's a very interesting observation! One worthy of further investigation... I wonder if it's a simple non-linearity of the gauge itself, or if it's temperature related? I imagine you could do some tests to determine that with the boiler cold, and simply increasing the pressure temporarily in the boiler... short of 'testing' the pressure relief valve of course!

CT3, I'm gonna look later... but if you can find it and post a link, muchas gracias!
 
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Old 01-24-09, 09:43 AM
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Originally Posted by gilmorrie View Post
But, I've noticed that there is a much greater disparity at higher pressure, when the boiler is fully hot: The old gauge reads 19 psi, the new gauge reads 14.5 psi. I wonder if bourdon tubes somehow lose their resistance over time?

Doug
It is normal for corrosion of the tube to cause this affect. Gauges are also fragile. Even light bounces will affect them. And calibrating them isn't all that easy, as I found out. There is both an adjustment for linearity and span.

Note that sealed liquid filled gauges are very temperature sensitive.

Al.
 
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Old 01-24-09, 12:58 PM
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Originally Posted by OldBoiler View Post
It is normal for corrosion of the tube to cause this affect.
Al, that makes sense to me. After nearly 60 years, the bourdon tube could have lost a little wall thickness - due to corrosion, erosion, etc. That could, I think, cause the gauge to read high.

My "old" gauge obviously was installed with the boiler in the early 1950s - it's a 3.5" (dial) combination pressure and temperature gauge. The temperature reads low. (I have a couple of those magnetic thermometers that I can attach to the supply and return pipes.)
Doug
 
 

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