pressuretrol/pigtail part 2

rtv

Well my friends..I have replaced the pigtail and moved the gauge(s) after it. now it seem the meters don't agree with each other. when the 0-30 reads 2psi the 0-3 is pegged so we are at least a pound off. the pressuretrol is cranked down as far as it will go (.5) with the Diff set at 1. SO.. if i understand correctly this means it should cut off at 1.5psi.
it doesn't!.. it cuts out at 2.5ish on the 0-30 gauge (and if the 0-3 gauge is correct that means its 3.5 or more)
Is the pressuretrol toast? are one or both of the gauges bad?

BobC

The 0-30 gauge is not at all accurate at that low a pressure and pressuretrols are by no means precision devices. There is a setscrew that may allow you to adjust it so it cuts out earlier.

This is the complete text of the pressuretrol calibration as posted by one of our board members a while ago -


If you see the pressure on the low pressure gauge go much over 1.5 to 2 lbs follow this procedure to re-calibrate the Pressuretrol:
Inside the Pressuretrol, right below the micro switch, there is a pivot arm. At the end of that arm you will see a screw pin that is activated by the diaphragm at the bottom of the Pressuretrol. If you look very carefully at that screw pin, you will see it actually has a tiny (I mean tiny) hex head on it. It takes a .050 hex wrench and you can turn it clockwise (Towards the bottom of the Pressuretrol to decrease the Cut-out pressure or counter clockwise to increase the cut-out pressure (which none of us want to do but who knows, your Pressuretrol may be really screwed up!). Turn the power to the unit off first. You may find the first attempt to turn that screw a little bit stubborn (relatively speaking) because it has some Locktite on it but it does turn. Don't turn too much, a tiny fraction of a turn goes a long way towards getting it adjusted where you want it (maybe 1/32 inch turn to start with) . You may need to play with it to get it exactly where you want cut out to be.

Bob

Captain Who

A few questions and suggestions:

1) Did you put a little water in the pigtail prior to installing the gauges and pressuretrol? Good idea to insure protection from the heat.

2) Did you make sure that the hole in the pressuretrol below the diaphragm (in the brass coupler) is clear? If it isn't that can give you a false pressure sense at the diaphragm.

3) I'd switch the positions of the 0-3psi and 0-30 psi gauge. Put the 0-3 psi gauge closest to the pressuretrol.

4) I'm assuming that you cleared out the piping between the new pigtail and the boiler?

rtv

thanks Bob! didn't know about the adjustment! so i should trust the 0-3 gauge?

rtv

captain:
1 yes
2 it looked clear
3 ok
4 there is no piping.. just the reducer fitting

BobC

As long as the 0-3PSI did not get slammed with high pressure I would be inclined to trust it. As long as you are sure that pigtail is clean I would try adjusting that setscrew, just adjust it in small increments.

Bob

Captain Who

Another thing you could try doing would be to remove the 30 psi gauge temporarily and install a hose barb fitting that fits some 1/4 in. clear vinyl tubing. Make a U-tube manometer shape out of it and attach the upper end to the ceiling somewhere with the bottom of the U near the floor. Pour some water into the bottom of the U. At 70 F you should have a difference in water column height of 1 inch for every 0.03605 psi, so 1 psi would be 27.74 inches of water. Use this to verify the accuracy of your 0-3psi gauge.

Fred

Do recalibrate the Pressuretrol. Find a .050 hex allen wrench and make very minor adjustments per the instructions above. I'd believe the 0-3PSI gauge as the 0-30 is just not very accurate at the low range.

rtv

thanks! i'll give the suggestions a try and report back.
Captain: just curious...why move the 0-3 closer to the pressuretrol? what difference would that make?

Captain Who

Only because temperature and pressure of a vapor are somewhat connected. If the tube length from the boiler through the pigtail and to the gauge is longer, the gauge will be at a lower temperature, especially when the temperature is climbing rapidly. It's best to have the gauge and pressuretol at the same temperatures. The time response of the gauge and pressuretrol will be more equal as well.

rtv

i see...thanks Captain! i took the 0-3 out completely till i get things under control so it doesn't hurt the gauge

Captain Who

Just to be clear though I wasn't saying I thought the gauge was in any danger of being damaged, especially if there is some water in the pigtail.

If you try the manometer trick, turn the elbow so it is horizontal and put the barb in that way, so it is also horizontal. Also, you might want to expel any water out of the pigtail for the short duration of the test. There won't be any short term damage with the length of pigtail you have even without water, at least not in the few minutes it takes to check the gauge accuracy and the water may interfere with the working of the manometer.

rtv

no...i was worried about the over pressure situation..putting 5psi into a 0-3 gauge!

Captain Who

Ahhh yes I'd be concerned about that also. You can test the gauge off of the boiler by rigging something up with a water column with an airspace between the water column and the gauge. I did a post on this years ago, I'll see if I can find it:

Makeshift Manometer

Captain Who

Here it is but it looks like my picture is gone :

http://forum.heatinghelp.com/discussion/115113/makeshift-manometer-for-testing-pressuretrol-vaporstat-gauges

nicholas bonham-carter

You can also hook the clear tubing up to the boiler drain, with a ball valve. When the valve is open, and the boiler has no pressure, (not running), mark the water level in the tube. When the boiler is firing, you will see the water rise in the tube, and by measuring the difference in height, between "at rest", and firing, you can determine the pressure, 1.75 ounces/inch).--NBC