Going commando!

ChrisJ

So,

After talking with @MarkS‌ I decided to try an experiment to see if my theory is right regarding not needing a pig tail.
I built this setup and so far the pipe towards the gauge stays ice cold and the only heat there is from the metal conducting heat from the block.

I plan on running it this way for a while before doing anything permanent and of course the pressuretrol will remain on a pigtail no matter what.

As far as why do this? Because my theory is because air cannot escape steam cannot enter and without the huge slug of water in the pipe the low pressure gauge should be far more accurate and responsive.

For now, I'm keeping the valve off until I have more proof that it will indeed stay cool.

BobC

Commando?

Watch the Zipper

Bob

gerry gill

i love experiments!

conversiontime

I think on well balanced systems there is just extremely low pressure, and therefore no gauge movement pigtail or not. Wika 0-3 psi barely flickers during burn, it takes a deliberate swing of 5+ degrees to build enough pressure to see needle finally rise even towards 1 lb. The vacuum is also an issue with mine, but believe related more to poor quality product. The gauge face easily pops off so you can readjust the needle back to zero. Since main vents take longer than rads vents to reopen, I have found that setting several key rad vents more open allows them to start allowing air back into the pipes when steaming ends asap, this has stopped the temp vacuum that forms at end of burning. Even so I have had to adjust the gauge annually.

ChrisJ

@conversiontime‌,

If you look at my picture in the original post you will see the Wika 0-3 PSI gauge on my experimental setup. In the backround is a Dwyer -8" to +16" WC gauge. One line on the gauge is 0.5"WC or 0.28 ounces.

As I pointed out to Mark the Dwyer gauge was showing a vacuum even though I had a 1 1/2" skimmer tapping open. When I installed the Wika gauge and fired the boiler it showed a slight positive pressure while the Dwyer still showed a slight vacuum, though it moved more towards 0 which indicated an increase in pressure but still claimed negative pressure.

The problem is not the quality of the gauges, it's the water in the pigtail creating either a slight vacuum or slight positive pressure between it and the gauge. As the vapor / air in between the water and gauge heats or cools, it causes the gauge's reading to be biased up or down and it stays there until conditions change. At least this is my theory right now.




Without the pigtail not only does this not happen, but the gauge is far more responsive, almost vibrating with the steam rather than slowly moving.

KC_Jones

I would tend to agree with that theory, I am seeing similar results on mine. When it was mild out I looked at my non vacuum gauge and it was well below zero like it was in a vacuum. I have not had to move my needle and it will just return to zero on it's own. I am running a 0-15 ounce gauge at the moment. What about a copper coil or something that would just condense or cool any steam before it hit the gauge. In addition set it up so that any condensate would drain back...I am thinking like the coil on a homemade still?! Which funny as it seems is a condenser?! hmm experiments indeed!

ChrisJ

KC_Jones said:

I would tend to agree with that theory, I am seeing similar results on mine. When it was mild out I looked at my non vacuum gauge and it was well below zero like it was in a vacuum. I have not had to move my needle and it will just return to zero on it's own. I am running a 0-15 ounce gauge at the moment. What about a copper coil or something that would just condense or cool any steam before it hit the gauge. In addition set it up so that any condensate would drain back...I am thinking like the coil on a homemade still?! Which funny as it seems is a condenser?! hmm experiments indeed!

If it drains back it would work, but if a straight pipe works why bother?

As they say, where there is air there can be no steam.

Fred

Chris, I think you theory is probably spot on. I get the same thing on my 0-3PS gauge but only when it is really cold outside and the boiler runs for an extended period of time. It does eventually return to 0.

vaporvac

I have my Vstat and low pressure gauges in my header with no pigtail in order to read my stystem pressure ( I have the standard gauge and pressuretrol on each individual boiler). I was going to suggest just doing that, but looking at your last pic, it almost looks like you have!

David Nadle

I don't think this theory holds water (pun intended). You can't maintain a vacuum or pressure differential on the gauge side because the water slug can move, which will change the volume of trapped air, which will equalize the pressure.

It is true that the water mass will slow down the gauge response. (It would slow down even more if you filled the pigtail with mercury...) I'm not sure that's a bad thing. A fluttering gauge is harder to read.

KC_Jones

If it doesn't hold water then what is the explanation for a gauge sitting in vacuum for hours after the boiler has been off, then later not being in vacuum or even showing a positive pressure? I messed with mine last night after it had been sitting in vacuum for at least 20 minutes. I cracked the union under the gauge, but above the pigtail and the gauge immediately jumped back to zero. That seems pretty definitive evidence something is going on...holding water or not.

David Nadle

I would question the gauge before questioning basic physics. The observed effect seems to be much smaller than the gauge tolerance.

ChrisJ

David Nadle said:

I would question the gauge before questioning basic physics. The observed effect seems to be much smaller than the gauge tolerance.

Hi David,

I don't think we are questioning basic physics at all.

When the slight vacuum is created between the gauge and the water it pulls the water up slightly. The gauge is now holding some of the water's weight therefore is reading negative.

I tried to show this in my drawing as well.

David Nadle

A vacuum is not pulling the water. The atmospheric pressure on the other side is pushing it. Pressure on both sides of the water must equalize or there will be a net force on the water and it will accelerate.

Captain Who

The water slug in the pigtail makes it act like a u-tube manometer. The density of water decreases as it's temperature is raised, so if you heated one side of the u-tube you would get a very slight height difference between the two columns, even though both sides are open to the atmosphere. In this case, the other side is closed, and has a pressure gauge on it, so it is showing a slight vacuum. Maybe this is what is going on.

KC_Jones

ChrisJ said:

If it drains back it would work, but if a straight pipe works why bother?

As they say, where there is air there can be no steam.

Because the coil would look "cooler" and you know on my boiler it has to work correctly and look cool.

nicholas bonham-carter

Won't the air gradually become displaced by steam during a long burn, as steam is lighter than air? Steam certainly gets up into the sections of steam-only radiators, by displacing the heavier air.--NBC

ChrisJ

nicholas bonham-carter said:

Won't the air gradually become displaced by steam during a long burn, as steam is lighter than air? Steam certainly gets up into the sections of steam-only radiators, by displacing the heavier air.--NBC

I'm not sure.
This is the kind of feedback I'm hoping to get and it sounds like you feel it very well might happen.

I have not seen any issues, but my run times are very short.

ChrisJ

Hatterasguy said:

ChrisJ said:

If it drains back it would work, but if a straight pipe works why bother?

As they say, where there is air there can be no steam.

This really begs the question:

What is the minimum length necessary for that pipe nipple? In theory, if there is air in a 6" pipe nipple (1/4" diameter), steam can't get to the gauge unless the gauge uses more air than the volume of that pipe nipple.............

Maybe we can dispense with these problematic pigtails altogether?

Your thoughts?

Right off the bat my initial thought is why do we have pigtails and have had them practically forever if it's not required. There must be a reason. My first assumption is a pigtail is much smaller than a long length of pipe. 6" probably won't work because, at least for 1/2" blackiron that was transferring a good amount of heat all on it's own from the block without steam.

The next test might have to be closing the king and return valves, building up 1.5 PSI and letting it sit there for a while to see what happens. The other problem is will a 1/2" pipe behave the same as say a 1/4" pipe, or even 1/8".


What I will say is no matter what I find I do not feel anyone should ever install a safety device such as a vaporstat or pressuretrol without a pigtail. Having a $30 gauge fail is different than having a safety limit fail and not knowing it.

KC_Jones

Would the pipe diameter influence this as well? The thinner the tube the less likely steam can "get around" or mix in with the air. What about a capillary tube from the boiler to the gauge similar to what is used on automotive gauges. You could mount the gauge where ever you wanted and just run the capillary tube (factoring in drainage) to the gauge. So many ideas..... And to all the people shaking your heads right now, yes I and all of us know this really isn't necessary. We just like to experiment...isn't that where innovation comes from?

KC_Jones

I was thinking more like the gauge would have to be self supported and not count the pipe (or tube) as support in this application. The idea being the connections don't see any load. 1/8" is definitely pretty flimsy. I get scared when my kids get close to a radiator vent.

KC_Jones

If we had to bend our own pigtails this idea wouldn't sound that fancy...lol

Dave in QCA

I believe I have an answer and a possible cure for your false readings on your VERY sensitive pressure gauge. First, the water level in the pigtail will equalize if BOTH ends of the pigtail are open. When there is a gauge or other device connected to one end, that end is not open. So, the water level in the pigtail may wind up not being at a level equilibrium, and this minor imbalance in the water level in the two sides of the pigtail is enough to produce a false reading on a gauge that is capable of reading inches of H2O Column. How does this happen? I had the same difficulty when I used one pigtail to feed multiple devices. When this is done, it will allow boiler pressure to push enough of the water through the pigtail, (because of the large displacement area on the outlet side of the pigtail). When this happens, steam enters the pigtail and condenses. When the boiler goes off the water tries to return, but it may not got back to equilibrium.

IF you were able to relieve the imbalance by cracking a union just beneath the gauge, why not install a tee at that location with another ball valve that is open to atmosphere. This way, when the boiler is off, you could vent the gauge side of the pigtail and it would allow the water in the pigtail to equalize.

KC_Jones

What about a vacuum breaker between gauge and pigtail? I don't know much about them so if this is crazy please educate me.

ChrisJ

I've thought about a vacuum breaker but could never find one for such low pressures.

Part of my concern is my Dwyer switch which is set to trip @ 4 ounces. Will it actually trip @ 4 ounces or will it be biased as well.

Dave,
Are you saying if I split my devices up to separate pigtails the problem will likely go away?

Captain Who

I think if you had a pigtail made of copper and it had heat sink fins on it, you may not observe the effect any more. I'm still leaning towards it being a temperature gradient issue within the pigtail.

ChrisJ

I emailed Dwyer Instruments and asked them what they thought. I figured since they made the devices I am using it couldn't hurt to ask. Below are their responses.


Chris,


That is not the first time I've heard of that happening with a pigtail siphon.

If we're thinking of the same rod heat exchanger setup, I don't see why it wouldn't work.

Feel free to give me a call if you'd like to discuss further.



After this I responded and attached a picture of my plan just to make sure we were on the same page.






And his final response.




Chris,


That picture is exactly what I was thinking.

I don’t see any problems with that. It should work better than a pigtail.



I'll be repiping in my Dwyer switch and low pressure gauge tonight, though I'll be using 1/2" black iron for the 18" piece for strength.

nicholas bonham-carter

Maybe the water in the pigtail is evaporating on the boiler side, but not the gauge side, thus causing the imbalance.
I have the same negative pressure for a while on my Peerless syphon, which is not a pigtail.--NBC

ChrisJ

nicholas bonham-carter said:

Maybe the water in the pigtail is evaporating on the boiler side, but not the gauge side, thus causing the imbalance.
I have the same negative pressure for a while on my Peerless syphon, which is not a pigtail.--NBC

That's an interesting theory too.
How do those internal siphons work?

Captain Who

@ ChrisJ - in the diagram above of your new test setup I'm thinking it might be interesting to have a tee near the base of that 18" riser leading to a pigtail with water in it and another identical Dwyer gauge, for which you know the relative calibration curve between the two. You could shoot a youtube video of that setup as the boiler pressure rises. Anyway, that would serve as a control. I guess an electronic pressure transducer right in the boiler would be the best reference, but I'm sure that isn't exactly inexpensive.

We know pressure of varying mixtures of gases can vary with distances and temperatures and mass density. The best example of that would be the atmosphere itself, where barometric pressure varies with location at any given time.

Charlie from wmass

Have you tried using a tapping on the header with a pigtail for your sensor where it is not exposed to the waterline turbulence?

David Nadle

What if the gauge side has a small leak, and some air is leaking out under operating pressure, but at atmospheric pressure the air takes much longer to leak back in? Then the gauge side could be acting a bit like a finger over a straw, leading to the water level difference ChrisJ described.

If that were the case then the vacuum error should increase slightly after each run.

ChrisJ

Got the gauge installed, no pigtail

For the first time ever I saw my boiler huff and puff on the gauge. I've heard this via the main vents if I've got my ear by them but never saw a gauge show it. Always does it when it first starts boiling.

http://youtu.be/Byjw5cBPTAc

If you want to skip the boring,
http://youtu.be/GqxFafLe_8w


According to this, I run around 1" WC. With pigtail it claimed 0.5" which even though small is technically a 50% error.

ChrisJ

-

Abracadabra

ChrisJ, you obviously need a -1 to +1" W.C. gauge.

ChrisJ

Abracadabra said:

ChrisJ, you obviously need a -1 to +1" W.C. gauge.

Perhaps for normal operation, but the Dwyer switch is set to trip at around 7" WC and it will see that during a recovery.

I had considered buying one of these to hook up with the one that is there, but probably won't.

LPG4-D8922N Low pressure gage, range -4-0-6" w.c. (-1-0-1.5 kPa).

ChrisJ

I'm also pretty happy with how far I'm come in regards to threading pipe together. All of my work from 2011 has way too much tape and dope on the threads. My recent stuff, which is still leak free has almost none showing after I tighten things down. I used 1/2" wide blue monster and a tiny dab of megaloc dope on all of the 3/4" and 1/2" black iron fittings. Brass I use either just the blue monster or white PTFE tape, no dope.

ChrisJ

Still working fine. Pipe is ice cold just above the valve.

bob_46

Chris, do you tap on the gage before you read it ? Why don't you get some clear plastic tube and build an inclined manometer you could read 100ths of an inch . Manometers are always more accurate than a gauge.

Abracadabra

bob said:

Chris, do you tap on the gage before you read it ? Why don't you get some clear plastic tube and build an inclined manometer you could read 100ths of an inch . Manometers are always more accurate than a gauge.

100ths of an inch pressure?

Now that's just crazy talk.

Captain Who

I think it might be reasonable if the tubing is marked along its length longitudinally (in hundredths of an inch of w.c.), but the tubing is at a 60 deg angle from the vertical, for example.