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Old 08-23-2016, 09:11 AM   #1
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Manometer guru's

This project started out with a family members trailer who had an issue with their dometic refrigerator flame going out down the road.
I went ahead and cleaned the gas orifice and the flame did get stronger but does not sound nearly as strong as the flame on our trailers Norcold refrigerator.
I thought I would troubleshoot a little deeper so I searched and found an article on testing an LP gas regulator with a DIY manometer.

LP Regulator testing.pdf

The lowest pencil mark is 0" WC

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I chose to tap into the gas line upstream of the refrigerator gas valve.
The static reading was 11" WC and dynamic was 8" WC. I dialed the regulator adjustment a bit (adjustment bottomed out) and was able to get the static to 13" WC and the dynamic to 9.5" WC.
As far as I know this regulator is original equipment on the 2007 Safari 25' trailer. (I tried this same test on my 94 AS with the original regulator and got similar results).

Does it matter how deep the dip is in the manometer tube? When I first tried the test it just blew all the water from the tube. I lowered the tube a few inches and it seemed to be have properly. The lowest pencil mark is 0" WC

Does this sound like a bad regulator? From my reading I understand that the dynamic pressure should be 11" WC and static can be anything under 14" WC.
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Old 08-23-2016, 09:19 AM   #2
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I have a 2008 Classic 25fb. I had a problem with the flame going out during travel. There was not enough shielding around the flame orifice. There was a Dometic recall that included adding more shielding around the lower part of the chimney that I had done in 2013. Luckily this fixed my problem and the flame hasn't gone out during towing. My trailer is now 9 years old from date of manufacture so I'm thinking I should replace the regulator and the feed hoses.

Kelvin
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Old 08-23-2016, 09:21 AM   #3
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Yes, there was a a recall, don't remember the years.
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Old 08-23-2016, 10:10 AM   #4
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The recall was not for operational issues but for fire issues if the heat exchange sprung a leak. That said it is far more effective at flame retention then it will ever be at fire suppression. It is an easy install so get one.
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Old 08-23-2016, 10:21 AM   #5
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The refrigerator has the extra shielding in place.
With the shielding in place I am still able to blow the flame out.......one click from igniter and it immediately relights itself.
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Old 08-23-2016, 10:28 AM   #6
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The next thing to look at is the thermocouple. Check to see if it is well positioned in the flame to detect the flame. If it is loosen the nut on the thermocouple line at the gas valve and retighten to insure a good connection.

If the thermocouple does not detect a flame or the signal is week the gas valve will shut down.
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Old 08-23-2016, 03:10 PM   #7
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Quote:
Originally Posted by crispyboy View Post
I chose to tap into the gas line upstream of the refrigerator gas valve.
The static reading was 11" WC and dynamic was 8" WC. I dialed the regulator adjustment a bit (adjustment bottomed out) and was able to get the static to 13" WC and the dynamic to 9.5" WC.
As far as I know this regulator is original equipment on the 2007 Safari 25' trailer. (I tried this same test on my 94 AS with the original regulator and got similar results).
What flow rate did you use for the dynamic reading? Just the fridge? That would be a fairly low reading then. If you're dumping gas at 40,000 btu/h, 9.5" is probably reasonable.

Quote:
Does it matter how deep the dip is in the manometer tube? When I first tried the test it just blew all the water from the tube. I lowered the tube a few inches and it seemed to be have properly. The lowest pencil mark is 0" WC
Doesn't matter as long as there's enough water that the gas doesn't blow it out.
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Old 08-23-2016, 03:40 PM   #8
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My understanding is to adjust to 11" dynamic with ALL appliances operating, save the stove. I think most techs use the stove line upstream from the stove's secondary regulator for their gauge tap.
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Old 08-23-2016, 03:45 PM   #9
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When I did this test, my regulator did like yours and blew the water out when I tried to adjust it. To answer your question, no, the depth of the U doesn't matter as long as you can get the required 11" difference in without blowing the the water out. I took that as a sign of a bad regulator and replaced it. All my propane problems went away.

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Old 08-23-2016, 11:17 PM   #10
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I was thinking about this again today, maybe overthinking.....As you can tell I am weak in the physics and science end of things.

Would it make a difference in the i.d. of the vinyl (manometer) hose? Should it be about the same as the copper pipe? The vinyl hose i.d. is 1/2" I think. I am guessing the i.d. of the copper pipe may be closer to 1/4" or 5/16". The reason I bring this up is consideration of the weight of the water in a 1/2" hose verses a smaller diameter hose. Wouldn't this make a difference in the readings?
To me there seems like there should be a correlation in the depth of the "U" in the manometer to the amount of pressure (water column) that your trying to read....

I think I am going to go back and hook the manometer up right at the regulator and perform the test with the appliances running versus using the orifice load test on the test rig I made up.
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Old 08-24-2016, 01:05 AM   #11
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My 1995 has a gauge in line right before the gas line enters the fridge. It makes regular adjustments a breeze.
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Old 08-24-2016, 06:04 AM   #12
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Crispyboy the diameter of the vinyl tubing is unimportant. You are trying to measure the 'difference' in the height of water in each leg of the 'U'. I can't tell from your picture posted in Post #1 if your graduations are 1/2" or 1" apart. Looking where your '0' mark is and how high the '11' vs the belly of the 'U' I think you could run the lower portion of your manometer a bit lower. you don't want the water to go into the bend of the 'U' section as it will give you invalid readings. Also do you fill the tube with water to the '0' mark before making any measurements? You could add a drop of food coloring to the water to make it easier to see.
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Old 08-24-2016, 06:55 AM   #13
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Quote:
Originally Posted by mehes View Post
I can't tell from your picture posted in Post #1 if your graduations are 1/2" or 1" apart.
The graduations are every 1/2" apart but numbered sequentially ie: 0, 1, 2, 3, 4....
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Old 08-24-2016, 08:49 AM   #14
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mehes has it right. I didn't notice your design before. Your 0" WC mark (where you fill to) needs to be in the middle of the straight vertical part, and the tube needs to be at least 5.5 inches tall above the mark but I'd make it substantially taller. The food coloring is a good idea as well.

The diameter of the tube doesn't matter. At equilibrium the atmosphere is pushing down with 14.7 psi on both sides. So what creates the difference in height is the increased pressure (not weight or volume) from the propane.

In my opinion, if you got a sudden increase in pressure when you tried to adjust the regulator, that is indicative of a bad regulator, probably something sticking. The risk is that when your appliances demand more pressure it could do the same thing, going to well over 11" WC and blow out the flame in one or more appliances.

Al
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