From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Very small propane leak in Motorhome
Date: Wed, 30 Jan 2008 01:19:46 -0500
Message-ID: <3i20q31p2rmd56vgik4he9piq8qj2836r9@4ax.com>

On Tue, 29 Jan 2008 19:15:02 -0600, "Don Hamilton" <CDHamil@excite.com> wrote:

>I have a small leak that I only smell while driving. It is not sewer gas
>because it is gone if the tank (propane) is off. I use the gas while
>traveling to run the fridge. After I stop I do not smell the gas. Is there a
>cheap sniffer available? Does anyone know where the best place to have it
>checked out? I feel a RV dealer will look at this as a open chance to charge
>as they will because of who knows how much time could be involved.
>Thanks Don
>

First question is, is the odor raw or burned propane?  If it's burned propane (the
two odors aren't all that much different sometimes), and assuming your rig is a
motorhome, then it's probable that you're picking up stack fumes from your 'fridge
while you're driving.  Do you leave a roof vent cracked when you're underway?  If so
then turbulence could push some refrigerator exhaust fumes back through the vent.  Or
you could have an air leak around your refrigerator.  If aerodynamic effects lower
the air pressure in the rig while you're underway (common enough) then combustion
fumes could be being sucked into the passenger compartment instead of up and out the
refrigerator roof vent.

If you're sure that it is raw propane, next question is, how are your gas tank(s)
configured?  Do you have removable 20 or 30 lb tanks in a compartment or is the tank
chassis mounted?

If you have removable tank(s) in a compartment then it is likely that when you're
driving low pressure develops in the RV from aerodynamic effects and/or high pressure
develops around the compartment.  This pushes propane odor through any leaks that
there may be in the compartment walls.

A fairly easy way to check for leaks is to put a smoke bomb (a fireworks stand type
will work fine) in the compartment, turn on your roof vent fan and look around the
compartment inside the rig for smoke streams.  Turning the gas off isn't really
necessary but you can if it makes you feel safer.

Do you smell propane at all when you're stopped?  Even when you sniff closely at the
regulator and connections?  If not then the "problem" is probably an aerodynamic
effect.  What happens is this.

Air pressure build up or drops slightly around the regulator from dynamic and ground
effects of air flowing around the moving rig.  When the pressure rises over normal
ambient air, the regulator raises the pressure just a little in the propane system
because it is designed to maintain 11" of water pressure in the system RELATIVE TO
ITS VENT PORT.  When the pressure around the regulator subsequently drops (you slow
down, speed up, stop or otherwise change the air flow around the rig), the excess
pressure diaphragm inside the regulator bleeds off a little propane to restore the
proper pressure.  This propane exits the vent hole.

This is normal operation.  The amount of propane vented is tiny.  However, if there
happens to be a stagnant air bubble around the compartment or frame-mounted tank AND
there is an air leak into the RV, some of this odor can be carried inside.

I ran into this problem several years ago on a rig I serviced for a customer.  It was
a small MH with removable tanks in a compartment.  Same complaint as yours - slight
smell of propane but only when underway.

I spent a LOT of time checking everything in the system.  Then, because his
compartment was at the very rear of the rig, the idea of pressure change occurred to
me.  I verified the problem by attaching the sensor of a high speed electronic
flammable gas meter directly to the regulator vent and letting him drive around while
I watched the meter.  I saw small erratic little puffs of propane coming out the
vent.  I tried measuring the air pressure in the compartment but the pressure inside
the RV was too unstable.  The manometer is referenced to the air pressure around it,
in this case, the inside of the RV.

My simple solution to the "problem" was to use the smoke bomb to find small leaks in
the compartment and seal them with plumber's putty and RTV.

If you brought your rig to me, the first thing I'd do is check for air leaks from
around the propane tank to the inside of the rig, and fix any I found.  I suspect
that a similar thing is going on here and the "fix" is to keep the stink outside the
rig.  If you can't smell propane when you're stopped then IMO, you don't have a
problem - just an annoyance.  I can't, of course, judge how strong the odor is when
you're moving, of course, so take that into consideration.

There isn't a sensor that I know of that is even remotely affordable that can rival
the common nose for leak detection.  The only thing you have to watch for is that
many people very quickly suffer "odor fatigue" and stop detecting the odorant after a
short exposure to it.  The solution is plenty of fresh air between short sniffing
sessions.  Neither soap bubbles nor any of the other common techniques will find a
leak that small.

If you want to confirm that there is nothing leaking downstream of the regulator then
a bubble flow sensor is the most sensitive instrument to low flow commonly available.
This sensor is placed in series with the gas flow.  IOW, it is hooked between the gas
regulator and the line going into the rig so that any gas flow passes through it.

You make the sensor out of a glass jar (a Mason jar is fine) with a tight sealing
lid, a couple of pieces of small tubing (1/16" diameter or thereabouts) and some
water.  I recommend the small brass tubing used by model builders.

Clean the lid of any paint, then poke two holes about an inch apart in the lid so
that the tubing will fit snugly.  Cut one length of tubing so that it reaches just
inside the jar and the other so that it reaches near the bottom.  If using brass
tubing, soft solder them to the lid.  If using something else, epoxy in place.

Fill the jar about half full of water.  No more than a couple of inches of water up
past the end of the long tube.  Rig up connections and hoses (surgical rubber tubing
is fine) so that the gas passes from the regulator to the tube that reaches to the
bottom of the jar.  Connect the other tube to the gas system.

Turn every gas appliance off, then very gently open the gas valve.  The water will
bubble vigorously for a few seconds as the system pressurizes.  Then the bubbling
should stop completely.  A bubble may hang on the end of the tube that reaches near
the bottom.  It may shrink and swell with minor changes in system pressure but it
should not break free.  At least, not more than once an hour or so.  Any bubble
activity indicates a leak.

If you want to check for dynamic effects, pass the hoses through a window on the rig
and have someone drive around while you watch the bubble.  If there is a dynamic
pressure effect at work, you'll see the bubble alternately swell and shrink at random
intervals.  It may break away and/or water may be sucked a little bit up into the
tube.  There may even be more than one bubble.

If the bubble doesn't remain stationary, more or less, then you can bet that a
dynamic air pressure effect is at work.  Nothing to worry about.  Seal any outside
air leaks to get rid of the odor.

Finally, I agree that this is the type of problem that an RV dealer either won't take
on or will hoover your wallet and probably still not find anything.  Narrowing down
the cause takes a lot of time and a not insignificant amount of detective work.  Best
that you do that at home where your time is free.

John