On Liquid Propane tanks, say, the 20 LB tank on my gas grill, what is the
pressure in the full tank and what is the pressure stepped down to on the
low pressure side of the regulator?
How does this differ from regulators used to deliver Propane to
Propane stoves, furnaces, and so forth?
I've heard that the "Outdoor-Use" regulators step down to deliver gas
at 1/2 PSI, and that "Internal -Use" regulators step down to deliver gas
at 2.0 PSI.
I'm wondering how to make an old propane furnace in my shop run on
20 LB cylinders, and need to understand the stepdown of the regulator
and where to get one inexpensively. I already have all of the fittings
to attach the tank directly to the furnace.
Let's leave code and inspectors out of this conversation for now.
There are TWO regulators on a propane system. The first stage drops
the tank pressure to 10PSI ( this is known as "high" pressure ) and
the second regulator drops the pressure to the required level for the
appliance. There are two types of "low" pressure regulators - a 2PSI
version for large heating units and an "11 inches of water" version
for most everything else.
Fisher makes both of these in many models depending upon how much gas
needs to be delivered. You do know that the tank has a reverse thread
on it, don't you?
Try eBay for regulators but only buy new in box units.
I have seen "10 inches" of water specified on a number of furnace
devices. I thought
I would just post what that means and what it means in psi.
10 inches of water means the weight of a square column of water 1
inch on each side which
is 10 inches high .
To determine the psi;
Remember 33.9 feet high (406.8 inches) and 1 sq inch on a side is
Therefore , 10 inches : (10/406.8) = (Xpsi/14.7) psi is
11 inches : 0.40 psi
So, these pressures represent a very low, but still positive pressure.
Obiously, these very low pressures would be a pain to measure with a
gauge, but really
easy with a manometer , hence "inches of water"....
If the bolus of water is 1 inch on a side, the bottom area
of the column is one square inch. Pounds per SQUARE INCH
is the measurement we are interested in, so I used this
as an illustration.
The WEIGHT of the water is dependent on the volume of
the container ---- density is not the issue we are interested
in. However, the DENSITY is what we are dealing with when
when we say that 33.9 feet of water in a column one inch
on a side, weighs 14.7 POUNDS...... Since the base of the
column is ONE SQUARE INCH, the pressure is 14.7 pounds per
This is also known as ONE ATMOSPHERE, or, the pressure of
a column of AIR , one inch on a side, extending from the
earth to space, at standard temperature and pressure, exerts
a force of 14.7 pounds per SQUARE INCH ,or ,ONE ATMOSPHERE....
Thats how I remember the value for water without having
to look it up...
If you are diving in water , at 33.9 feet below the
surface, the water pressure on you is 2 ATMOSPHERES, or,
29.4 psi.... ( Easy to remember if you take a scuba class)
Go look in the CRC tables, and it will list all this stuff.
Also, a general science book from high school should give
a similar explanation....
Sorry if my explanation was not in the manner that allowed
you to understand it..... it worked for me, and that's how
I remember it......
Finally,1 gallon of water = 231 cubic inches = 8.34 pound avor
You can work this backward and get the same answer.......
By the way, here's a GREAT rule of thumb.
1 foot of water is 1/2 psi. Not strictly precision,
but real easy to remember and close enough for alt.home.repair.
Andy , BS, MS, PE etc.etc.etc
Andy adds :
One more thing.
"Eric" sent me an Email telling me that he worked for a gas company
that used a little
tube of clear plastic to determing the pressure. It was partially
filled with water and
one end open and the other connected to the pressurized line. A simple
ruler, in inches,
was used to determine the pressure......
Sure... That's called a "manometer". If you mount it on a nice
stainless steel plate and
put a calibrated ruler on it, you can buy one for between 10 and 1000
on the quality. Personally, I use the little plastic tube and my kid's
ruler, since I don't
need calibration to 4 decimal places (grin).
For very low pressures, you can't beat this method.
By the way, the LARGER the diameter of the tube, the higher the
accuracy. It has
to do with capillary action...... If you have a half inch diameter,
it's pretty good......
As an out of class assignment, do a google search on "water levels",
which is a
way to us a very long plastic tube to determine that a very long piece
is level...... This stuff gets interesting when you start applying
these principles to
stuff you might need to do......
When I read Andy's statement I had the same impression as
Terry. 10 inches of water has nothing to do with size of
the column. On the other hand, you have to know the weight
of a 1 inch square column to convert to PSI (pounds per
square inch), which is what Andy said he was doing.
Pressure measurements are confusing because so many
different measures are use. PSI is common to everyone,
inches of water are used for low pressures. Inches of
mercury are used for somewhat higher pressures. Atmospheres
is a convenient measure for some work.
And, it is not easy for many people to find the conversion
factors. Here are some:
Atmosphere -- 14.70 psi -- 1.058 tons per square foot--
29.92 inches of mercury -- 33.90 feet of water
Inches of water -- 0.03613 psi -- 0.07355 inches of
mercury-- 0.002458 atmospheres
By the way, the inches of water measure is based on 4 deg C
and mercury is based on 0 C.
You know, I had never thought of that. It's obvious from
the state table that this temp is the max density of water at standard
Thanks for pointing it out. It seems there are a number
of things that I haven't run into before.......
That's distilled (H2O) water I presume, not "heavy water" (D2O) which
has a specifig gravity of 1.107. <G> Ice cubes made from D2O sink in
You can even buy it here:
The town water where our business is located has enough bad taste to it
that I bet it even has a different density.<G>
There's so much salt in it (from years of road salt finding its way into
the well water.) that the town has to tell all property owners that if
they are on a restricted salt diet, "Don't drink the water."
Interestingly enough, there's no town regulation requiring the property
owners to inform office tenants like us, or apartment tenants. Seems
like they should though. I happen to know about it because we lived in
the town about 19 years ago and they were doing it then. Friends who
live in the same town now tell me they still get notices in the mail
from the town once a year.
We use a "MultiPure" brand filter ahead of our office water dispenser,
and we use the same one at our home in the next town, though the
unfiltered water at home tastes a lot better than the stuff at the office.
Ha. Ha. If you have heavy water, you've got too much time
on your hands, or maybe your hands have been where they
weren't supposed to be.
I thought the actual numbers might be interesting. But, yes,
distilled water would be best, and put that water in a
vacuum bottle to keep it the right temp! Actually for all
practical purpose the difference in density within normal
living temperatures and the difference in solutes of normal
drinking water aren't going to make much difference in the
measurement. Besides, the guy that decided 11 inches was
correct or 7 inches in an RV system, just picked a ball park
number. Stuff I've looked at that uses low pressures often
allows a 20-30 or even 50 percent difference in allowable
pressures. So even it you use water so salty you can't
drink it to set the regulator, it is unlikely it would make
any measurable difference to operation.
Wait till you get to a system where they decide the best way
to add chlorine is to add a months supply in one day. That
way you don't need to bother with adjustments for the rest
of the month.
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