Fred Paget on sat 23 aug 97
Hi Jon,
No the diameter of the tube does not make any difference. But by all means
put a valve between the manometer and the gas supply and only turn it on
while you are there reading it, otherwise there is danger of a gas leak if
you get a pressure surge or the thing falls over or something.
You have to have enough height (length) of water column to balance the
maximum pressure. If you don't it will blow all the water out the open top
end.
Calibration is reading the difference in level between the water surfaces
in the leg of the U exposed to the gas and the top of the other leg which
is exposed to open air.That gives the pressure directly in inches of water
column.You do not want to shorten the gas side leg of the U and put so much
water in it that it will overflow the gas end into the pipes if the gas
pressure drops to zero. It is a balancing act!
>Does the diameter of the tube have any effect on the distance
>he water moves? Hope somebody out there is familiar with
>the physics of water columns. We're also wondering if there
>has to be certain amount of water in the column and how
...>.to calibrate, by volume? Length?
Fred Paget---Mill Valley,CA,USA
Never try, never win!
Vince Pitelka on sun 24 aug 97
>Calibration is reading the difference in level between the water surfaces
>in the leg of the U exposed to the gas and the top of the other leg which
>is exposed to open air.That gives the pressure directly in inches of water
>column.You do not want to shorten the gas side leg of the U and put so much
>water in it that it will overflow the gas end into the pipes if the gas
>pressure drops to zero. It is a balancing act!
Fred -
This might be seen as a bit misleading. Correct reading of the manometer is
just based on the number of inches which the pressure raises the water above
the zero mark in the vented side of the U-shaped tube. With no pressure the
zero-mark is calibrated at the point where the water seeks its own level.
When pressure is applied, we ONlY read the number of inches the water rises
in the vented side of the U.
That is very good advice about placing a small valve at the manometer
hookup. It eliminates the riskiness of the manometer, since it is only
under pressure when you open the valve to check the pressure.
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
1560 Craft Center Drive, Smithville TN 37166
Vince Pitelka on tue 26 aug 97
>Sorry Vince , I have to dissagree. Your readings are repeatable but are
>only about half of the true reading. I got out my Handbook of Engineering
>Fundamentals to refresh my recollection and from the article on manometers
>the true reading is a function of the distance between the fluid meniscus
>under pressure and the meniscus that is vented to the air.Your method of
>reading could also suffer from inaccuracy due to evaporation or loss of
>fluid if it beads up on the walls.
>Best regards, Fred Paget
OK, now I'm confused. I received the above message from Fred, and it is
obvious that he has done his research. I have made a number of manometers
over the last three decades, starting in the late sixties when we had the
chemistry department at Humboldt State University make us a manometer to
check blower pressure on several cupolas we built in Steve Daly's iron
foundry. And I have seen a number of them installed on kilns over the
years. All of the manometers I have seen had actual inches marked off on
the vented side, above the zero level where the water seeks its own level
(of course this requires that you regularly replenish any water lost to
evaporation to keep the zero level at the same point). So, were all of
these manometers wrong, in effect reading half the actual inches of
pressure? For in fact, if the reading in inches is the difference between
the water level in the two legs of the tube, when the water rises one half
inch in the vented side, it would drop one half inch in the pressure side,
giving a reading of one inch? Is that correct, and have I always been wrong
in all the manometers I have used?? Why is it that a manometer set up as I
was taught, read a pressure of seven water column inches on house natural
gas pressure, as is in fact the actual pressure? Can someone clear this
up?? If one reads a manometer as Fred instructs, is the reading still in
actual inches, or is some other scale used? I don't mean to drive this into
the ground, but I really want to know the correct answer?
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
1560 Craft Center Drive, Smithville TN 37166
Fred Paget on tue 26 aug 97
Vince- I hate to come out on a public forum and tell someone they are
wrong. It is too muvh like flaming. Therefore I sent the private message.
Now I am sure you have 1000% more experience with the use or manometers
than I . The last time I used one was about 1953 when I was a test engineer
at G.E. My later experience was in electrical and power electronic
engineering.
Your practical experience does cast doubt on my reasoning. However the
Handbook is quite plain in explaning how the measurement is taken .It does
not discuss units of measurement. There is a figure of a simple manometer
open to the air and a differential manometer (one where the vented arm
terminates in a closed tank). They give a mathematical equation for the
differential manometer/ pA+sg1y1-sg2y2-sg3(y3-y2)=pB
where sg1 is the specific gravity of the gas in the side A of the tube
sg2 is the specific gravity of the water
sg3 is the specific gravity of the gas in the closed tank
y1 is the distance from the top of the tube to the meniscus on the
gas side
y2 is the height between the two meniscuses
y3 is is the distance from the gas side meniscus to the tank
pA is the gas pressure
pB is the tank pressure
There is no equation for the simple manometer so I had to derive it myself
as follows:
Simple manometer:
sg1 the specific gravity of gas
/
/-------\ / B
--|--|--- A |_____________________ | |
| | _____________________ | | |
| \-------/ | |---|---** .
| | | | **
y1 | | | **
| | | | ** ,
| | | y2 **
| | | | **
| | | | **
--|---------------------------------- ** ---|- **
** **
** **
**********
/ **********
/
sg2 the specific gravity of fluid
A manometer reading is converted into a pressure reading by writing an
equation from one end of the manometer to the other
pA + sg1y1-sg2y2=pB
since specific gravity of the gas is so small the term containing it can be
neglected and by a little algebra we have :
pA - pB = sg2 y2
pB is the atmospheric pressure and is the arbitrary zero in pressure
reading so it drops out and we have:
Gas pressure =specific gravity of water x y2
expressing it in conventional terms:
Gas pressure in inches of water = y2
Hope the picture comes through unscrambled. It has given me fits preparing it.
That is about the best I can do with the books I have available to me. If I
had access to an engineering library I am sure I could find books or at
least chapters on the subject.
With regard to your practical experience in correllation of the manometer
readings you took with the published gas pressure, is it possible that your
manometer scale was reduced to one half inch to the inch? That would give
about the correct reading if you used your method. Another explanation may
be that the practice is to read the things as you describe and call that
WCI even though it is not technically correct. I don't know as I know very
little about gas. Maybe we should talk to an engineer at the gas company.
Regards- Fred Paget
>
>Fred - I appreciate your response, although I wish you had posted it on
>Clayart, because I want to get to the bottom of this. I have no way to
>explain why the manometers I have used have worked just fine, even to the
>extent of measuring natural gas house pressure at seven WCI, which is
>exactly right. How can that be?? Now I'm confused. I have posted an
>inquiry to the list, including your post to me, and maybe someone else out
>there will have an opinion on the subject. In your handbook of physics,
>does it specifically say that the differential between water levels on the
>vented side and pressure side in inches is the actual measurement of water
>column inches, or is there some conversion factor??
>
>Please believe that I do not post opinions to Clayart unless I am pretty
>sure of myself, and thus my thoroughness here in trying to get to the bottom
>of this.
>- Vince
>
>Vince Pitelka - vpitelka@DeKalb.net
>Home 615/597-5376, work 615/597-6801, fax 615/597-6803
>Appalachian Center for Crafts
>1560 Craft Center Drive, Smithville TN 37166
Fred Paget---Mill Valley,CA,USA
Never try, never win!
Bill Aycock on wed 27 aug 97
May the old rocket scientist step in here and stop this nonsense?
Pressure is the question, and the units of measure is the problem- People
who HAD to do this thing regularly, for ages, stepped in and set the units
to the height of water in a tube, not anything having to do with different
specific gravities, sizes of tank or any of that junk- That is why they
call the units they use:
"WATER COLUMN INCHES"
What the force per unit area, acting on a liquid of some specified density
will do to raise the liquid is an excersise best left to those who delight
in calculation for its own sake. I KNOW- I WAS ONE-- FOR A LIVING!!
With a simple "U"tube, the areas effecting the weight of water and the
force generated, cancel out- ie, makes NO difference- You dont worry about
the Specific gravity, because WATER (ie H2O) is the reference used, and is
almost universally available.
Tank size is not a part of the problem because there IS NO TANK. We are
talking "U" tubes here. They are dirt-simple to make, using plastic tubing
and clear plastic tape. (duct tape tends to hide the water.) The last one
I made was residual Breather (Oxygen) tubing left over from my last
hospital stay, taped to a piece of 1"x2" and leaned (almost vertical)
against the stove. My problem was that the stove regulator had two
positions, but was not marked. One position was 6 WCI, the other was 10
WCI-- (WCI means "WATER COLUMN INCHES).
It makes no difference what gas you use, because you are not measuring a
property of the gas, but its condition, ie, its pressure.
Bill, getting all carried away, telling others to NOT get all carried away,
but safely out of the way, on Persimmon Hill
At 02:28 PM 8/26/97 EDT, you wrote:
>----------------------------Original message----------------------------
>Vince- I hate to come out on a public forum and tell someone they are
>wrong. It is too muvh like flaming. Therefore I sent the private message.
>Now I am sure you have 1000% more experience with the use or manometers
>than I . The last time I used one was about 1953 when I was a test engineer
>at G.E. My later experience was in electrical and power electronic
>engineering.
>Your practical experience does cast doubt on my reasoning. However the
>Handbook is quite plain in explaning how the measurement is taken .It does
>not discuss units of measurement. There is a figure of a simple manometer
>open to the air and a differential manometer (one where the vented arm
>terminates in a closed tank). They give a mathematical equation for the
>differential manometer/ pA+sg1y1-sg2y2-sg3(y3-y2)=pB
>where sg1 is the specific gravity of the gas in the side A of the tube
> sg2 is the specific gravity of the water
> sg3 is the specific gravity of the gas in the closed tank
> y1 is the distance from the top of the tube to the meniscus on the
>gas side
> y2 is the height between the two meniscuses
> y3 is is the distance from the gas side meniscus to the tank
> pA is the gas pressure
> pB is the tank pressure
>There is no equation for the simple manometer so I had to derive it myself
>as follows:
> Simple manometer:
> sg1 the specific gravity of gas
> /
> /-------\ / B
> --|--|--- A |_____________________ | |
> | | _____________________ | | |
> | \-------/ | |---|---** .
> | | | | **
> y1 | | | **
> | | | | ** ,
> | | | y2 **
> | | | | **
> | | | | **
> --|---------------------------------- ** ---|- **
> ** **
> ** **
> **********
> / **********
> /
>
> sg2 the specific gravity of fluid
>
>A manometer reading is converted into a pressure reading by writing an
>equation from one end of the manometer to the other
>pA + sg1y1-sg2y2=pB
>since specific gravity of the gas is so small the term containing it can be
>neglected and by a little algebra we have :
>pA - pB = sg2 y2
>pB is the atmospheric pressure and is the arbitrary zero in pressure
>reading so it drops out and we have:
>Gas pressure =specific gravity of water x y2
>expressing it in conventional terms:
>Gas pressure in inches of water = y2
>Hope the picture comes through unscrambled. It has given me fits preparing
it.
>
>That is about the best I can do with the books I have available to me. If I
>had access to an engineering library I am sure I could find books or at
>least chapters on the subject.
>With regard to your practical experience in correllation of the manometer
>readings you took with the published gas pressure, is it possible that your
>manometer scale was reduced to one half inch to the inch? That would give
>about the correct reading if you used your method. Another explanation may
>be that the practice is to read the things as you describe and call that
>WCI even though it is not technically correct. I don't know as I know very
>little about gas. Maybe we should talk to an engineer at the gas company.
>Regards- Fred Paget
>>
>>Fred - I appreciate your response, although I wish you had posted it on
>>Clayart, because I want to get to the bottom of this. I have no way to
>>explain why the manometers I have used have worked just fine, even to the
>>extent of measuring natural gas house pressure at seven WCI, which is
>>exactly right. How can that be?? Now I'm confused. I have posted an
>>inquiry to the list, including your post to me, and maybe someone else out
>>there will have an opinion on the subject. In your handbook of physics,
>>does it specifically say that the differential between water levels on the
>>vented side and pressure side in inches is the actual measurement of water
>>column inches, or is there some conversion factor??
>>
>>Please believe that I do not post opinions to Clayart unless I am pretty
>>sure of myself, and thus my thoroughness here in trying to get to the bottom
>>of this.
>>- Vince
>>
>>Vince Pitelka - vpitelka@DeKalb.net
>>Home 615/597-5376, work 615/597-6801, fax 615/597-6803
>>Appalachian Center for Crafts
>>1560 Craft Center Drive, Smithville TN 37166
>
>
> Fred Paget---Mill Valley,CA,USA
> Never try, never win!
>
>
Bill Aycock --- Persimmon Hill --- Woodville, Alabama, US 35776
--- (in the N.E. corner of the State) ---
also -- W4BSG -- Grid EM64vr
baycock@hiwaay.net
Jon Pettyjohn on wed 27 aug 97
Hi
thanks to Fred, Vince, Ward and Bill for sending me advice on
building a manometer. Will put this to good use especially
the safety tips, until we can get can get some good dial
gauges.
One of the things I hoped to clear up was the point about
how to read the thing correctly, so I was surprised to
see there is some confusion on this.
Just did a search from Infoseek on "manometers" and was surprised
to get 600+ hits back. Narrowed the search to "u-tube-manometers"
and got back 16 but seem to have found some answers from the following
urls:
http://www.olemiss.edu/-cmprice/lectures/pressure.html
http://www.mes.umn.edu/documents/d/c/dc5716.htm
Where I found the following:
>Figure 1. Using a u-tube manometer to measure pressure in a grain bin.
>When a fan generates pressure, it forces water in the tube to move in
>the direction of lower pressure. The height difference of the water
>levels on the two sides of the tube, measured in inches, {cut}
This would seem to say that the water movement on both sides of the
tube are measured.
Interestingly one other use for this device I found is measuring draft
for fire place chimneys. I wonder if there is some type of
manometer that could be used to measure kiln back pressure too?
Jon Pettyjohn Manila jon@mozcom.com
David Woodin Set Clayart Digest on wed 27 aug 97
A manometer can read low pressure, vacuum, and differential pressure. All
potters are interested in is pressure. Leave one side open to atmosphere
connect the other side via a valve to the gas line. The pressure is read in
inches on the side that goes to atmosphere. If you were reading differential
pressure then both sides of the u tube are hooked up to the orfice or
whatever, thru valving and the differential is read by readingboth sides of
the u tube. Only the water level not the meniscus is read. As stated in
other posts, shut off the valve when you don't need to read the pressure, it
would also be helpful to have a bled off valve so the zero can be checked and
a movable scale so that you don,t have to keep adding water.
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