Manometer theory

[Huzo]

Please guys, help me settle a bet.
My mate has "balanced" the TB's on his BMW R1100S.
He has selected one of the methods and something in my dim distant Uni Physics tells me that his method is in error.
Ignoring if you will, the crappy diagram, does one of these apparatus have an inbuilt anomaly?
I won't say (yet) which one I think is crap. But his set up rings a slightly off tune bell.
I favour one over the other.

[Muzz]

My understanding says that the right hand one is correct.

I have been known to be wrong before.....

[Chesterfield]

As always, Google is your friend!
https://en.wikipedia.org/wiki/Pressure_measurement#Liquid_column_.28manometer.29
https://sciencing.com/manometer-2718.html

[rodekyll]

Either is good if you know how to use it.

[Huzo]

As always, Google is your friend!
https://en.wikipedia.org/wiki/Pressure_measurement#Liquid_column_.28manometer.29
https://sciencing.com/manometer-2718.html

Yeah, most of that is not new, but I wouldn't have been able to recall it.
I just have a problem for some reason with the "closed loop" (as it were) apparatus. He says that when both are pulling equal vacuum, the menisci will be level, but I can't get away with thinking that one sides reading will be corrupted by the other'.
Dunno...there's just bloody something.

[Huzo]

Either is good if you know how to use it.

Would you tell us in a bit more detail RK ?

[Kiwi_Roy]

Please guys, help me settle a 

Assuming the chamber in the middle is the manifolds the one on the left is measuring difference, it could be filled with any light fluid like oil or water, could be any height between 2 and 6 feet, my choice.

The one on the right would have to be filled with mercury nothing else would work and be at least 4 feet tall, more likely 5 feet.
This is really two gauge pressure manometers (comparing the manifold to atmosphere) this would be very unsafe IMHO
One thing we forget about is momentum, the mercury will rush up the tubes and well exceed the height the vacuum is calling for spilling into the manifold to be vaporized by the hot engine.

[Old Jock]

I'm with RK

One is an absolute pressure measurement the other a differential.

If what you are trying to do is achieve equal vacuum then all you need to do is get the meniscus level to the other, the pressure difference is the height between the levels.

Of course one might "interfere" with the other if they are both pulsing like crazy, but providing they are damped, usually through an orifice I can't see the problem

The absolute version will require bigger tubes or denser fluid as they are measuring individual values relative to atmosphere and not to each other.

Now some whizz kid will come along and make me look stupid............. .........

[rodekyll]

OJ's got it.

The one on the left does relative balance and is ready to go as you drew it.  I posted a pic of one I built in another topic (http://wildguzzi.com/forum/index.php?topic=92964.0).  It looks just like your left shark.

The one on the right needs to have the meniscus in each tube indexed to an external "absolute" reference.  If you know how to "zero" the tubes it will work.  I have a MOTION PRO 4-pot balancer that looks like your right shark +2.

The advantage of left shark is that it balances naturally -- no calibration needed.  The disadvantage is that it works best on a two-pot system.

The advantage of right shark is that you can make it up for any number of pots.  The disadvantage is that you need to calibrate each tube to each other via an absolute index before each use. 

Right shark is also sensitive to pulses and can suck a tube dry if the system is dramatically out of balance or the throttle gets blipped (good argument against mercury).  So we use long tubes with restricting orifices (olives) to damp the pulses.  Left shark isn't prone to meniscus suck* as long as it is connected and disconnected with the system shut down.

*I knew that if I could sell you on the left-shark/right-shark bit I could get the phrase "meniscus suck" to work, too.   

[Kiwi_Roy]

Now I see by double clicking the sketch it blows up to show the boxer engine R1100S
Assuming the sketch is somewhat to scale.



 Double Click Here

The one on the left will work if its almost in balance to start with, really has too much of the tube filled with oil.
There should be restrictors where the tubes join the manifolds to provide some damping.

But the one on the right will suck the fluid, actually atmospheric pressure will blow - be it oil or mercury out of the manometers into the left and right manifolds so you had better be prepared for that. Your manometers could never be long enough for even mercury unless the bike is up on a lift table as the tube will hit the floor. Again there should be restrictors to damp the fluid.


Restricting the fluid movement can be either by tiny holes pinching the gas at the manifold or just squeezing the tube at the bottom of the "U", it's much easier to slow down the liquid.

[Wayne Orwig]

Please guys, help me settle a bet.
My mate has "balanced" the TB's on his BMW R1100S.
He has selected one of the methods and something in my dim distant Uni Physics tells me that his method is in error.
Ignoring if you will, the crappy diagram, does one of these apparatus have an inbuilt anomaly?
I won't say (yet) which one I think is crap. But his set up rings a slightly off tune bell.
I favour one over the other.

They are both crap.

The cylinders have sagged down from the perfect 90 degrees.

[Sasquatch Jim]

 It is always entertaining when the mercury is sucked into a running engine.
 First the YAAG! of surprise, followed by the oooh shyt, and finally the despair of knowing that your freshly rebuilt engine will need an upper cylinder job so soon.

[n3303j]

http://www.powerchutes.com/manometer.asp
Works like a charm.
Deadly accurate.
Cheap.

[Rough Edge racing]

 I put away the devices measuring manifold pressure/vacuum and use this arguably better tool. The Synchro Flow measures air flow into the carb or throttle body . This eliminates the slight variances in manifold pressure due to cylinder sealing..  it's a few inches long and may not fit the carb/throttle body air horns in all situations.

     

[dxhall]

Why would mercury cause damage if sucked into a motor?

[Old Jock]

Why would mercury cause damage if sucked into a motor?

https://www.youtube.com/watch?v=Z7Ilxsu-JlY

[Rusnak_322]

http://www.powerchutes.com/manometer.asp
Works like a charm.
Deadly accurate.
Cheap.

I have a homemade one like this that I used auto trans fluid in. Works fine for me.

[Orange Guzzi]

Balancing two or more intakes also includes making sure all are pulling the correct "inches of mercury" at idle.  This us usually listed in a service manual.  I have seen intake balancing specs. that gave a tolerance of 1 inch between intakes.  That is a lot.

 With a home made set up, you cannot see how many inches of vacuum difference you are getting between intakes.  A light fluid level that is 4 inches higher on one intake may be much less than 1 inch of mercury out of balance.  Not as dramatic as it appears in the tube.  I would suggest investing in "real" gauges with a real calibrated scale.  Using a clear tube and fluid set up is not as accurate as it appears.  If anything, it is very confusing and extremely sensitive it changes in the carb. opening.  The "usual" fluid of choice is much less dense than mercury.  1 inch of mercury in a tube weights much more than 1 inch of transmission fluid. 

Many factors need to be taken into consideration.  Tube I.D. and fluid weight per volume are part of the equation. That is why it is called "inches of mercury" not inches of transmission fluid mixed with alcohol. 

I have tried the homemade set up.  It is a waste of time and effort and not accurate.  Buy real gauges.  Do it the right way. 

[kirb]

You are trying to get the vac balanced between left and right. You need to measure the pressure difference. The closed loop is the easiest way to make that happen. There is little need to have the liquid quantity exact on the closed loop. That matters quite a bit on the open loop version.

It's better to have the liquid more vertical (think 'U' shaped) where the levels of the right and left tube are close enough to get a good balance point.

My home made version uses fork oil as the liquid with about 30-36" of vertical. The oil level is about half-way up the tube.

[Tom H]

Gotta ask:

Why not just use vacuum gauges? I bought a set and seems to work fine.

Tom

[Rusnak_322]

Gotta ask:

Why not just use vacuum gauges? I bought a set and seems to work fine.

Tom

$90+ vs $6 for something that is very rarely used.

[Tom H]

For $90, they must be some very nice gauges!

The set I bought was in a kit with 2 gauges, hose and all and was about $30, might have been less, can't remember.

Kinda thought that would be the answer. Thanks,
Tom

[Kiwi_Roy]

Why would mercury cause damage if sucked into a motor?

Never mind the damage to the motor, that is insignificant compared to the environmental damage, mercury vaporized by the hot motor, a hazmat crew would have a field day.

Let alone  "what were you thinking, having that dangerous material where our grandchildren play"?

[Kiwi_Roy]

Balancing two or more intakes also includes making sure all are pulling the correct "inches of mercury" at idle.  This us usually listed in a service manual.  I have seen intake balancing specs. that gave a tolerance of 1 inch between intakes.  That is a lot.

 With a home made set up, you cannot see how many inches of vacuum difference you are getting between intakes.  A light fluid level that is 4 inches higher on one intake may be much less than 1 ounces of mercury out of balance.  Not as dramatic as it appears in the tube.  I would suggest investing in "real" gauges with a real calibrated scale.  Using a clear tube and fluid set up is not as accurate as it appears.  If anything, it is very confusing and extremely sensitive it changes in the carb. opening.  The "usual" fluid of choice is much less dense than mercury.  1 inch of mercury in a tube weights much more than 1 inch of transmission fluid. 

Many factors need to be taken into consideration.  Tube I.D. and fluid weight per volume are part of the equation. That is why it is called "inches of mercury" not inches of transmission fluid mixed with alcohol. 

I have tried the homemade set up.  It is a waste of time and effort and not accurate.  Buy real gauges.  Do it the right way.

As an instrumentation Technician, I can't let that go
Why can't you tell the difference in inches of oil with your home-made setup, if you want to use a classic manometer inches of water is another standard

"1 ounces of mercury out of balance", there's no such unit that I'm aware of

Are you saying the old tape measure you used as a scale was not calibrated properly?
(not that you need a scale for carb balancing)

True, the SG of the oil has some effect on the column height but no more so than the inaccuracy of the average bourdon tube gauge, most hydrocarbons are around 0.8. Besides you are usually trying to make both sides the same not set to some unit of pressure, why do you care what the vacuum reading is in inches of mercury?

Yes tube ID is important to avoid capillary attraction but 1/4" ID is enough to make it irrelevant

What do you consider a Real Gauge?, manometers are used to verify other types of gauges and transmitters , bourdon tube gauges in low ranges are very fragile.

From the Merium website, they have made calibration manometers from the year dot
"The U Type Manometer is a versatile, economical instrument and is the primary basic standard of pressure measurement. Pressure measurements are accomplished by balancing a vertical head of indicating fluid with the pressure to be measured. As there are no cams, gears or levers to operate in the manometer, the readings obtained are always accurate"

Oil is one of the indicating fluids they sell

[Wayne Orwig]

Gotta ask:

Why not just use vacuum gauges? I bought a set and seems to work fine.

A. When I tried a set years ago, the needles bounced around a LOT.
B. It was hard to compare the needles. Parallax and all that.
C. You have to hope they are calibrated properly.
D. A hose with a bit of fluid is so cheap and eliminates most of those issues.

[Tom H]

A: Good point, they do bounce a lot. The kit I bought came with little valves to reduce/remove the bounce.

With all the talk of mercury. Why can't water be used instead? You just need some kind of fluid to see how much it moves, right????? Just curious. We are trying to balance, not look for a specific number for each to be at, right??

Thank you,
Tom

[Orange Guzzi]

As an instrumentation Technician, I can't let that go
Why can't you tell the difference in inches of oil with your home-made setup, if you want to use a classic manometer inches of water is another standard

"1 ounces of mercury out of balance", there's no such unit that I'm aware of

Are you saying the old tape measure you used as a scale was not calibrated properly?
(not that you need a scale for carb balancing)

True, the SG of the oil has some effect on the column height but no more so than the inaccuracy of the average bourdon tube gauge, most hydrocarbons are around 0.8. Besides you are usually trying to make both sides the same not set to some unit of pressure, why do you care what the vacuum reading is in inches of mercury?

Yes tube ID is important to avoid capillary attraction but 1/4" ID is enough to make it irrelevant

What do you consider a Real Gauge?, manometers are used to verify other types of gauges and transmitters , bourdon tube gauges in low ranges are very fragile.

Why do you care about inches of mercury at a specific engine speed?  The service manual specifies it.  "Set to some unit of vacuum".  If you look in a service manual, you will usually find this intake vacuum pressure set at idle.  With a tolerance of 1 inch between intakes. 

Making a set using tubing and mercury, the scale would have to be in coordinate with the the tubing i.d./volume per inch of column.  The tape measure scale reference on a homemade set up has nothing to do with "inches of vacuum", it is just a scale with numbers. 

I will go back and correct my previous comment to read  "1 inch"   My apologies for the confusion.   

[Wayne Orwig]

With all the talk of mercury. Why can't water be used instead? You just need some kind of fluid to see how much it moves, right????? Just curious. We are trying to balance, not look for a specific number for each to be at, right??

It can be. But water weights a lot less then mercury, so the water column has to be a LOT longer. I don't have the numbers, but it is going to be a number of feet I suspect, instead of inches.

[Tom H]

I understand now. Thank you!!
Tom

[Kiwi_Roy]

Why do you care about inches of mercury at a specific engine speed?  The service manual specifies it.  "Set to some unit of vacuum".  If you look in a service manual, you will usually find this intake vacuum pressure set at idle.  With a tolerance of 1 inch between intakes. 

Making a set using tubing and mercury, the scale would have to be in coordinate with the the tubing i.d./volume per inch of column.  The tape measure scale reference on a homemade set up has nothing to do with "inches of vacuum", it is just a scale with numbers. 

I will go back and correct my previous comment to read  "1 inch"   My apologies for the confusion.

Ok I'll give you that a service manual might ask you to set at a certain inches of mercury but we don't usually balance the throttle bodies using a service manual, I will have to see what my Griso manual says

No, Volume per inch has nothing to do with it Head is head for 1/4" or 4 inch ID, no difference in the pressure at all.

Inches are inches are inches, pressure is measured in these units because it can be replicated anywhere, It may be effected by the gravitational constant at various points in the globe but that's insignificant, if we are going to worry about that we should also worry about temperature effect.