Pressure guage inaccuracies!

[dznf0g]

I am a new member, although I have been reading on the site for some time. I have also posted this thread on iboats and received some comments, but no real resolution.

Ok, so I had an old SIB which was worn out. It had a hand pump with a guage and life was good.

I got a new SIB this fall which also came with a nice high pressure hand pump with a guage. Life was still good. I decided to get a manometer to use on the water as it is more convenient than pulling out the pump just to check pressure. Upon comparing the new pump guage with the new manometer, I found a 1 full PSI difference between the two. THAT's HUGE when you factor in possible pressure increases in the sun.

The Manometer reads 1 pound higher than the new pump (meaning I had been overinflating my SIB with the pump?????)....or was the manometer wrong?

Then I remembered I had retained the guage off of my old worn out pump from the old boat. It matched the manometer. So, I guess the odds are that the new pump guage reads 1 PSI low???!!!!!

It seems when I inflate the boat per the manometer, it is soft....or is it just that I got used to the "feel" of the boat overinflated?

Can anyone think of a good accurate way to "calibrate" my guages, short of buying several more and working the odds?

Before you say take it out and see if it handles right at the manometer pressure.....the water is a little cold and extra hard here in the northlands!

Thanks

[chewy]

You could pay to get the gauge calibrated but it would probably out weigh the cost of a new gauge.
I know I'll get slated for saying this but just pump it up till your fist bounces off it. No doubt someone will come along and point out the keel needs pumping to a certain pressure.

Whats the range of the gauge your using?

Welcome to the forum!

[dznf0g]

They are all three standard 15 psi inflatable boat gauges. The gauges that agree are a Bravo, off of my old pump and the new manometer, which is a k-pump kwik check. The one which doesn't agree is "no name" which came attached to the pump that came with my Mercury SIB. The pump says "Q" on it (for Quicksilver????). I do run an airfloor so I need a gauge which reads at least 12 psi.

[chris123]

I think you are probably just seeing the limitation of pressure gauges.

Mechanical pressure gauges are always inaccurate at the lower end of the range. Where on other devices (eg electronic pressure transducers) the errors are a percentage of the reading, the mechanics of a pressure gauge mean that the errors are a percentage of full scale. For a good quality pressure gauge the best you can expect is +/-10% of full scale, ie +/- 1.5 psi across the range for a 15 psi gauge.

I wouldn't worry too much about it - you could try and get someone to validate it against a calibrated pressure transducer / manometer but it is probably not worth it.

If you wanted a third opinion you could get one of these:

http://rib.net/forum/showthread.php?...ighlight=gauge

They are from a 'name brand' and prbably as good as anything.

Cheers

Chris.

[chewy]

Originally Posted by chris123

I think you are probably just seeing the limitation of pressure gauges.

Mechanical pressure gauges are always inaccurate at the lower end of the range. Where on other devices (eg electronic pressure transducers) the errors are a percentage of the reading, the mechanics of a pressure gauge mean that the errors are a percentage of full scale. For a good quality pressure gauge the best you can expect is +/-10% of full scale, ie +/- 1.5 psi across the range for a 15 psi gauge.

I wouldn't worry too much about it - you could try and get someone to validate it against a calibrated pressure transducer / manometer but it is probably not worth it.

If you wanted a third opinion you could get one of these:

http://rib.net/forum/showthread.php?...ighlight=gauge

They are from a 'name brand' and prbably as good as anything.

Cheers

Chris.

10% you sure, I'd be sending it back if it was that bad.
I'd expect the gauge to be accurate right the way across its range although the operating pressure should be roughly mid-range.

[Poly]

Originally Posted by chewy

10% you sure, I'd be sending it back if it was that bad.
I'd expect the gauge to be accurate right the way across its range although the operating pressure should be roughly mid-range.

I'd i've thought for the sort of money involved that this is about what you can expect.

[chewy]

Originally Posted by Polwart

I'd i've thought for the sort of money involved that this is about what you can expect.

Might as well pump it up and whack it with fist then.

I've calibrated £30 gauges and they've been fine.

[dznf0g]

Chewy, How did you "calibrate"....just compare against a known good and accurate pressure and compensate against your gauge? What did you use as a known good pressure?

[chewy]

Originally Posted by dznf0g

Chewy, How did you "calibrate"....just compare against a known good and accurate pressure and compensate against your gauge? What did you use as a known good pressure?

A Druck which is a calibrated pump/gauge. You are effectivly comparing it against a known accurate pressure.

I usually stand in the boat when pumping the keel up, if it lifts me then its pretty hard. I honestly wouldn't worry about it. I haven't used a gauge on any of my SIB's or RIB and never had any problems.

[dznf0g]

OK, "druck" may be a British term....I have never heard that term before....what kind of pump is that...a low pressure water pump? What kind of application is a "druck" used for?

[chewy]

Originally Posted by dznf0g

OK, "druck" may be a British term....I have never heard that term before....what kind of pump is that...a low pressure water pump? What kind of application is a "druck" used for?

Drucks the make of the pump.
I wouldn't bother messing about with it to be honest. You can buy a new gauge cheaper than you could get one calibrated.
If you bounce it about then it could be out of cal again.

[Poly]

Just a suggestion but could both gauges be telling the truth - but giving you slightly different info. A pressure gauge is essentially measuring a pressure difference between the test side (in your case the boat) and some reference point. One of your gauges may be measuring against a sealed reference point and the other relative to atmospheric pressure. A 1 psi pressure fluctuation is consistent with atmospheric changes.

[chewy]

Originally Posted by Polwart

Just a suggestion but could both gauges be telling the truth - but giving you slightly different info. A pressure gauge is essentially measuring a pressure difference between the test side (in your case the boat) and some reference point. One of your gauges may be measuring against a sealed reference point and the other relative to atmospheric pressure. A 1 psi pressure fluctuation is consistent with atmospheric changes.

Wouldn't a gauge that measured between two pressures be a differential pressure gauge.
The gauge he's using will have a sealed bourdon tube in it which moves a quadrant and then the pointer.

He's using one gauge and then the other so he won't have an atmospheric pressure change over the time he's using it.

[Poly]

Originally Posted by chewy

Wouldn't a gauge that measured between two pressures be a differential pressure gauge.

no all pressure gauges are measuring a pressure difference. A "differential pressure gauge" is a bit more complicated and has connections to two different systems.

However the general concept of the bourdon tube is that it measures a pressure difference between the pressure inside the tube and outside it (inside usually connected to the test system, and the outside is often open to atmosphere). It is measuring pressure relative to current atmospheric pressure. Sometimes more correctly referred to as gauge pressure, written as psig (or Barg).

But it is also possible to have a sealed environment on the otherside of the tube that contains a reference pressure (this might be a vacuum, a chosen pressure, or whatever the atmospheric pressure when the device was made and sealed), if the reference pressure is vacuum, or known, then the pressure measured can be absolute, but the scale can be 'offset' so that it appears to read gauge pressure.

[chewy]

Originally Posted by Polwart

no all pressure gauges are measuring a pressure difference. A "differential pressure gauge" is a bit more complicated and has connections to two different systems.

However the general concept of the bourdon tube is that it measures a pressure difference between the pressure inside the tube and outside it (inside usually connected to the test system, and the outside is often open to atmosphere). It is measuring pressure relative to current atmospheric pressure. Sometimes more correctly referred to as gauge pressure, written as psig (or Barg).

But it is also possible to have a sealed environment on the otherside of the tube that contains a reference pressure (this might be a vacuum, a chosen pressure, or whatever the atmospheric pressure when the device was made and sealed), if the reference pressure is vacuum, or known, then the pressure measured can be absolute, but the scale can be 'offset' so that it appears to read gauge pressure.

Thanks for explaining my own job to me, did you google the above?

[Poly]

Originally Posted by chewy

Thanks for explaining my own job to me, did you google the above?

nope

[Locozodiac]

Being a pressure gauge fan, thought that buying more expensive suppossed state of the art gauges will give precise readings, will not mention brands, although bigger, more accurate and easy to read than small ones, some can have up to 0.5 psi innaccurracy differences. It's a real shame that manufacturers don't pass a strict a lab test before sending units to clients.

The only way to test accurracy would be to buy 2 of the same brand, inflate an air mat, keel to 3.0 PSI with one and test the other and compare readings. Opting for rocket science and having lot's of gauges, went to a digital pressure lab, a tank was filled with controled 3.0 psi, tested 3 different gauges one of them read 3.0 exact, it's the master gauge, now can compare the reading of any gauge that will be given with sibs/ribs to the master reading and see their + / - 3.0 psi innaccurracy.

Most of the inflatables gauges on the market uses burdon tubes for measuring air pressure, the problem is located where the tube is solded to the gauge body, the slightest tube angle innaccurate left/right soldering position will give innaccurate readings, see pic for understanding.

Happy Sibbing

[Zubenelgenubi]

Originally Posted by chewy

Thanks for explaining my own job to me, did you google the above?

I bet Polwart subscribes to pressure gauges monthly, though the reply does look very "Googled".
Polwart knows everything about everything dont you know, he would have put the Oracle at Delphi to shame..........

[Poly]

Originally Posted by Locozodiac

It's a real shame that manufacturers don't pass a strict a lab test before sending units to clients.

if you need accuracy you buy a calibrated gauge (or pay someone to calibrate it) if you need a pressure indication then this is probably wasted money; if I remember rightly you were promoting gauges which cost less than 10 dollars.

The only way to test accurracy would be to buy 2 of the same brand, inflate an air mat, keel to 3.0 PSI with one and test the other and compare readings.

won't really tell you anything about accuracy though. it will simply show that either the two gauges disagree (which is more accurate ?) or that they agree (but both may be innacurate by the same amount) - you need to compare to a calibrated gauge if you want to understand accuracy.

[Poly]

Originally Posted by Zubenelgenubi

I bet Polwart subscribes to pressure gauges monthly,

I seem to have missed that one from my subscriptions list... ...but I did used to read "vacuum and coating technology" for a while.