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Old 15 January 2010, 15:02   #1
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Pressure guage inaccuracies!

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
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Old 15 January 2010, 17:14   #2
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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?

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Old 15 January 2010, 20:27   #3
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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.
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Old 16 January 2010, 05:14   #4
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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.
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Old 16 January 2010, 05:50   #5
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Originally Posted by chris123 View Post
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.
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Old 16 January 2010, 05:57   #6
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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.
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Old 16 January 2010, 06:18   #7
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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.
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Old 16 January 2010, 09:15   #8
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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?
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Old 16 January 2010, 09:23   #9
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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.
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Old 16 January 2010, 09:27   #10
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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?
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