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Old 18 January 2010, 10:28   #21
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Originally Posted by Polwart View Post
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.

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.
Seems I write in Nepalese, you cannot know is the gauge you have just bought is accurate or not until you compre it to a second one or to a well calibrated lab gauge, we have matched a accurate gauge to match same 3.0 psi on a digital lab facility comparison, now can compare same or any other brand to this one and see their inaccuracies, this is not rocket science, but al least much better than not using a gauge at all.

Polwart, would be stupid to pay extra to have it lab calibrated, when the gauge should have come ok and well tested from the manufacturer. You are supposed to be paying for a precise gauge, anyway with just one unit, wouldn't know if it's accurate or not, need at least 2 units to compare betweem them or to a perfect lab calibrated gauge. Gauge detractors will sure be laughing about this post, we live on an imperfect world. And the actual cost is 10.30 Euros + shipping.

Happy Sibbing
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Old 18 January 2010, 13:52   #22
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Quote:
Originally Posted by Locozodiac View Post
Seems I write in Nepalese,
no its quite clear english - its the factual content that is wrong
Quote:
you cannot know is the gauge you have just bought is accurate or not until you compre it to a second one
wrong - as I said all you know is if those two gauges are agreeing not - you don't know which, if either, is right or wrong
Quote:
or to a well calibrated lab gauge, we have matched a accurate gauge to match same 3.0 psi on a digital lab facility comparison, now can compare same or any other brand to this one and see their inaccuracies, this is not rocket science, but al least much better than not using a gauge at all.
yes so all your doing is creating a calibrated gauge of your own to use as a reference. So now we need an extra gauge, a friendly 'lab' to calibrate it for your etc. Would be cheaper and easier just to have bought a calibrated gauge in the first place if you care that much. And you've only calibrated at 3 psi so you've no idea how accurate it is at 1 or 2 psi.

Quote:
Polwart, would be stupid to pay extra to have it lab calibrated, when the gauge should have come ok and well tested from the manufacturer. You are supposed to be paying for a precise gauge,
precision and accuracy are totally different things. at your price point I think your being unrealistic expecting it to be calibrated. Chewy's experience suggests that 30 GBP gauges calibrate well (but I think may well be supplied without calibration certificates) and my experience is that it would cost about that price to send a simple gauge away for laboratory calibration. Expecting a gauge that costs 1/3rd of that to be just as well calibrated is niave.

In reality it probably doesn't matter anyway - inflate the boat to a pressure where it performs correctly/well (by trial and error) and use an uncalibrated gauge to determine the value that works for you. From then on inflate to this pressure on this gauge and it will perform the same! The number itself is irrelevant.
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Old 18 January 2010, 14:54   #23
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Originally Posted by Polwart View Post
Chewy's experience suggests that 30 GBP gauges calibrate well (but I think may well be supplied without calibration certificates) .

In reality it probably doesn't matter anyway - inflate the boat to a pressure where it performs correctly/well (by trial and error) and use an uncalibrated gauge to determine the value that works for you. From then on inflate to this pressure on this gauge and it will perform the same! The number itself is irrelevant.
The 30 gauges did cal ok but most gauges will when new, the 30 gauge was crap.

I would just pump it up till you find whats best like Polwart said.

Loco, how do you know your "accurate gauge" is accurate, it should be cal'd every six months if your doing things right. If you drop the gauge or knock it it may put it out of cal.


If you get really bored you could join this: http://www.instmc.org.uk/
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Old 19 January 2010, 10:50   #24
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Seems we are bitting own's tails, will start from the beginning so the general sib audience follows. A complete newbie just bought his brand new sib, and wants to use a guage as recommended, he has read so, a buddy using one has recommended to use one, etc, he would order one online, at a nautic store, whatever, and buy one, alleluya have solved all his inflating issues forever, wrong, having purchased one will not now if factory accurate or not as he doesn't have another one same brand to compare with.

In my "particular local case" needed to confirm the 3.0 psi accuracy or +/- inaccuracy of every single gauge that is delivered with each new sib/rib against my well calibrated gauge as to state a pressure refference, most of the stocked 30 gauges are accurate or have inaccuracies of 0.25 to 0.5psi when matching a source (air mat, keel) inflated to 3.0 psi, so it's not a big difference issue, so depending on the reading, instruct client to inflate to this final reading, to make it kindergarten a tag is placed on every gauge to infalte to a given pressure, it could be 3.0/3.25/3.50 psi, but in reality will be inflating to 3.0 psi. The only way to obtain perfect reading, that is with no air release whatsoever, is to have/make an appropiate gauge adapter that slightly pushes the central pin valve.

Want to state clear that inflating a sib to 3.0/3.5 psi is not a value that have invented, read any sib owners manual or look at a transom plate, you will find tech data written there, if you want to obtain best sib/engine performance. The purpose to best technically explain was just to answer dznf0g thread, assume now more confused than ever and with no valid tech answer whatsoever as nothing is absolutely perfect or precise.

Answering Chewy: I know that my master gauge is correct as it has been compared to a digital pressure gauge reading in a reputed local lab that does calibrations services and matched perfectly a digitally monitored 3.0 psi air source. Was previously mentioned before, you seem to read posts very light !!! And yes, my well calibrtaed gauge is always kept properly stored, never goes to sea and avoid dropping on floor, just in case. And have been reading perfect for the last 2 years.

Conclusion: if you want to use a gauge, finger touch/cone bend, drum tune sound method ; if gauge has flooded or sunken in deep salt water and recovered, if your son smashed the gauge open wanting to play with inside needle, it's your problem. No matter what you state, we seem to have completeley different appreciations, will continue to use gauges as been doing for the last 14 sibbing years, and please, let's end this useless ping/pong writing

Happy Sibbing
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Old 19 January 2010, 12:02   #25
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Want to state clear that inflating a sib to 3.0/3.5 psi is not a value that have invented, read any sib owners manual or look at a transom plate, you will find tech data written there, if you want to obtain best sib/engine performance.
Firstly most people probably don't care that much about the last 0.5 knots or the 1/2 second time saving to get on the plane from having a perfect pressure in the chambers - much the same way as I have never got round to fine tuning the height of my engine (the cav plate is 1" below the keel) - it does exactly what I need it to do and life is too short to worry about if it could be a few percent better!

Secondly - does it not strike you as surprising that all sibs have suggested inflation pressures which are "round" numbers - why is is 3.0, not 2.95 or 3.15 why 3.5 not 3.4 or 3.7 ?? Because the guy writing the manual/plate understands precision and accuracy and also that the average user has no way of achieving an exact number - so has written it rounded to the nearest 0.5 psi!

Thirdly - what makes you think that the factory who came up with the 3.0 or 3.5 number actually used a calibrated gauge to arrive at the number. 3.0 probably means "the needle was pretty close to 3 psi on the cheap gauge we used in our workshop, which wasn't calibrated so was probably somewhere between 2.5 and 3.5 psi in reality"

dznf0g already knows the answer it was in the final paragraph of his message - since he can't use the boat till the ice melts he just has to wait till then!
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Old 21 January 2010, 11:10   #26
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Wow! I didn't mean to open a "can of worms" here! What's with all the hostility?? (humor here...don't get all bound up) I'm not confused...just anal retentive! I actually learned quite a bit through the discussion. I guess my only REAL concern is this. I really don't care what pressure I am at, I just don't want to realize sometime I have forgotten to let some air out while the SIB is left in the sun and come back to find a rupture. Maybe I should ask these questions:

According to my manual a boat inflated to 3.6 psi (spec) left in the sun can rise to 4.8psi.
If I use may gauge and it is in fact reading one psi low and my actual pressure is 4.6 psi (cold), would my actual pressure hot be 6.1 psi?

I have forgotten my physics...is it air volume or density which prescribes thermal expansion and pressure increases/decreases?

Is 6.1 psi enough to weaken or rupture seams and deform the fabric?
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Old 21 January 2010, 11:58   #27
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Physics:

(P1*V1)/T1 = (P2*V2)/T2

You are at as near as d@mmit constant volume, so V cancells out & P2 = (P1*T2)/T1. As T2 is in this case higher than T1, P2 rises. (T is absolute = Kelvin)


I also reckon that to stop a lot of warranty claims that pressure advice is likey to carry a fair safety margain.
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Old 21 January 2010, 13:24   #28
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Physics:

(P1*V1)/T1 = (P2*V2)/T2

You are at as near as d@mmit constant volume, so V cancells out & P2 = (P1*T2)/T1. As T2 is in this case higher than T1, P2 rises. (T is absolute = Kelvin)


I also reckon that to stop a lot of warranty claims that pressure advice is likey to carry a fair safety margain.
Its not just T that is absolute so is the Pressure - so you need to add atmospheric pressure to the number on your gauge.
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Old 21 January 2010, 14:30   #29
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So what about petrol forecourt airline gauges???

More of a critical application, I believe???
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