Tube pressure, do you use a gauge?

[boristhebold]

No i dont use a guage other than feel each time I go out.

[Orwell boy]

I may be living under a rock when it comes to Chinese ribs ,but having been in pressure measurement all my working life I do understand Chinese pressure gauges ,ha ha , you are all happy to fill your tubes using such devices ,ha ha ,let me explain .....
these 15 psi gauges you are using will have a build spec of probhably plus or minus 5% of full scale ( 15x1.05= 0.75 psi +or _ ) apply to target 2psi and you realise it could be 1.25psi or 2.75 psi or indeed anywhere in between (maybe even spot on)
the simple and 100% accurate way is to make a pressur loop out of a length of clear tube ....to make a 2psi capable loop you will need 6 metres of clear tube and a piece of wood 3metres long , attach the tube to the wood in a loop with two open ends at top, make provision to stand the wood upright ,mark the tube at 1.5 metres from the floor (tube loop) link your pipe from your rib valve but don't connect yet ,note the mark @1.5 metres will become your zero pressure reference , add a broken tape measure leading up from the reference point to the top of the wood ....... fill the tube from the top with a funnel till water is eqal in the u tube to the referece mark (add a bit of food colour if you wish to make it easier to see ) .........connect the rib valve maintaining the loops vertical position ,wont matter how long this pipe is
now the maths .....2psi =1406mm of water ,so if the non connected side of the loop rises 1.4metres (10cms short of the top) then 2 psi is spot on and every measurement in between is perfectly divisible and accurate , if you need to be a real anorak you could take a temp reading in the morning and see what a rise in temp does to the tubes over the day
hopefully for those who have 6 metres of clear tube a piece of wood and a tape measure kicking about this is a cheaper solution to an accurate result ,on the other hand for the rest to get a remotely sensible answer you will need a pressur gauge with a max scale 3 psi

[69cmw]

Originally Posted by boristhebold

No i dont use a guage other than feel each time I go out.

[emoji106][emoji106]

[Pikey Dave]

Originally Posted by Orwell boy

I may be living under a rock when it comes to Chinese ribs ,but having been in pressure measurement all my working life I do understand Chinese pressure gauges ,ha ha , you are all happy to fill your tubes using such devices ,ha ha ,let me explain .....
these 15 psi gauges you are using will have a build spec of probhably plus or minus 5% of full scale ( 15x1.05= 0.75 psi +or _ ) apply to target 2psi and you realise it could be 1.25psi or 2.75 psi or indeed anywhere in between (maybe even spot on)
the simple and 100% accurate way is to make a pressur loop out of a length of clear tube ....to make a 2psi capable loop you will need 6 metres of clear tube and a piece of wood 3metres long , attach the tube to the wood in a loop with two open ends at top, make provision to stand the wood upright ,mark the tube at 1.5 metres from the floor (tube loop) link your pipe from your rib valve but don't connect yet ,note the mark @1.5 metres will become your zero pressure reference , add a broken tape measure leading up from the reference point to the top of the wood ....... fill the tube from the top with a funnel till water is eqal in the u tube to the referece mark (add a bit of food colour if you wish to make it easier to see ) .........connect the rib valve maintaining the loops vertical position ,wont matter how long this pipe is
now the maths .....2psi =1406mm of water ,so if the non connected side of the loop rises 1.4metres (10cms short of the top) then 2 psi is spot on and every measurement in between is perfectly divisible and accurate , if you need to be a real anorak you could take a temp reading in the morning and see what a rise in temp does to the tubes over the day
hopefully for those who have 6 metres of clear tube a piece of wood and a tape measure kicking about this is a cheaper solution to an accurate result ,on the other hand for the rest to get a remotely sensible answer you will need a pressur gauge with a max scale 3 psi

You beat me to it, I was just about to say " manometer" I suppose you could make a one off to calibrate your cheap eBay gauge & occasionally use it as a datum.

[LaurenceTaylor]

I must be a failure. As an engineer my "gut feel" should be in the correct ballpark for tube pressure change due to temperature. For some reason I don't get it. I have had to register so that one of you can help me out.


In a basic simplified calculation pressure should be proportional to temperature in Kelvin. If so a temp change from 10C (start of day) to 30 C (now sunny) is only 20K change from 283K to 303K. This is only 7% or so.


If you pump tubes to 2psi a 7% change is not an issue + or - . So why do tubes go over hard and soft if we don't adjust ????

[jeffstevens763@g]

i must be getting old
my boats for over 40 years have been a bit like a famous shampoo.
pump & go
if the manufacturer [well known] sells a pump with a gauge thats good enough for me i know there's a tolerance in the tube and has to be for heat effect so a slight discrepancy on the gauge is nowt.
opinion based on experience & common dog.
i am off to scotland this weekend my boats pumped up no relief valves i wont touch it again until i get back pressure wise. all IMO of course

[Fenlander]

>>>if the manufacturer [well known] sells a pump with a gauge thats good enough for me i know

Agreed... it's what most of us rely on.

When I had my Honwave the OE gauge reading was near enough to the setting on my Bravo 12V pump to be considered the same. A couple of years ago I had three OE Zodiac gauges (from new to 18yrs old) which were also near enough identical in reading to the Bravo pump.

I think the chances of them all being similarly wrong was pretty remote so happy to assume they are all correct... as near as they need to be anyway.

[Pikey Dave]

Originally Posted by jeffstevens763@g

i must be getting old
my boats for over 40 years have been a bit like a famous shampoo.
pump & go
if the manufacturer [well known] sells a pump with a gauge thats good enough for me i know there's a tolerance in the tube and has to be for heat effect so a slight discrepancy on the gauge is nowt.
opinion based on experience & common dog.
i am off to scotland this weekend my boats pumped up no relief valves i wont touch it again until i get back pressure wise. all IMO of course

+1 my pump has a max pressure of 2.5 psi, tubes are rated to 2.5 psi, tubes have over pressure valves. Pump them to the max & go. At full "pump" pressure, they feel right.

[Last Tango]

Originally Posted by LaurenceTaylor

I must be a failure. As an engineer my "gut feel" should be in the correct ballpark for tube pressure change due to temperature.

In a basic simplified calculation pressure should be proportional to temperature in Kelvin. If so a temp change from 10C (start of day) to 30 C (now sunny) is only 20K change from 283K to 303K. This is only 7% or so.


If you pump tubes to 2psi a 7% change is not an issue + or - . So why do tubes go over hard and soft if we don't adjust ????

This theory is only going to work on a fixed volume container.
The nature the tubes means the pressure is going to increase logarithmically
rather than linearly with the volume (and the pressure increase is a consequence of increase in volume) of air put in it. I'd also suspect that in direct sunlight the internal air temperature will reach a lot more than ambient particularly with a dark coloured boat.

[jeffstevens763@g]

if you google lorry tyers there's a formula for drivers for temp increase effect

[Last Tango]

Originally Posted by LaurenceTaylor

I must be a failure. As an engineer my "gut feel" should be in the correct ballpark for tube pressure change due to temperature. For some reason I don't get it. I have had to register so that one of you can help me out.


In a basic simplified calculation pressure should be proportional to temperature in Kelvin. If so a temp change from 10C (start of day) to 30 C (now sunny) is only 20K change from 283K to 303K. This is only 7% or so.


If you pump tubes to 2psi a 7% change is not an issue + or - . So why do tubes go over hard and soft if we don't adjust ????

Right.....been thinking about this. Theory is right but the figures are wrong.

The pressure in the boat isn't 2 PSI. It's 17 PSI. (1 ATM + 2 PSI) Giving a pressure difference of 2 PSI to external pressure.
7% of 17 PSI is 1.2 PSI pushing internal pressure to 18.2PSI. This gives a change in pressure difference from 2 PSI to 3.2PSI with the 20 degree temp change ........Significant?

[Last Tango]

Originally Posted by jeffstevens763@g

if you google lorry tyers there's a formula for drivers for temp increase effect

Yes, but you can't relate the two as the 60-100psi that lorry tyres use make atmospheric pressure largely irrelevant, whereas the low pressures we are dealing with mean you absolutely have to take it into account.

[jeffstevens763@g]

It was an explanation of Charles law, atmospheric pressures changes by millibars on a day to day and within the day basis if that worried you you would forever be getting the gauge out to check pressures then for some that poses another problem is the gauge correct.
If this was so crucial manufacturers would fit relief valves to all craft I bet you could pump up to twice the max pressure before they burst.
Just reading my manual 21 boats in the range from 2.2-4.7 m all have 0.25 bar 3.6 psi
Paragraph states max inflation pressure 3.6 psi depending on climate and operating conditions pressures may require monitoring during use. No other warning.
Next paragraph if boat used for 2-3 days tubes may lose pressure it is important to maintain operating pressure to ensure boat stiffness.
Only other warning is don't use compressed air for fear of over inflation.
Last it just mentions filling by balancing between chambers to avoid baffle damage or bulkheads as they call them

[Last Tango]

Originally Posted by jeffstevens763@g

It was an explanation of Charles law, atmospheric pressures changes by millibars on a day to day and within the day basis if that worried you you would forever be getting the gauge out to check pressures then for some that poses another problem is the gauge correct.

As you say the change from day to day in atmospheric pressure is totally insignificant and irrelevant......however it's absolute value is around 15 PSI and it is the effect of heating that, that creates the the biggest part of the increase in pressure in the tubes. The "extra" 2PSI that you've put in there with the pump to inflate the boat is pretty insignificant. But if you're going to use Charles law to do any calculations on this then the value you use can't be "2PSI" it has to be 17PSI because that's the actual pressure in the tube.

Using Lawrance's example the "gauge measured pressure", which is in effect the difference between the pressure inside and outside the tube, would have changed from 2PSI to 3.2PSI and that's not insignificant.

My point being, had that boat been inflated to 3 PSI to start with at 10C then it's pressure is likley to be over the maximum by the time the temperature reaches 30C. Probably almost 4.5PSI

[chipko]

New to ribs but have had more than a few sibs over the years and so fairly hot on tube pressures as found 90% of a sibs performance is in the last 10% of pressure.
Although not performance related on a rib, still like to keep tubes up to pressure as work on the assumption low pressure can seriously stress joints.
With that in mind spec'd prv's at build, check/top up to 0.2 bar religiously with battery operated pump with digital gauge (from sib days) before each use in bow to stern order to preserve baffles. Only takes 5 or so mins to top up.
All sounds a bit OCD compared to the touchy-feely approach but always surprised how hard 0.2 bar actually feels.
PS; Prv's probably leach a bit of air between use but a small price to pay for peace of mind.

[jeffstevens763@g]

Totally agree with your point and the pressure value change with temperature your scenario of a 20c temp change is at the extream a more extreme scenario is with a diving cylinder pressurised to 300 bar all I am saying is even without relief valves there's some leaway before maximum advised pressure is increased significantly to blow the toobs which is past test pressure. expeariance shows when they need letting down only once have I done it whist in storage.when I had my last rib with relief valves I only needed to pump it up once a year beginning of the season that told me the pressure difference + or - during the summer wasn't significant enough to effect the stiffness of the toobs

[simonl]

Originally Posted by Orwell boy

I may be living under a rock when it comes to Chinese ribs ,but having been in pressure measurement all my working life I do understand Chinese pressure gauges ,ha ha , you are all happy to fill your tubes using such devices ,ha ha ,let me explain .....
these 15 psi gauges you are using will have a build spec of probhably plus or minus 5% of full scale ( 15x1.05= 0.75 psi +or _ ) apply to target 2psi and you realise it could be 1.25psi or 2.75 psi or indeed anywhere in between (maybe even spot on)
the simple and 100% accurate way is to make a pressur loop out of a length of clear tube ....to make a 2psi capable loop you will need 6 metres of clear tube and a piece of wood 3metres long , attach the tube to the wood in a loop with two open ends at top, make provision to stand the wood upright ,mark the tube at 1.5 metres from the floor (tube loop) link your pipe from your rib valve but don't connect yet ,note the mark @1.5 metres will become your zero pressure reference , add a broken tape measure leading up from the reference point to the top of the wood ....... fill the tube from the top with a funnel till water is eqal in the u tube to the referece mark (add a bit of food colour if you wish to make it easier to see ) .........connect the rib valve maintaining the loops vertical position ,wont matter how long this pipe is
now the maths .....2psi =1406mm of water ,so if the non connected side of the loop rises 1.4metres (10cms short of the top) then 2 psi is spot on and every measurement in between is perfectly divisible and accurate , if you need to be a real anorak you could take a temp reading in the morning and see what a rise in temp does to the tubes over the day
hopefully for those who have 6 metres of clear tube a piece of wood and a tape measure kicking about this is a cheaper solution to an accurate result ,on the other hand for the rest to get a remotely sensible answer you will need a pressur gauge with a max scale 3 psi

I fell asleep half way through that! Then I woke up abruptly worrying how the temperature fluctuations in the water would impact the height of the reading, particularly the non-linear coefficient of expansion where density variations are exacerbated by the presence of contaminants. I then figured the parallax errors introduced against an uncalibrated tape measure (particularly a broken one) would be further compounded by changes in the radius of curvature of the miniscus introduced from changes in the internal diameter of the tube. I'm quite short though - can I offset all these errors by standing on the tube to take the reading?

Good point about the inaccuracies of a bourdon gauge where the instructions are in Chinglish.

[Pikey Dave]

Originally Posted by simonl

I fell asleep half way through that! Then I woke up abruptly worrying how the temperature fluctuations in the water would impact the height of the reading, particularly the non-linear coefficient of expansion where density variations are exacerbated by the presence of contaminants. I then figured the parallax errors introduced against an uncalibrated tape measure (particularly a broken one) would be further compounded by changes in the radius of curvature of the miniscus introduced from changes in the internal diameter of the tube. I'm quite short though - can I offset all these errors by standing on the tube to take the reading?



Good point about the inaccuracies of a bourdon gauge where the instructions are in Chinglish.

You forgot to factor in the gravitational pull of the moon, if it's springs, the water gauge will read under[emoji848]

[Orwell boy]

Originally Posted by simonl

I'm quite short though - can I offset all these errors by standing on the tube to take the reading?

Good point about the inaccuracies of a bourdon gauge where the instructions are in Chinglish.

I personally usually stand on the wifes shoulders to eliminate reading errors