mmHg/atm/psi/Under Pressure.

General Discussion
21

page 563 not glycerin filled but you don’t really need it to be glycerin filled the glycerin is only there to prevent a shock load binding the needle if it hits the stop, and that is not easy to do with a small pump and a large volume like you vacuum bag.

Quote:
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McMasters-Carr item #3941K53 $9.37. Check it out, by the way the one you were looking at is in inches of Hg, and 30 inches of Hg is outerspace, and I have pumped for 24 hours to get down even close to that with more then just a little vacuum pump. Either one of these gages will work just fine believe me.

Did a product search at MM-C for Item # 3941K53 and no such item found. Closest guage (0->30 in vacuum, glycerin-filled) would appear to be Item # 384645K11 at a little over $17.00 (+shipping). As I recall, there was another vacuum guage for around $11 (+shipping), but was not glycerin-filled.

mtb

22
Quote:

page 563 not glycerin filled but you don’t really need it to be glycerin filled the glycerin is only there to prevent a shock load binding the needle if it hits the stop, and that is not easy to do with a small pump and a large volume like you vacuum bag.

The glycerin is nice to have if the vacuum pump seals get worn or the valves start sticking as then each stroke/revolution introduces a pressure pulse into the vacuum line. If the line between the intake port of the pump and the pressure gage is large, compared with the subsequent hose/pipe/tubing to the vacuum chamber, the vacuum meter needle can undergo large fluctuations (in some cases the guage needle rapidly oscillating between 0 and 30 inches of Hg). The glycerin helps to dampens the magnitude of these excursions so that one can get a better estimate of the averaage value. Alternatively coupling the vacuum guage to the vacuum line with a small diameter hose/orifice will also reduce the fluctuations that the guage experiences.

23

Thank you guys for your information and views.

MTB, thanks for your math on the pressure required, I couldnt sort the figures out.

Surfer_Dave, I planned to set the board and the rocker table onto a steel cradle, flipped into the tank and then fill the tank with water.

End view pic, but this was planned with a shallow tank, maybe up to 1 metre deep.

So the cradle that holds the board is part of the tanks structure, galvanised steel frame or similar.

Get it all prepared, totally in place and then add water.

Have the water rise, rather than forcing the board down under water.

I thought it might be a simpler technique to use after the bugs are worked out.

I did plan to have a retrieval/dunk system with compound pulleys but maybe thats not required except for emergencies. If at all.

And then just drain the water at the endand flip the cradle up.

Bagman, how about if the board is angled as the water rises, and then is levelled out, do you think it would need to be angled throughout the whole process?

Or just until the minimum or maximum pressure is reached?

I appreciate any critical analysis from you guys because it makes me see problems that I hadnt thought of before.

Water would be the easiest medium to use, any suggestions of a denser fluid that I could use so less depth is required ?

MTB, would you know whats the correlation or difference between density and specific gravity of a fluid ?

Thanks guys, I will certainly post pics when I get it sorted.

SF.

edit: NERD REFERENCE for liquids and on the RHS are comparison tables for everything.

     <a href="http://www.simetric.co.uk/si_water.htm" class="bb-url"><a href="http://www.simetric.co.uk/si_water.htm">http://www.simetric.co.uk/si_water.htm</a></a> and <a href="http://www.simetric.co.uk/si_liquids.htm" class="bb-url">http://www.simetric.co.uk/si_liquids.htm</a> 

                   Right click/create shortcut on desktop?/yes
24
Quote:

…… MTB, would you know whats the correlation or difference between density and specific gravity of a fluid ? ……

The density of a material is its weight divided by its volume, e.g.: lb/ft^3, gm/cubic-cm, etc.

The specific gravity of a material is its density divided by that of pure water–usually at 3.98 deg Celsius, where water has it’s maximum density (although sometimes densities at 60 deg. F are used instead).

Specific gravity is a dimensionless quantity (i.e. a number without any “units”, such as gm/cc associated with it). Hence in a sense, it is more invariant than density. By way of an example, if (hypothetically speaking) you were on the moon and you had one cc of pure water at 3.98 deg C at one atmosphere pressure enclosed in a weightless container, its density would be roughly 1/7 gm/cc instead of about 1 gm/cc (because of the moon’s reduced gravitational acceleration relative to that of earth). However, the specific gravity of the water would still be 1 (same as on earth).

Actually, the maximum density of pure water is slightly less than unity (see table below). Hence at 3.98 degrees F, the density of pure water is ~0.9999720 gm/cc while the specific gravity is exactly unity (1.0000000… ) .

deg-C…gm/cc

10…0.9997026

4…0.9999720

0…0.9998395

25

Hi Surffoils,

It is a nice thought, but if a air bubble gets trapped under pressure when the resin hardens,

A lower pressure will make the air want to expand and create a bump in the laminate and risk of delams.

Vacuum works the other way, if a trapped bubble gets under normal pressure it will try to suck air in so no bubble appears.

A combination of both could allow you to use a smaller under pressure for the same result.

Wouldn’t it be better to use all that iron and space to make a very large vaccuum tank?

Thoughts from Soul

26

MTB, thanks again for the science lesson, surfing is steeped in idiot design gurus high on a mountain with 2 tablets ( and an IM injection).

But the reality is, (after all the confusing bullshit), is that reality works, and I thank you for the help.

Soul, After all my pontification, I think my setup is possibly suited to an autoclave. Ive done autoclaving for specialised dental and orthopaedic implants for quite a while and an enclosed surfboard system might be the go. Maybe just use a pressurised gas ( instead of water) like vac-forming with temp to accelerate cure?

SF.

Nothing seems to be new here, we are just adapting techniques from other industries, but its a brave new world for am board building.

SF>

my computer is dying so I may not post for a month or so. Thanks guys.