WARNING AND DISCLAIMER:  If you are underage, then consult with your parents or guardians before attempting any of this.  You are on your own - I'm not responsible for your actions or harm you may bring to others because of your actions.  Making the items described below  can result in injury or death to you or people in your vicinity. Some things mentioned here may be illegal to make in your city, county, state, or country so check the laws that apply to you before you attempt anything described here. These notes are not complete on purpose. If you are reading them and new to pyrotechnics, then you are making a mistake. Stop now - this page is not for you. Get a beginning book on fireworks (see Skylighter or American Fireworks News (very quick shipping)  for a start) and read up. You can't make any of this work without more information so read up or join a club or ask someone to help you.

Making a Pressure to Force Gauge

The Cheating Way


This article was inspired by Andy H's post on Passfire. Others have done this so this article is not original

Why do you want to make a Pressure to Force gauge, that is, rig up a hydraulic gauge so it can measure how hard you are pressing something?  Well.. mainly, you want to make consistent rockets and press BP and comets accurately.  You must use some sort of pressure measuring device. One way is to add a pressure gauge to a hydraulic cylinder. You can buy a Pressure-to-Force (PtoF) gauge but it will cost almost $100 plus shipping. This setup will cost about $39 and be every bit as good.

Ok.. Here is how to make a Pressure to Force Gauge by Cheating

10 ton Harbor Freight short bodied ram. Part number 
95979. This is setting in a tray lined with paper towels
and butcher (Kraft) paper on top. Even small amounts 
of hyd. fluid are messy -  set up your bench  in advance

Blunt the end of a metal screw and stick it in
the hole in the middle of the ram. It only has to 
tighten a little. You will use it to pull the ram 
up so it has some clearance.

Brace the ram and take off the quick disconnect
(save the quick disconnect for another project)

Pull the screw so the ram comes up out of the
body by 3/8" or so

Refill the cavity with fresh hydraulic fluid - make sure there
 is no air in the cavity. This may take a bit of squishing 
of the ram to expel the air.


Put some Teflon tape on a 3000 psi gauge and
screw it into the body. The gauge is available from
many sources including Harbor Freight, Surplus 
Center, and eBay. . It should be liquid filled with 
a bottom opening.  Note the hydraulic fluid on the
table - left over from squishing the ram too hard (see
the panel to the left). ;-}

Calibrate the gauge (PSI will be approximately
2.3 x the gauge reading if you don't want to calibrate
it on a press or other device. The big deal is to
be consistent so close will be good enough)

You now have a finished PtoF gauge - it took five
whole minutes to build and you can now press
rockets like a pro!

 

Using the PtoF Gauge

The short bodied ram has a lip on the bottom so the ram acts like a bigger jack - you can see that in the picture below. The diameter of the cylinder is a tad over 1.69 inches. I rounded it to 1.7 inches and then found the area - which was ~2.27 square inches. I rounded that to 2.3 to simplify calculations. The rounding of values will make the actual readings a little high but the gauge accuracy will shadow slight differences anyway. 

Here is a dissected ram:

Example calculations for typical rocket sizes 

Let's say you are using a 3/4" ram and the readout on the gauge is 1000. That means the actual pounds per square inch is 2300 and because the 3/4" ram is only .44 square inches, the actual pressure is 2300/.44 or about 5227 on the composition. That is usually good enough for most rocket mixes but, on this 3/4" example, you might want to go to 7000 (1300+ on the gauge)  for whistle - especially if you are going to store them for any length of time.

The following calculations are approximate but close enough to give you consistency.

 


 

ram=3/8 (0.375) area = 0.1104

217 on the gauge = ~4527 on comp
326 on the gauge = ~6790 on comp
434 on the gauge = ~9054 on comp

ram=1/2 (0.500) area = 0.1964

326 on the gauge = ~3819 on comp
434 on the gauge = ~5092 on comp
543 on the gauge = ~6366 on comp
652 on the gauge = ~7639 on comp
760 on the gauge = ~8912 on comp

ram=5/8 (0.625) area = 0.3068

434 on the gauge = ~3259 on comp
543 on the gauge = ~4074 on comp
652 on the gauge = ~4889 on comp
760 on the gauge = ~5704 on comp
869 on the gauge = ~6518 on comp
1086 on the gauge = ~8150 on the comp
1304 on the gauge = ~9778 on comp

ram=3/4 (0.750) area = 0.4418

652 on the gauge = ~3395 on comp
760 on the gauge = ~3961 on comp
869 on the gauge = ~4527 on comp
1304 on the gauge = ~6790 on comp
1739 on the gauge = ~9054 on comp

ram=7/8 (0.875) area = 0.6013

869 on the gauge = ~3326 on comp
1304 on the gauge = ~4989 on comp
1739 on the gauge = ~6652 on comp
2173 on the gauge = ~8315 on comp
2608 on the gauge = ~9978 on comp

ram=1 (1.000) area = 0.7854

1304 on the gauge = ~3819 on comp
1739 on the gauge = ~5092 on comp
2173 on the gauge = ~6366 on comp
2608 on the gauge = ~7639 on comp
3043 on the gauge = ~8912 on comp (recommend 5000 psi gauge)

ram=1 1/8 (1.125) area = 0.9940

1304 on the gauge = ~3018 on comp
1739 on the gauge = ~4024 on comp
2173 on the gauge = ~5030 on comp
2608 on the gauge = ~6036 on comp
3043 on the gauge = ~7042 on comp (recommend 5000 psi gauge)
3479 on the gauge = ~7952 on comp (needs 5000 psi gauge)

ram=1 1/4 (1.250) area = 1.2272 (5000 psi gauge needed)

1739 on the gauge = ~3259 on comp
2173 on the gauge = ~4074 on comp
2608 on the gauge = ~4889 on comp
3043 on the gauge = ~5704 on comp (recommend 5000 psi gauge)
3479 on the gauge = ~6557 on comp (needs 5000 psi gauge)
3913 on the gauge = ~7334 on comp ( needs 5000 psi gauge)
4347 on the gauge = ~ 8149 on comp (needs 5000 psi gauge)

ram=1 1/2 (1.5) area = 1.767 (10000 psi gauge recommended)

3043 on the gauge = ~3954 on comp (recommend 5000 psi gauge)
3479 on the gauge = ~4527 on comp (needs 5000 psi gauge)
3913 on the gauge = ~5093 on comp (needs 5000 psi gauge)
4347 on the gauge = ~5659 on comp (needs 5000 psi gauge)
4782 on the gauge = ~6225 on the comp (needs 5000 psi gauge)
5217 on the gauge = ~6791 on the comp ( needs >=6000 gauge)
5652 on the gauge = ~7357 on the comp (needs >= 6000 gauge)
6086 on the gauge = ~7923 on the comp (needs >=7000 gauge)