Compressed Air Pressure Meter

Well, I want to make an automated compressed air rocket launcher… To do that, I need to know what the current pressure is in the rocket… Basically I need a compressed air pressure sensor that I can use with arduino. It needs to thread into a standard sized thread; 1/4", 3/8", 1/2" or soemthign close… Can someone please link me to a sensor?

Also, any recommendations for solenoids? 1/2" would be best, and 12 volts is highly prefered…

What pressure range?

0-150 would be good, I think. I’m not sure what pressure I would ever be using, and most compressors only go to like 125…

You’re thinking of using a compressor?
Are you aware of the potential dangers of using compressed air?

Well I built a 2 liter (plastic coke) bottle launcher in the 80s from a mag article I had come across. Those plastic coke bottles will take a lot of pressure but I never got high enough to burst one as I put a 100 psi pressure relief valve on my setup. The tricky part is of course the pressure sealing device you build to fit into the neck of the bottle, that will allow pressure to enter the bottle but then has a quick release to allow fast discharge for launching.

There was no electronics used in my launcher. I used a rubber bottle stopper sealer that used a lever to release or tighten from inside the bottle’s threaded neck. I took quite awhile to find the right modifications to allow a 1/4" filling tube to be fitted inside and through the stopper and still allow for compressing and releasing the stopper. I then had a one way check valve between the stopper and the foot pump with a 100psi relief valve T-ed between those two parts. Just pumped away until I heard the relief valve start to release and it was ready for launch. If I didn’t pull on the release string in a min or so the bottle did have a tendency to gradually crawl off the stopper and self-release! Again the stopper mechanism was the most difficult part to get right.

I would fill using a foot bicycle pump up to around 90-100 psi and then release the stopper with a pull string. The sound it would make on release was great, people would come out of their houses to see what the hell the sound came from. The bottle, having somewhat poor aerodynamic characteristics, would only go up about 200 ft max and with such a light bottle any wind would take it pretty far from launch site. However the acceleration and sound was fantastic. Once I did one launch. any avalible area kids would be so excited that they would supply the foot action for pumping air and retrieving the bottle for all future launches in a afternoon! The acceleration was so fast that often the glued paper coke label would be left at the launcher!

Anyway it was a real fun project that would hold a crowds attention for a long time. Never any safety problems, however I always tried to make sure no kid was standing over the bottle when filling or launching as I’m sure if launched into a peering face it could cause injury.

Tell us more about your launcher ideas.

Lefty

Well, I’m in 7th grade… As my 5th preiod class, I have “Mission Space”… We launch 2 liter bottle rockets and try to recover the egg inside without it cracking…

The launcher is made of 2 hinges with notches carved in… You set the rocket on a little plastic stopper thing, put the hinges over the lip of the bottle. You use a little bracket with a chain attached to hold the hinges together… You pull the chain, and rocket to WHIZ!

I want to make one, btu have it timed, pressure meatured, ect.

Such items are somewhat standard, but tend to be a lot more expensive than you’d think (you have your pressure sensor, and then you have the carefully designed housing designed to contain at least 5x overpressure, and you have some electronics that conditions the bare signal from the sensor to convenient voltages, with temperature correction and stuff.)
OMEGA is one popular manufacturer with a wide range of products. This one looks like it would work:
http://www.omega.com/ppt/pptsc.asp?ref=PX209_PX219

($200 !) A nice request from a teacher on a school letterhead might get you a free sample… You can also check on eBay; all sorts of “pressure sensor” and “pressure transducer” there. You want one with a 0-5V output.

Mere visual pressure gauges are mechanical and much cheaper.

100psi is pretty high for 2l rockets. I was surprised just how low a pressure makes for impressive altitudes, using a mechanical gauge and foot-operated bike pump…

An interesting idea to look into is the pressure sensors you can now get for monitoring automotive tires. Some of those are available as add-on kits. Or you could use/convert a digital tire pressure gauge…

Yeah… I would like to divide that price by about 10… There is no way in heck that my mom will get that for me…

See if your teacher will request a sample. It can’t hurt, and sometimes companies are very generous toward educational institutions…

Also, this looks OK: http://cgi.ebay.com/TI-PRESSURE-SENSOR-PPE-PA-GF30-0-100-PSI-PIGTAIL-/290477233182?pt=LH_DefaultDomain_0&hash=item43a1cad41e

That is good, but 125 or 150 would be better… 125 is like the highest it will ever go, so I would like to have some extra room for if there is a fualt, it doesn’t break.

If you check your local Wal-Mart or PepCheckerZone, you can probably find a digital tire gauge with a sensor that you can cannibalize. Although you might need to hit a truck supply house to get one with a high enough maximum.

Jeremy1998

I think that this little $15 beauty from Harbor Freight might be the way to go:

http://www.harborfreight.com/air-tools/air-accessories/digital-air-flow-regulator-98426.html

While it is a digital regulator, I think that it will display the actual pressure delivered rather than the pressure set. You could crank it up to your maximum safe working pressure by setting it with a high-pressure shop source then install it into your model rocket system and just watch the gauge.

Or you could Arduinoize it and hack into the display to get digital signals and relay them to your Arduino.

Ironically, I was just looking at that… The problem is, I think that it measures speed (40MPH).

Brain blast! (Jimmy Nutron!!!) How do air compressers do it? If I get oen of those controller boxes, would I be able to hackify it?

Knock Knock Knock! Anybody out there?

I need a range of 0 - 125 Psi for under $50. That $200 one would be nice, but there is just no way…

Jeremy1998,

What gave you the idea that the Harbor Freight Digital Air Pressure Gauge measured “speed”?

It is an air pressure gauge, not an airspeed gauge.

Take a look at the manual:
http://manuals.harborfreight.com/manuals/98000-98999/98426.pdf

From the looks of it . . . and I have physically examined it since I was contemplating its suitability as a hackable device . . . it should be as easy as any other small electronic device to hack.

Open up the box and try to pick of the appropriate signal going to the LCD then decode it to your hearts content.

I was also looking into this for a system I’m (kinda) working on for controlling animatronics for a halloween theme park I work at. The animatronics are basically steel frames with pneumatic cylinders on them to move hinged parts in certain ways. These hinged parts have custom made foam/fabric designs on them to look like monsters. One example is a warewolf we have where a cylinder lifts an arm connected to his neck causing him to stand up from a crouching position with another cylinder to raise and lower it’s head.

Each of these uses a solenoid valve which sends air to the cylinders, close the 12 or 24v circuit to the solenoid, he stands up, close the other circuit, his head moves up, open it, the head drops back down, etc. These units were professionally made, but their electronics leave something to be desired, hence the reason I’m here.

I already have them under my control, but what I’d like to do is be able to see how much pressure is in the line before I trigger each one. I figure that if I know how much pressure it’s going to take to run the whole scene, I can delay the trigger until while the air compressor is charging up and activate the trigger only when there’s enough available pressure to run the full cycle. The standard way to do this is with an air transducer, but the cheapest one I could find is over $100 so that option’s out.

I could use a pressure switch that would close at a certain pressure, they’re cheap enough, but then I’d have to find the right one for each unit, we currently have 20 of these and are planning on getting more of varying sizes, so I would far prefer to be able to detect a whole range and set the minimum pressure digitally within the Arduino.

Hacking a digital tire pressure gauge seems like a good way to go, I’m gonna have to look into that more when I get a chance to buy one and rip it apart. I’m curious as to the range that these sensors can handle. I’d probably somehow remove the sensor, and hopefully remove the tire stem coupling. Then I’m thinking I would get a 1/2" T connector where one side would be the female quick-connect to their air line and the other side would have the screw thread into the solenoid valve block. The T itself would contain the sensor, probably glued into place so that it constantly monitors the pressure inside the T while the air being used would flow right through it. Having it after the regulator would give me the pressure for each animatronic.

Another thought, albeit half-assed, would be to take a “dry” (as in non-liquid-filled) pressure gauge apart and connect a potentiometer to the needle shaft. This would give me precisely the data I need, but then it comes down to how hard it would be to build this setup for each animatronic.

how about connecting the ‘needle’ shaft of a $3 pressure gauge with the shaft of a $1 pot ?

Excellent! ::slight_smile: Just one thing: it won’t work…