Vacuum Chamber on the Cheap

[ecarson]

It's amazing what one finds in a basement after living in the same house for over 25 years. A major component of this system I forgot I had. I was converting a car from R12 to R134 refrigerant quite a couple of decades back, and in a bin with my regular motor driven vacuum pump, I found a red box called "Air-Vac" that I had given up on. At the time, I just did not have big enough a compressor to use it for A/C use. Well, I put in a 60 gallon 7 hp compressor perhaps 15 years ago, but by that time I had forgotten about the air-vac.

I wanted to test my EggTimer under simulated barometric conditions, just to make sure all the system worked OK.

I started with a 1 qt. Mason jar, found under the kitchen cabinets. Drilled in a couple of 7/16" holes, and threaded in a couple of 1/4" nylon barbs. A copious amount of hot glue sealed them up solid to the lid. I remembered I had a large engine vacuum gauge (I do probably 95% of my own automobile maintenance and repair), that would be peachy for measuring chamber pressure.

A quick run, and I found that even though the air-vac would evacuate the Mason jar very quickly, as soon as the air was shut off, air leaked back into the Mason jar rapidly. So, another hunt through the bins, and in my lawn mower stuff, I found a 1/4" fuel shut-off valve to put between the Mason jar vacuum port, and the air-vac pump.

Amazingly, the fuel valve holds vacuum very well. So well in fact, it held 25" of vacuum overnight without moving the needle at all. Today, I put the EggTimer in the jar, and after a few runs to get the hang of it, I got an interesting chart from the data download. A couple of things I found. One, somehow my red LED failed on the EggTimer after a couple of runs. I have no idea why, but it was replaced with one I had from an assortment, and good to go again. I found a couple more issues with my soldering during the testing, and again those were quickly corrected. I think stuffing the board, wires, batteries, and switches in and out of the jar revealed these issues, so that is a good thing to find on the ground, and not while bird is flying.

On the attached chart, point "A" is where I shut off the red valve to the jar. This holds the altitude for a second. Then I shut off the air from the compressor, and then at point "B" I crack open the red valve ever so slightly to let the air back in the jar. Note the curve of the air returning into the chamber, reflects a curve on the altitude measurement. I had the EggTimer programmed to trigger CHB (drogue) ejection at apogee, and then Main at 500 feet. Both circuits worked OK triggering the lights, and data reflected this.

I think you can still buy these Air-Vac pumps for $20 or so, and the red fuel shutoff valve for $4 or $5. I don't know about the gauge, but I have had that for 30 years now. The big item though is having an air compressor that has the pressure and volume. I will try the system on my small air compressor next, just to see if it can do the job. All this cost me just a couple of dollars for the two nylon 1/4" barbs from Lowes. The rest of the stuff I had in my collection of odds and ends.

 

[dhbarr]

View attachment 323287

That's a nice air pressure curve. Would make a decent fin planform as well. Hrmm, need a thrust curve to match it

 

[Voyager1]

That's a very useful setup. I started acquiring a few bits & pieces for my own vacuum chamber which is still a work-in-progress. The pump I purchased from Sparkfun for about $15. My aim is to have it in a similar glass chamber and feed the data from a reference sensor (and the altimeter if it has altitude telemetry) out to an Arduino-based display/control module. I think this pump is only good to about 15,000', though.

https://www.sparkfun.com/products/10398

 

[Coop]

Interesting! I fabricated a vacuum chamber for altimeter testing --merely verifying operation with christmas tree lights in place of ejection charges-- by stretching a nitrile glove over the top of a plastic cup, securing with a rubber band, and pulling the fingers until I saw the apogee light go, then slowly letting go until the main light went. For me, that's easy and can be fabricated just about anywhere. I may or may not (mostly may) have done this at Medic 4 on my last shift there as I was prepping my Phoenix for flight on Sunday.

Yours is nicer in that you can actually measure the vacuum at a given time. Well done!

Later!

--Coop

 

[cerving]

I'm still looking for a decent way to simulate a "real" flight. The probem with vacuum pumps is that they tend to suck out the air very quickly, so you get a very sharp "ascent", which is great for testing mach-immunity but not so good for some other things (like getting a good apogee reading). They do work fine for testing your charges, though. The syringe type pumps pull out the air slower, but it's hard to get a smooth and consistent pressure reduction with them.

 

[dhbarr]

Syringe on a lever arm would smoothe out the action quite a bit.

 

[Voyager1]

There is another thread about this from 2012

https://www.rocketryforum.com/showthread.php?34388-Portable-Vacuum-Chamber-for-Altimeter-Testing

 

[ecarson]

I'm still looking for a decent way to simulate a "real" flight. The probem with vacuum pumps is that they tend to suck out the air very quickly, so you get a very sharp "ascent", which is great for testing mach-immunity but not so good for some other things (like getting a good apogee reading). They do work fine for testing your charges, though. The syringe type pumps pull out the air slower, but it's hard to get a smooth and consistent pressure reduction with them.

Back when I was working in industry, we had some leak testing pressure decay units, that had to be precisely calibrated and certified. They all worked with a metering orifice. When I get a bit further on in this project, I may experiment with some drilled orifices to put in either the vacuum line, or gauge line. One orifice for the vacuum would slow down the evacuation, and perhaps two orifices in the gauge line. One for drogue descent, and the other for main descent. Just thinking (writing) out loud here.

 

[OverTheTop]

That's a very useful setup. I started acquiring a few bits & pieces for my own vacuum chamber which is still a work-in-progress. The pump I purchased from Sparkfun for about $15. My aim is to have it in a similar glass chamber and feed the data from a reference sensor (and the altimeter if it has altitude telemetry) out to an Arduino-based display/control module. I think this pump is only good to about 15,000', though.

https://www.sparkfun.com/products/10398

View attachment 323293

Sometimes you can get a better vacuum if you have two in series. This is done frequently in chemical analysis equipment. Works well enough if the seals are up to the task.

 

[Voyager1]

Sometimes you can get a better vacuum if you have two in series. This is done frequently in chemical analysis equipment. Works well enough if the seals are up to the task.

As it happens OTT, I have 2 of these pumps for the job.

Like Cris, I also wish to simulate a 'real' flight profile. These DC pumps appear to work happily down to about 2V (without load!), so I'm hoping to control the suction using motor control. Watch this space!