Altimeter Testing

[KL2IE]

Good Evening All,

I've been trying to test altimeters in preparation for a dual deployment flight. In my most recent test, I had my altimeter set for drogue deployment at apogee + 0 seconds and main chute deployment at 800 feet. During my testing while pulling vacuum I am able to achieve better than 2000 feet as indicated by my Jolly Logic Altimeter One, however there is no ignition event until about 300 feet, and then both my drogue and main igniters go off at the same time.

My altimeters are Missile Works RRC2+s. I have had successful tests before, but now the result I just described above seems to be happening with 3 of 4 of my altimeters including one of the ones I just bought. I haven't tested the 4th one because I'm not sure if I've done something to mess up the other three. Not sure what, but how sensitive are these things to ESD?

Admittedly, my test setup is pretty rudimentary; my vacuum chamber is made from (what is supposed to be) an airtight food container, and I'm pulling vacuum with an automotive brake bleeder.

I appreciate any advice that is offered,

Shawn C.

 

[user 30299]

How are you controlling the release of the vacuum? Are you able to slowly bleed it and watch the altitude change on the Jolly Logic ? Or are you just letting it go wide open? Have any of the RRC2's flown in a rocket?

 

[David Schwantz]

Mason jar and syringe is what I use. As long as the Xmas tree lights light up, they're "good to go".

 

[Bat-mite]

If you are having the problem on three units, then the problem is definitely with the testing technique, not the units.

The keys are to make sure you are really pulling a vacuum, pull the vacuum rapidly to indicate ascent under thrust, release the vacuum more slowly to simulate descent under drogue.

 

[jderimig]

You have a leak while you are pumping out your chamber.

 

[Buckeye]

I've been trying to test altimeters in preparation for a dual deployment flight. In my most recent test, I had my altimeter set for drogue deployment at apogee + 0 seconds and main chute deployment at 800 feet. During my testing while pulling vacuum I am able to achieve better than 2000 feet as indicated by my Jolly Logic Altimeter One, however there is no ignition event until about 300 feet, and then both my drogue and main igniters go off at the same time.

These ground vacuum tests are not very precise. You are probably pulling the vacuum too fast. I had the same observations:

https://www.rocketryforum.com/threads/weird-slcf-deployments-during-vacuum-test.165455/post-2120025

 

[lowga]

Agree 100%. Testing multiple altimeters there is no chance that all three are bad. It's the testing methodology that is flawed. I ground test outdoors in the rocket, using a shop vac to pull a vacuum on the vent hole. This also allows me to gauge the correct amount of black powder to use in order to get the laundry reliably outside the airframe.

Even this method isn't foolproof. Sometimes, the charges will fail to deploy, always because the vacuum is leaking somewhere. For instance, I have learned that I have to have a motor in place with the retention on for some rockets, otherwise it's hard to draw an adequate vacuum inside the rocket.

You'll figure it out with a little trial and error.

 

[KL2IE]

How are you controlling the release of the vacuum? Are you able to slowly bleed it and watch the altitude change on the Jolly Logic ? Or are you just letting it go wide open? Have any of the RRC2's flown in a rocket?

I have, but only for single deployment.

 

[KL2IE]

How are you controlling the release of the vacuum? Are you able to slowly bleed it and watch the altitude change on the Jolly Logic ? Or are you just letting it go wide open? Have any of the RRC2's flown in a rocket?

There is a lever on the brake bleeder that I'm using vary the "descent" rate. I'm using a JL Altimeter 1 in Realtime mode to observe the affects of my pulling and releasing vacuum.

 

[cerving]

If your altimeter can give you a graph, or preferably a CSV or similar file with the actual data, take a look at the "up" and "down" slopes. You'll probably see that they are very steep, it's not easy to get a "real" flight out of a vacuum test. I have people email me about this all the time. A vacuum test is at best a qualitative test... you can use it to check that the baro sensor is working, and the outputs are working, but don't expect to get the same results as a real flight. If both channels appear to be firing simultaneously, especially on the way "down", it's because the pressure is equalizing to ambient so fast that the time difference between the two outputs is imperceptible.

 

[jderimig]

If all 3 altimeters do the same thing in your test then they are all good. Fly it.

 

[Zman1961]

Mason jar and syringe is what I use. As long as the Xmas tree lights light up, they're "good to go".

Dave, can you a few more details on exactly how you set this up? Thanks!

 

[David Schwantz]

I soldered a brass nipple to the lid of a large Mason jar. Then a piece of silicon fuel tubing to the syringe. Put all in the jar, screw lid on, and then just pull the plunger on the syringe. Release plunger and the Xmas tree lights will light up. If not then something wrong with setup. As long as the lights come on, alt is sending voltage to the outputs for the matches to fire.

 

[cerving]

You might try putting a little silicone grease on the o-ring for the plunger, they can "chatter" when you move the plunger and the baro sensor will pick up the vibrations. They are VERY sensitive...

And again, if it arms when you gently pull back the plunger and hold it for a few seconds, and the channels fire when you gently push it back, then you're good to go.

 

[Bat-mite]

Adept Rocketry, when they were around, sold an altimeter testing kit. Plastic jar with sealing lid, wiring harness built in, rubber hose and syringe. I bought one and still use it for every new altimeter I get. Too bad they went out of business.

Someone with a rocketry business should consider selling them.

 

[John Beans]

I'll pitch in my two cents here, for what it's worth.

People generally over-engineer altitude test chambers. We certainly did when we started. For instance, we started with a very high quality vacuum chamber that can go to super low pressures and has a certified pressure sensor interfaced to a high speed computer DAQ for recording. But that's massive overkill for ground testing a deployment setup up. We rarely use it any more.

What I'd recommend it to get a clear plastic container with a screw on lid (think big snack container from Costco). Drill a 1/8" hole in one corner opposite the label (so that when the container is on a table the label is on the bottom of the container, not blocking your view).

1. Put your AltimeterOne in Realtime mode
2. Add your device to test
3. Screw the lid on
4. Suck the air out of the vent hole (can easily do 2,000 feet)
5. Quickly put your finger over the hole
6. Watch AltimeterOne and adjust pressure on the hole to achieve desired descent rate (20-100 fps)
7. Either notice when deployment occurs, or take video

It's a great setup because it's cheap, reliable, really convenient and actually quite accurate.

 

[aerostadt]

I ended up using the syringe altitude chamber that ManixFan describes. Worked really good for me.
https://www.rocketryforum.com/threads/weird-slcf-deployments-during-vacuum-test.165455/#post-2213026

 

[RocketRev]

I used a gallon pickle jar with a brass valve (not just a brass nipple) from a fish tank air bubbler soldered to the lid, to connect to a vacuum/pressure pump that I got at a scientific equipment surplus store. It had a pressure gauge as well as a vacuum gauge, but I never had any means of validating their readings. I used the heavier duty fish tank air-hose-tubing rather than the silicon because it is almost impossible to collapse, unlike the silicon air hose. I used small 12v light bulbs on the altimeter outputs to indicate ejection charge firing. I also epoxied "feet" on the pickle jar so that it didn't roll around on my bench.

When I turned on the vacuum pump it quickly took the altimeter up over 20K in altitude, measured with all sorts of different altimeters over the last 25+ years. If I let it go for as "high" as it would go, I've gotten several altimeters to read over 50K. Some altimeters won't regardless of how much vacuum to which they are exposed. One of my early altimeter purchases was an Adept Alts-2-50K designed for reliable readings up to 50K AGL. So I did question the over 50K readings but there was no doubt that they were higher than the 49K readings I was getting regularly.

I used the fish tank brass valve because it allowed me to open up the chamber very slowly which made reading/timing deployment charges much easier.

The larger volume of air to be removed from the pickle jar makes for a slower descent rate when I slowly cracked opened that little bitty brass valve.

The brass valve also allows one to make the ascent slower by leaving it a crack open during the vacuum cycle and then slowly closing it. Much easier to control than a finger over a hole in the jar.

You may have noticed above that I said "used" in the past tense. It worked great until my pickle jar got broken in my last move. I haven't replaced it yet but I ought to one of these days.

Oh yeah, one last thing. The metal lid on the pickle jar would always make a clicking noise when it got the jar up to a good vacuum. Pretty much the same sound that the 1/2 gallon Mason jars of tomatoes my mother canned for years after they came out of the boiling bath and started to "cool down." Each one made that clicking sound when it cooled enough to draw a vacuum to seal the jar of tomatoes, or pickles, or beets, or any number of other things my mom canned back in the day. When the jar's lid made that click you knew that it was successfully sealed.

Without some kind of a slow release valve, you won't get good measurements. And while I'm sure the manufacturer recommended the syringe vacuum pump, I have my doubts about being able to reliably draw enough vacuum on a Mason jar to validate any sort of real testing. But maybe that's just my mild OCD rearing its ugly little head.

As others have said before me, your results may vary.

Brad, the "Rocket Rev.," Wilson

 

[AlexBruccoleri]

Another thought. I modified an old rocket with new payload bay and used the rocket as a test rocket for altimeters. This is a nice way to do end-to-end testing, and its FUN.

 

[Kelly]

1. Suck the air out of the vent hole (can easily do 2,000 feet)

And, to be clear, what do you suggest we suck the air out with? I've done something similar with plastic hose and using my mouth to suck, and been able to get to about the 2000' pressure you mention. It doesn't require a pump to do this test, which I think is what you are also saying.

 

[David Schwantz]

I think John is saying use your mouth and then plug the hole with your finger.

 

[John Beans]

I think John is saying use your mouth and then plug the hole with your finger.

Correct.
Super easy and super fast.

 

[waltr]

And, to be clear, what do you suggest we suck the air out with? I've done something similar with plastic hose and using my mouth to suck, and been able to get to about the 2000' pressure you mention. It doesn't require a pump to do this test, which I think is what you are also saying.

I have also done this by placing the tube directly over the Baro sensor. Works fine fine and is quick and easy for a basic test.

Also use a jar with a tiny hole in the lid. Then a vacuum cleaner to draw a vacuum. To release I pull the vacuum cleaner's nozzle a little bit away so pressure increase is slower. Have been able to get pretty close to a typical flight profile curve on the data (Eggtimer Quantum) with seperation of the Drogue and Main Lamps lighting.