Altimeter mounting location question

[Lugnut56]

I had always assumed that an altimeter needed to be mounted in a payload bay to protect it from the ejection charge, but I've seen several videos that show the altimeter just attached to the nose cone. My question is this--does the ejection charge cause the altimeter to give incorrect readings, and could it damage the altimeter?

 

[Speaknoevil]

Attached to the nosecone? In a nosecone compartment is more likely.

 

[Lugnut56]

Nope. Attached to the lug on the nosecone. I believe the video I saw was produced the guy behind BPS.

 

[Speaknoevil]

Typically you need to protect the altimeter whether inside an NC or another compartment.

 

[markjos]

Nope. Attached to the lug on the nosecone. I believe the video I saw was produced the guy behind BPS.

Might it have been a data-collecting altimeter, like a Jolly Logic Altimeter 1, 2, or 3? Those are in their own housings, and often just clipped to the nose cone lug. A FlightSketch Mini could also be tethered that way, in a little pouch. The brief ejection charge event can be filtered so that the data aren't adversely affected, and the pouch or housing can protect the electronics. Deployment altimeters would more commonly be mounted to a sled in an av-bay.

 

[Lugnut56]

I'm using an Estes altimeter in LPR. Even protected from shock, does the ejection charge PSI cause the altimeter to give false readings?

 

[markjos]

I'm using an Estes altimeter in LPR. Even protected from shock, does the ejection charge PSI cause the altimeter to give false readings?

No, because there's data leading up to the ejection charge, and data following it, so it can be filtered and smoothed to practically ignore it. That's not to say that the altimeter should be in the direct line of fire from the charge - but a normal amount of wadding and the parachute should be between them. And there should be a vent in the body tube, if so instructed.

 

[Rocketjunkie]

The Estes altimeter will work just hanging it from the nose cone. However, I never could get mine to work. They would work fine when tested in the motel or at home, but doing exactly the same thing on the field no luck. I ended up using PerfectFlite Fireflies. Over the long term, you may have issues with the ejection causing corrosion of the pressure sensor.

 

[Buckeye]

I'm using an Estes altimeter in LPR. Even protected from shock, does the ejection charge PSI cause the altimeter to give false readings?

Absolutely it will. The charge will give a false pressure reading at time of ejection, usually low altitude, but sometimes high altitude. If your altimeter only reports a single peak value, then it may be wrong. You can't filter anything unless you have a recording altimeter to visually inspect the data.

Shock/jerk can also throw off a barometric altimeter.

If good flight data is important to you, then secure the altimeter in a payload bay sealed from ejection gases.

 

[Lugnut56]

Thanks for all the replies. I've been wondering about this issue for awhile, but couldn't find a definitive answer.

 

[Wayco]

I will only comment on my experiences with Estes altimeters in LPR rockets, qualified with documented results. We have over two dozen small rockets, some with static ports, some without. I use these altimeters to adjust the Cd in Thrustcurve, making my motor delay estimates more accurate. After about three flights with good data, I will adjust the Cd for that rocket to match the reading I get with the estimated altitude (in Thrustcurve) for the motor I flew. Once that adjustment is done, almost all readings I get match up pretty well with the simmed altitude in Thrustcurve, regardless of the motor I fly, and my motor deployments are within a second of apogee. All of these flights are with the altimeter attached to a loop at the nosecone, and the little retainer is taped down with some electrical tape covering the top of the altimeter, but leaving the switch and display visible. I just don't trust the clip by itself. I have broken a few with bad landings, and had the tab break off, before I started adding the tape, but if it is set correctly before flight, and has a good battery, you can usually expect good results.
This has been working for me for about five years, and hundreds of LPR flights.