Altimeter two vent holes question.

I always like to use my Jolly Logic Altimeter Two as a back-up to my installed altimeter. The instructions say to make three evenly spaced 1/16" to 1/8" holes for proper venting. Does this "leakage" at ejection require a significant more ejection charge needed to make certain the chute deploys properly? My hunch is that it isn't significant, but still thought I'd ask. Thanks!!

I guess the speed and configuration could make a difference.
I have a universal AV bay I use on everything from 3' up to 9" diameter.
They always say 2 or more holes, mine has 1 large hole, about 0.75" diameter, never had an issue on any size rocket.

Thanks Dave...That's kinda what I figured. I just clip the Altimeter Two, to the nose cone and it always reads super close to what my Stratto logger reads.

I'm nowhere close to you guy's levels, but on my mid and low power stuff I just drill a 1/8" hole straight up from each fin. No ejection issues yet.

You do have to have (a) vent hole(s) in the compartment containing the altimeter otherwise it does not work properly.

You shouldn't need to have extremely large hole(s), the recommended (3) 1/16" to 1/8" holes is adequate for what it does and will not effect the ejection charge efficiency if you keep to the recommended hole size and number.

Having (3) or more smaller holes instead of (1) or (2) holes is recommended because of gust rejection. If the altimeter compartment has only one hole a wind gust can raise or lower the chamber pressure depending on direction. With (3) or more holes you get significant differential pressure mode rejection. All three ports measure the static pressure identically but each individual port measures the differential pressure differently. Since each of n port measures 1/n of a gust while all ports equally measure the static pressure, the gust induce pressure variations are minimize and there is a different time constant for common mode response versus differential mode responds. That's how the foam microphone windscreens work as each open cell is a very small fraction of the pressure sampling area.

Bob

My alt bay is an aluminum sleeve, 2.75" x 8.0".
It comes with 1- 0.75" hole, it is through there I can reach the 2 micro-slide switches for the 2 altimeters with a small screwdriver.
Works like a champ.

I had a similar question, so I asked John.

"You can clip AltimeterTwo to the shock cord, many people do. You need to make vent holes that won’t be blocked to let the air pressure into your fuselage. Usually three holes around the diameter of the tube, low enough that they are not blocked by the nosecone when it’s inserted, but high enough that they are not blocked by the chute. As long as the holes are about 1/8” or less in diameter, the chute deployment won’t be affected."

Hope that helps?
Kind regards,
–John

John Beans
President, Jolly Logic
[email protected]
Phone: 925-399-1510
Fax: 925-553-2627

V