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BigRIJoe 06-09-2018 03:51 PM

Altimeter 2 Vent Holes
I'm mounting a Jolly Logic Altimeter 2 in a BT-30 payload section. How many and what size vent holes should the payload section have?

BEC 06-09-2018 04:16 PM

How long is the payload section?

That said - unless it's huge - all you need are three or four large pinholes.

BigRIJoe 06-11-2018 04:21 PM

Well it's a 5 inch section of BT-30. And aren't pinholes, by nature, "small"?

BEC 06-11-2018 07:48 PM

Well, yes. But I’m thinking use a fairly large T-pin rather than a small sewing pin. In other words, something on the order of 1/32 of an inch in diameter or perhaps a little smaller. As a long time aeromodeller I have lots of big T-pins and I use one like that for this purpose.

Basically what you are trying to do is provide enough of a leak path for air inside the payload compartment to get out and allow the pressure to equalize at apogee before the model has fallen back so much as to mess up your reading unacceptably. If you do the math and assume a ballistic path (and neglect drag), you fall back 16 feet in the first second after reaching apogee (1/2at**2) where a in this case is the acceleration due to gravity. In the first half second it’s 4 feet.

So you want the payload compartment to “leak down” pretty quickly.

astronwolf 06-11-2018 09:10 PM

You need only make a couple "small" pin holes, at most. You may not need any pin holes at all.

Newbomb Turk 06-27-2018 08:46 PM

This is all very interesting. I've never used an altimeter before, but would like to try one.

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