Need advice on size of holes to drill for pressure for altimeter. Please advise.

[user 30299]

There’s nothing wrong with using metric measurements, but it’s just as easy to simply use decimal inch measurements.

Frankly, you’re way overthinking this. All altimeter manufacturers understand that high velocity air moving past the ports causes turbulence. They include filters or (in the olden days) a Mach delay to disregard data during that velocity region. The only time that the altimeter really needs to work accurately is when the rocket is flying its slowest vertical velocity.
Drilling too large of a hole will actually result in having more sensitivity to turbulence or Bernoulli effects during his velocities. The inability for the air within the av-bay to instantaneously flow out of the av-bay at highest velocity (which is also when ambient pressure is changing the fastest) is a good thing. It acts as a filter - sort of a mechanical accumulator, analogous to a capacitor). Having the av-bay completely open would make the altimeter very vulnerable to turbulence. Of course if you want the utmost accuracy at high velocity larger holes are necessary, but for our purposes that’s just not usually needed. If you do use larger holes understand that your flight data will look much more chaotic.
If you want something that works and has been used in thousands of flights by hundreds of flyers, just drill three holes spaced equally circumferentially, that are not at the same level as the barometric sensor, 3/16 works well in a four inch diameter rocket, 1/4 inch in a 7.5 inch diameter rocket. I use 5/32” for small rockets because that’s the other drill bit I haven’t broken or misplaced yet.

Time tested rules-of-thumb, time tested handy equations, time tested experience - works for me.

(It would make a great science experiment for some HS students interested in rocketry . . . .)

 

[gtg738w]

Steve said it well. Don’t overthink it. If the altimeter is for deployment and is in a dedicated bay, the hole size will just add a time delay that is proportional to the vertical velocity. Near apogee when the vertical velocity is near zero, it really doesn’t matter much. If you’re looking for velocity data from the barometer it will act like low pass filter and might underestimate it with small holes.

The biggest concern is if the the altimeter is in the main bay with the recovery gear. Things shifting around will have a much bigger impact on the internal pressure and bigger vent holes will minimize that. In that scenario you’re usually not depending on the altimeter for deployment so it’s safe to just fly it and look at the data. Adjust if needed.

 

[jderimig]

For apogee detection about any reasonable hole size works. It simply is not that critical. I have one drill for drilling vents. It's a 1/8". For 3 inch rockets I drill one. For 5 inch I drill 3.