RRC2L Altimeter Vent Holes

[Steven88]

I am trying to figure what size of vent holes I need in my Avbay. I have two RRC2L altimeters in my FiberMax. The Avbay diameter measures (inside) 3.75 and inside length is 7.83”. Yes, I, we, have read the instructions over and over and either there is a missing information or we are not following the formula correctly for finding vent holes sizes. It appears you need to find the single port diameter first even if you are going with multiple holes. I want to go with three holes. However, it doesn’t seem to give enough info for me to figure one vent hole first. The formula inside my Eggtimer manual for my TRS is much simplier. Is it alright to just use that formula or is each brand of altimeter different and needs different vent sizes?

 

[schneijt]

There’s no difference between various barometric altimeters when it comes to vent hole sizes—it’s just that there’s a margin for what’s acceptable for hole sizes, so charts for vent sizing will usually vary a bit. For an avionics bay that size, I’d probably drill three 3/16” or four 1/8” holes, depending on the number of fins.

 

[Steven88]

I purchased it pre built and it has one aprox 5/16th hole so not sure what to do. I could fill it and drill it out at 3/16 and add two more vent holes. That might be the best.

 

[Cameron Anderson]

I purchased it pre built and it has one aprox 5/16th hole so not sure what to do. I could fill it and drill it out at 3/16 and add two more vent holes. That might be the best.

You're way overthinking the whole process. You just need a few holes to let the air out during ascent. Grab a drill, get a few holes between 1/8" and 5/16" and you'll be fine.

 

[FMarvinS]

As stated by Joe, the vent hole size determination is independent of the altimeter used. Tommy, the creator of the Adept family of altimeters, provided a simple way to determine vent hole size:

"The general guideline for choosing port size is to use one 1/4 inch diameter vent hole (or
equivalent area, if multiple holes are used) per 100 cubic inches of volume in the altimeter
chamber. For instance, an eight-inch long four-inch diameter tube has a volume of about 100
cubic inches. Use one 1/4 inch port, or three or four 1/8 inch ports evenly spaced around the
tube. .... Try to keep hole sizes within -50% or +100% of the general guideline. Do not make the holes too small, and especially do not make them too large."

If you use Jim's RCL2 instructions then:
1. the bay volume = 3.14 x (3.75/2)squared = approx. 87 cubic inches
2. Then for volumes < 100 Single Port Diameter (inches) = Volume / 400 = 87/400= 0.2175 inches
3. Then Single Port Area = (Single Vent Diameter / 2) x (Single Vent Diameter / 2) x 3.14
which is (0.2175/2) squared x 3.14 approx = 0.037 sq inches
4. and finally, each Multi Port Diameter = 2 x SQRT ((Single Vent Area / # of holes) / 3.14)
or 2 x SQRT (0.037/3 holes/3.14) approx = 2 x 0.063 =0.126 inch diameter per vent hole
which is approx 1/8th inch.

So even with the approximations both Tommy's and Jim's methods agree at three 1/8th inch diameter vent holes. Joe's recommendation also works fine consistent with Tommy's comment that hole sizes be kept between -50 and + 100% of the general guideline.
I hope this helps.
Fred, L2
ICBM, Camden, S.C.
KG4YGP

 

[Banzai88]

Many just follow the guidelines here. Hard to argue with the success that's been reported.

https://www.vernk.com/AltimeterPortSizing.htm

 

[Bat-mite]

I use this chart for all of my rockets. Hasn't failed me yet.

 

[Conway Stevens]

https://www.vernk.com/AltimeterPortSizing.htm