Figuring altimeter port holes

[firemanup]

Bay is 4” diameter by 12” long giving me a volume of 150.72.

MW gives me a formula of:

Multi Port Diameter = 2 x SQRT ((Single Vent Area / # of holes) / 3.14)

What is SQRT, i’m assuming square root. Ok, square root of what? The volume? That would be 12.27.

What is “single vent area” the diameter of the correct single port size?

Volume is 150.72. SQRT of that is 12.27. Single port diameter is .5781”

2x12.27((.5781/3)/3.14)=1.5?? Im doing it wrong ...

What am i screwing up?

 

[firemanup]

Rocketrycalculator.com figures it at three 3/16” vent holes which coincides with another chart i found, will go with that.

What happens if you are a touch big??

 

[SecondRow]

150.72 inches cubed is the volume of your bay. You need a formula that gets you from the volume of the bay to the area needed for a single vent. For instance, I’ve seen that the total area should be about 1/1500 of the volume. There’s probably other guidance elsewhere. In this case, that would equate to an area for a single vent of 0.100 square inches. But you want multiple vents.

The formula you have will then tell you the diameter you need for each vent. For three holes, that would be 2xsqrt(0.100/3pi) = .21”. Which is fairly close to 3/16”.

 

[tsmith1315]

Square root of

Multi Port Diameter = 2 x SQRT ((Single Vent Area / # of holes) / 3.14)

What is SQRT, i’m assuming square root. Ok, square root of what?

The square root of this: ((Single Vent Area / # of holes) / 3.14)

So, first find A=πr² for the normal "one big hole" that you would use. That "A" is the Single Vent Area, and the formula uses that, along with the number of holes you *want* to use, and tells you what diameter the desired holes would need to be.

The size of the "one big hole" needed for your chamber volume isn't determined by this formula.

edit: SecondRow, looks like we overlapped posts.

 

[firemanup]

Thanks Tim,

Single vent area is? Its not the diameter of a single port hole for my volume is it?

 

[firemanup]

Nevermind now ive got it, thanks.

 

[Rocketjunkie]

For an avbay the size you describe (4" x 12") i use 4 1/4" holes. 1 is covered by my 'twist and tape' arming switch. Not all altimeters tolerate this due to lots of bounce.

 

[John Beans]

Keep in mind that sizes calculated are MINIMUM sizes that allow the air to escape at a rapid enough rate so that the pressure readings won't lag the ambient pressure by too long.

Bigger is better, as long as they don't interfere with deployment. There's no such thing as an "optimum size," just a too small size.

I've always thought a pretty exhaustive treatment of this was done by Vern Knowles:

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

 

[Handeman]

Keep in mind that sizes calculated are MINIMUM sizes that allow the air to escape at a rapid enough rate so that the pressure readings won't lag the ambient pressure by too long.

Bigger is better, as long as they don't interfere with deployment. There's no such thing as an "optimum size," just a too small size.

I've always thought a pretty exhaustive treatment of this was done by Vern Knowles:

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

+1 I completely agree with "There's no such thing as an "optimum size," just a too small size."

If you see an altimeter deployed rocket that seems to arch over past apogee before the charges go off, it's probably because the holes are too small.