Miniature Pressure Port

[jonnycowboy]

Hi all,
I'm looking for a threadable pressure port, the smallest possible. The pressure sensing and "working" lines have very small diameters (~20 thou ").
I need the sense line to be 90 degrees to the "working" line I'm wondering if I have to use something fairly large or is there something available in this size range?

To clarify what I mean by "threadable", the ideal would be to have a threaded hole on the part in which the pressure line is drilled, and then screw in an insert to a more standard sense line size (80 thou " or ~2mm).

Thanks

 

[hacksaw]

plenty of methods to make your pressure tap, but more importantly you need to define what pressure you are actually trying to measure and at what mach no.

 

[jonnycowboy]

The pressure will be low (total and dynamic pressure measurements), my sensors are capable of 0-30psia max.
I have found some threadable ports but they have M5 threads, that's too big. I was hoping for M3 (or similar imperial size) with 0.025" holes.

 

[jonnycowboy]

I found something:

http://www.beswick.com/products/product_details.php?pid=8

The flow-through size is a bit bigger than I hoped but I think it'll be OK.

 

[hacksaw]

that is not going to give you a valid pressure measurement unless your velocity is really low

is this for process piping

 

[jonnycowboy]

No it's for AOA probe use.

NACA report 1364 shows that small pressure ports are the most effective (0.020"-0.030" generally). That's tiny, I know and unfortunately I won't be able to do it on my lathe, I'll need to use EDM drilling.

Unless you have experience otherwise, that using a small drill hole (1/16) would be acceptable?

 

[IRstuff]

??? It's not my field, but I don't get that conclusion from that report. It seems to be talking about having smaller orifices on a relatively larger tube. It mentions 25mil and 32mil as being better than 52mil. It also seems to stress the importance of having good profiling of the orifice:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930092348_1993092348.pdf

TTFN
faq731-376

 

[hacksaw]

there are smaller drills for manual drilling but that is all it would be and not a proper pressure port

you might consider a tube insert rather than a drilled port

how do you plan to test your custom AOA/pitot measurement?

 

[jonnycowboy]

I never thought of using a tube insert, that's a really good idea. I guess I would bond it in? Or simply heat the big tube, chill the small one and insert it with a small interference fit?

Testing: intially calibrated in freestream (ie: outside) and then in a wind tunnel.

 

[hacksaw]

sounds like you are just making a pitot rather than angle of attack probe, regardless, there are a number of methods to secure the tube depending on the temp range

for both measurements the edge effects of the port are important

 

[jonnycowboy]

I am doing both pitot and AOA in a single probe. AOA will be measuring the pressure difference between the bottom and the top probes (known as a smart probe).
That's good information for the edge effects, I was just going to bevel the edges of the holes but I'll take a look at detailed information on this point.
For the temp range, the probe will be unheated (limited to "sunny day" operation). so probably -20C to +55C (DO-160 CAT A2 for temp)

 

[IRstuff]

I still don't get why you need a separate tube. The paper seems to suggest that a straight, unbeveled hole is all that's needed.

TTFN
faq731-376

 

[hacksaw]

just buy an AOA probe, you are getting into a can of worms with a beveled port , but given that you were going to use a hose bib initially, maybe thats all you really need

sunny days -20C to 55C, famous last words, that's not enough, just get a heated probe that has been design and tested. ice crystals are very small but they can really make a mess

 

[pmrobert]

Any chance a modified carburetor jet might be of value? Small hole, threaded insert, etc...

 

[jonnycowboy]

pmrobert, another great idea, thanks!!