Airspeed/Altimeter calibration

tonycondon

tonycondon

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Tony
For my senior project i need to calibrate the altimeter and airspeed indicator on my glider. Lots of discussion in chat so heres a picture of my completed manometer.
 

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Here is the first hack at calibration on the airspeed, im sure ill refine my methods and give it a few more tries in the next week or so. not too bad in the key range where i want to test.
 

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tonycondon said:
Here is the first hack at calibration on the airspeed, im sure ill refine my methods and give it a few more tries in the next week or so. not too bad in the key range where i want to test.
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What are the axes labels? Airspeed and altitude in thousands of feet for x and y, respectively?
 
nope, airpeed on the horizontal, and error on the vertical. all in mph
 
tonycondon said:
Here is the first hack at calibration on the airspeed, im sure ill refine my methods and give it a few more tries in the next week or so. not too bad in the key range where i want to test.
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This is the instrument error or the Static / Pitot system error?
 
Wow from 45 to 65 it looks ok, but there is quite a bit of error outside of that zone. Is that normal for Air speed indicators? I always thought that below 200 kts the pitot pressure/vs airspeed relationship was pretty much linear. What do you think is causing this behavior?
 
tonycondon said:
Here is the first hack at calibration on the airspeed, im sure ill refine my methods and give it a few more tries in the next week or so. not too bad in the key range where i want to test.
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I did a full-house article on airspeed calibration in the July '89 issue of Kitplanes if you can get your hands on a copy of that issue.

If not, the government publication AN-05-10-24 is the seminal definition of ASI calibration. There is a relatively simple equation that will translate inches of water to airspeed:

Airspeed = sqrt (inches of water * 1526.5)

I use a drop or two of red food coloring in order to be able to read the meniscus accurately; a drop of dishwashing soap will flatten out the meniscus a little bit. The ultimate California wine snob will use Gamay Beaujolais with the standard -1.2345% wine correction factor.
 
the calibration was done on the instrument only. it was removed from the glider.
 
i hooked the altimeter up to the test stand tonight and ran it up to 10,000 feet in 500 foot increments and then back down to 1000 feet. averaged out the two values at each 500 foot mark and plotted the error. a fair bit of error down low, around 50 feet, but from 2500-10,000 no more than 20 feet of error either way, not bad for the altimeter that is marked U.S. Navy and manufactured by Pioneer, who made all of the old WWII instruments that I have seen.

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tonycondon said:
i hooked the altimeter up to the test stand tonight and ran it up to 10,000 feet in 500 foot increments and then back down to 1000 feet. averaged out the two values at each 500 foot mark and plotted the error. a fair bit of error down low, around 50 feet, but from 2500-10,000 no more than 20 feet of error either way, not bad for the altimeter that is marked U.S. Navy and manufactured by Pioneer, who made all of the old WWII instruments that I have seen.
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Using what as your standard?

Jim
 
tonycondon said:
i used the test set at the airport that is used to calibrate altimeters for IFR static system checks. it was calibrated in June.
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That'll work, but only if you know the allowable error for the test set. When it is calibrated, what error is allowed?

Jim
 
And, then, for extra credit, you can come up with a process for calibrating wind socks. Was that on this board, or the predecessor to the red board?
 
Ghery said:
And, then, for extra credit, you can come up with a process for calibrating wind socks. Was that on this board, or the predecessor to the red board?
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Classic thread on the orginal AOPA web board. I wish I'd kept it. Don't remember her name but she even wrote a set of realistic contract specs.
 
flyingcheesehead said:
Karen something, wasn't it?
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Yes. I'll have to dig through my e-mail at home Sunday night and see if I still have the e-mail I exchanged with her. She had gotten very busy working on her IR and didn't have time for the boards. Unfortunately, she hasn't come back. Karen Cozzolino, or something like that. And it was a classic thread.
 
Tony, how did the calibration go? I think my LP-49 is too slow (that's the excuse I use for landing so fast at MUT). I'd be interested in calibrating mine.
 
Ghery said:
Yes. I'll have to dig through my e-mail at home Sunday night and see if I still have the e-mail I exchanged with her. She had gotten very busy working on her IR and didn't have time for the boards. Unfortunately, she hasn't come back. Karen Cozzolino, or something like that. And it was a classic thread.
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That's her real, or real close to the last name. I liked her style. I think she was put off by some of the 'rougher' elements of those days. Hope she's doing well.
 
Just bumping this, and throwing a question wrench in the works...:)

What reference temperature is used for altimeters/airspeed indicators, and does it matter (I suspect it doesn't, per se)?

Allow me to explain: If a column of water 100" tall is kept at a 4*C temperature uniformly throughout the column it will be approximately 0.17" shorter than a column of water that is connected to the same pressure source but is kept at a uniform temperature of 20*C (i.e. the column that's 20*C will be about 100.17" tall).

Ask how I know :redface::p. This bit us several years back in my former place of employment when we forgot to bother with the temperature correction while calibrating some test pressure gauges/manometers whose accuracies were +/-0.1% of full scale.

One tech would use one calibration standard which had been cut for a reference temperature of 20*; another (yours truly) would use the more accurate standard which had been cut for a reference temperature of 4*C. The error was always about 0.17% of indication which, at full scale, would cause the Unit Under Test to be off by 0.17%.

Thus an unnecessary Out of Tolerance Report, followed by analysis/retesting in the field by our "customers/stakeholders"--until I sat down and nuked it out, then came up with a correction table dictating how much trim mass to apply to the 20*C standard which, after we started applying, knocked our OOTRs down to nearly zero for that particular product line.:target:

So, question: At what temperature is the water column supposed to be for the calibration manometer when verifying/calibrating ASIs and altimeters? I'm suspecting the FAA has dictated that somewhere...:smilewinkgrin:
 
There are three "standards" I know of. The scientific world uses 4C giving .036126 PSI/inH20 (@ "standard gravity). ASME-3M calls for 20C giving .036063 PSI/inH20 (This is the number used by "steamfitters"). And ANSI 2530 says 15.6 C (60F) giving .035092 PSI/inH20 (this is the number used by your gas company).

My guess is that the aeronautical standard is the scientific one.


jshawley said:
Just bumping this, and throwing a question wrench in the works...:)

What reference temperature is used for altimeters/airspeed indicators, and does it matter (I suspect it doesn't, per se)?
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the good news is that the equation i used to determine what the airspeed should indicate based on water height included temperature and atmospheric pressure as variables.
 
Lance, I'm inclined to agree, and am too lazy to go looking any harder:p. We ran into all three temperatures back at the nuclear power plant where I worked; 4*C, 20*C and one supplier of differential pressure gauge/transmitters, ITT Barton, decided to use the 60*F ANSI standard, just to be obnoxious:nono:.

I don't think any of the other nuclear plants ever were as anal about this as I tried to be. It was a lot of fun, though, playing with the toys the company gave me back in the day.

Just remembered, DH Industries has a downloadable units of measure converter which I just remembered I had: There they show, for example, that 100kts is good for 6.545014 inWa@4C, or 6.556562 inWa@20C, or 6.551177 inWa@60F. Should anyone really care down to the .00000X inch of water, lol.
 
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Did you see that this thread made it into "Kit Planes".

Dan
 
tonycondon said:
no, i dont get Kit Planes. what was in there?
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The author started out by saying that an in depth discussion was going on on the POA board. He wanted to dispel any of the myths that calibrating your ASI had. I'll try scanning it and sending it to you. basically he said the posters were making it sound way too complicated.

Dan
 
Jim Weir,
I tried calibrating my asi using the manometer. You indicated using tubing the same size as the port going into the instrument. This is about 3/16" ID, but I couldn't get the water to flow down the small tubing without blocking up and getting bubbles between slugs of water, even with some soap in the water.

My question is; it really shouldn't make any difference the size of the tubing should it? A column of water exerts .434 psi regardless if the tubing is 2'' or 1/4" in diameter, I THINK.

Anyway, I was doing something wrong because the error was too great. I have always felt the asi was pretty close to right. Just thinking aloud here.

Dale
 
Stretch said:
Jim Weir,
I tried calibrating my asi using the manometer. You indicated using tubing the same size as the port going into the instrument. This is about 3/16" ID, but I couldn't get the water to flow down the small tubing without blocking up and getting bubbles between slugs of water, even with some soap in the water.

My question is; it really shouldn't make any difference the size of the tubing should it? A column of water exerts .434 psi regardless if the tubing is 2'' or 1/4" in diameter, I THINK.

Anyway, I was doing something wrong because the error was too great. I have always felt the asi was pretty close to right. Just thinking aloud here.

Dale
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The only difference that tubing diameter will make is that a larger diameter will lessen the effect of surface tension (capillary action), but AFaIK as long as both ends are the same diameter that gets canceled out. A more likely source of error is the density of the water varies with temperature and contaminants.
 
Stretch said:
Jim Weir,
I tried calibrating my asi using the manometer. You indicated using tubing the same size as ...

My question is; it really shouldn't make any difference the size of the tubing should it? A column of water exerts .434 psi regardless if the tubing is 2'' or 1/4" in diameter, I THINK. ...


Dale
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No, it shouldn't make any difference what diameter tubing you use, but the practicality of using larger diameter tubing and then some sort of reduction mechanism to connect it to the ASI fitting is a bit of a problem.

I've done that tubing trick thousands of times and never got "slugs" of water yet, even raw water with no detergent. What i.d. tubing were you using?

Jim
 
Jim I was using 3/16" ID and I was getting air locks between the water. I used 1/4" and the water worked fine. I tried again today and after putting water in the tubing and push the tubing on the back of the ASI the needle goes up some and I think it is making my reading too high. Tommorow I will put a valve in line, put water in the tubing, push it onto the ASI, open the valve to bleed the air and let the water level out. Then close the valve and hopefully the needle will start from the stop in the guage when I start adding water to the manometer.
Thanks for the help
 
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