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  #31  
Old 11-30-2011, 09:15 AM
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rocketbob rocketbob is offline
 
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Quote:
Originally Posted by lostpilot28 View Post
Less than .4 inch/H2O...I can live with that. My guess is that if there is error, it's not going to swing wildly from 0 to the .4 with each reading. It'll most likely be "off" by somewhere between 0 and .4 and pretty much stay there. I take it you don't like cheap Chinese tools.

It's instrument error...you have that with every instrument.
Lets say you have a pressure gauge that measures 0-100 psi and the accuracy is +-1% FS, which is 2psi total error. Now lets say you are measuring a range of pressures from 0 to 5 psi. That equates to 40% accuracy in that narrow range, which is not good at all. Same thing applies here. If you are measuring 0-5" then you will have an accuracy of 13.2% and thats ignoring resolution or hysteresis errors based on the specs of the manometer if you believe them. If you got one that had a narrower range you would have far better results.
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  #32  
Old 11-30-2011, 09:34 AM
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Quote:
Originally Posted by rocketbob View Post
Lets say you have a pressure gauge that measures 0-100 psi and the accuracy is +-1% FS, which is 2psi total error. Now lets say you are measuring a range of pressures from 0 to 5 psi. That equates to 40% accuracy in that narrow range, which is not good at all.
You sure it's not 2 psi across a range of 100, thus 1/20 of 2 psi across 5?
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  #33  
Old 11-30-2011, 09:50 AM
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Originally Posted by DanH View Post
You sure it's not 2 psi across a range of 100, thus 1/20 of 2 psi across 5?
No, it'd be 2 psi across a range of from 1-5. In any event, mil standards want you to avoid the top and bottom 20% of any gauge while measuring...stick with the middle 60. So I'd never measure 5 psi on a 100 psi reference gauge.
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  #34  
Old 11-30-2011, 10:10 AM
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Quote:
Originally Posted by rocketbob View Post
Lets say you have a pressure gauge that measures 0-100 psi and the accuracy is +-1% FS, which is 2psi total error. Now lets say you are measuring a range of pressures from 0 to 5 psi. That equates to 40% accuracy in that narrow range, which is not good at all. Same thing applies here. If you are measuring 0-5" then you will have an accuracy of 13.2% and thats ignoring resolution or hysteresis errors based on the specs of the manometer if you believe them. If you got one that had a narrower range you would have far better results.
Understood...but it's .3% across a range of 110 in/H2o. That's 3x better than what you're analogy is using. Maybe it's not ideal, but I'm OK with it. From what I've seen, the erros in digital instrumets are usually one way or the other (not randomly selected each time you measure). I don't really care what the values are...I want to know the difference between the values. I don't care if my upper plenum has +10 in/H2o or if it's +8. As long as it reads the same each time for the same test conditions.
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  #35  
Old 11-30-2011, 12:26 PM
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Originally Posted by Mark12A View Post
No, it'd be 2 psi across a range of from 1-5.
I think you're saying the spec refers to offset error rather than gain error.



Obviously I was thinking gain. However, I suspect offset is the correct view because a close look finds "FSO" in the spec:

Accuracy: +- 0.3% FSO @25C

Does FSO stand for "Full Scale Offset"?
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  #36  
Old 11-30-2011, 12:39 PM
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I think FSO stands for Full Scale Output.
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  #37  
Old 11-30-2011, 03:06 PM
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Quote:
Originally Posted by DanH View Post
I think you're saying the spec refers to offset error rather than gain error.



Obviously I was thinking gain. However, I suspect offset is the correct view because a close look finds "FSO" in the spec:

Accuracy: +- 0.3% FSO @25C

Does FSO stand for "Full Scale Offset"?
My apologies, I'm not educated on instrument calibration. Can you explain this in layman's terms? I can make a guess about the error being smaller the closer you are to "zero", but I'd probably be wrong.
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  #38  
Old 11-30-2011, 07:38 PM
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Quote:
Originally Posted by CMW View Post
I think FSO stands for Full Scale Output.
Thank you Chris. Leads me back to thinking it's a gain spec, but no matter.

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Originally Posted by lostpilot28 View Post
My apologies, I'm not educated on instrument calibration. Can you explain this in layman's terms?
I too am learning, but this part isn't hard.

Offset is a constant error. The readout is always some fixed amount too high or too low.

Gain is a multiplier. The error gets proportionally larger as you move up the scale. Gain error is listed as a percentage of the full range value ("FSO"), so the possible gain error for any given application is (percent of range being used multiplied by gain value).

Apparently the good instruments come with both an offset and a gain percentage stated in the specs. So you calculate possible gain for the part of the range you plan to use, then add the stated offset to know the total possible error.

In the case of the Chinese meter, we don't know if the stated 0.3% FSO is offset or gain. Doesn't matter. Check it with a tube full of water. It may be perfectly accurate. Or the 0.3% could be a total lie and error is far worse.

Me? I've been flying around with tubing zip-tied to a yardstick
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Last edited by DanH : 11-30-2011 at 07:43 PM.
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  #39  
Old 11-30-2011, 08:41 PM
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Dan, you're awesome. Thanks for that explanation. I may indeed test the instrument prior to flying with it just to see how far off it is. I still think that getting consistent results is better than actual numbers in this case because I just want to know the difference between two values. I hope that's not out of line - and if it is, then I'm sure someone will correct me.

Thanks again!
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  #40  
Old 11-30-2011, 09:12 PM
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What are you going to do with the data once you get it? What is good...what is bad?

Dave (Swift driver) and practical engineer in the pressure and flow measurement business for 37 years.
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