| [cm3215-l] Fwd: Accuracy of DP meter; a
student's question |
Friday, January 29, 2010
10:40:38 AM |
| From: |
fmorriso@mtu.edu |
| To: |
cm3215-l@mtu.edu |
>Dear Dr. Morrison:
>
>I have a question about the precision of the DP meter in CM3215.
>
> Say you let the manometer sit for a
> long time with both ends exposed to the atmosphere.
Presumably after
> this the two liquid levels on both sides of the manometer would be
> exactly equal. Then, say you plugged in your DP meter,
bleeding the
> valves of the meter for a long time before you took a reading.
Say it
> was something like 4.2 mA. Wouldn't this be the lowest
reading you
> could take? And couldn't you be fairly confident that the
difference
> between the two liquid levels was 0?
> Sincerely, A Student
Good morning,
Imagine you do your experiment and the reading is 4.2 mA as you say.
So apparently zero pressure drop is 4.2 on the meter.
Now you have two chambers. From some other method you know that
the pressure in the first one is 1.0000 atmosphere and the pressure in
the second one is 1.0001 atmosphere. What will your DP meter
read?
Maybe based on your calibration it's supposed to read 4.2002, but
you
don't have that many digits on your digital multi meter, and besides,
even with your experiment the 4.2 number bounced around alot, maybe
reading 4.1 or 4.3 sometimes. A pressure difference of .0001 is
too
small for you to measure.
Now repeat the two chamber test. Now the first chamber is at
1.0000 atmosphere and the second one is at 1.0010 atmospheres.
Now
what does the DP meter read? is it precise enough to notice this
difference? no, it's not.
Third test. First chamber is 1.0000 atm and the second is 1.0100
atm. Now is there a difference?
If the pressure difference is small, below some threshold, you do
not have the precision in this device to get the measurement. All
of
the above differential pressures, which are all different, would all
look like zero to you. You can only tell the difference between
the
two vessels if the difference is large enough. How large does the
difference have to be before you can reliably tell the difference?
best wishes,
Dr. Morrison