Re: Manometer calculation

 On 12/16/2010 2:38 PM, Curbie wrote:


I came back to this to calculate:

   h = (P - P0) / (d * g); where

   P  = 21718 kPa = 21718000 Pa
   P0 = 101325 Pa
   d  = 1000 kg/m^3 (for water)
   g  = 9.81 m/s^2

and ended up with h = 2203.5346 m

Wow! Can that be right - did I screw this up?

--
Morris Dovey
http://www.iedu.com/DeSoto/
PGP Key ID EBB1E70E

 
1 Atm ~= 10 m of water ~= 100 KPa.

Memorize a few easy approximations and use them to make quick
estimates or double check the calculator.

jsw

 On 12/28/2010 6:21 AM, Jim Wilkins wrote:

I'm making progress - but I still have a _long_ way to go. :)

Your 10m/Atm approximation has been added to the heap. I did at least
quintuple check the calculator, without spotting any errors - but a 2.2
km water column seemed too good to be true.

Usually when something looks too good to be true and there don't appear
to be any errors, it's time to talk to someone who's got a better handle
on things.

Thank you!

--
Morris Dovey
http://www.iedu.com/DeSoto/
PGP Key ID EBB1E70E

 wrote:


Those are approximations but I'd rather just memorize the actual
values.  One of the handiest value to memorize is that 1 atmosphere is
exactly 760mm of mercury at standard gravity.  101.325 is the
conversion to kPa.  101.3 is easy enough to remember.


It is.  Sanity check.  1 atm = 760mm of mercury.  mercury's density is
13.54 (another handy one to remember)  so 760 * 13.54 = 10,290 mm of
water.  About 10 meters or 33 feet.

Morris, something that you'll find invaluable, especially if you
insist on working in those execrable SI units is this little program

http://joshmadison.com/convert-for-windows/

Convert converts just about anything to just about anything.  I'd hate
to think about life without it.

I didn't go through your calculation but I'd say that you probably
mixed up units, especially with the SI pascal tossed in there for bad
measure.

John

 
I do that too. The approximations are for when some engineer stops me
in the hall and asks for help on his latest brainstorm, etc. I
sometimes find fundamental flaws while listening and politely steer
them into reconsidering. It's a relic of learning to do engineering
calculations on a slide rule.

I was sending 10 million test bits over a satcom link at 2400 / sec
when a Ph.D asked me when I would be done. By mental math I quickly
told him the test would take 1 hour, 9 minutes and 26.7 seconds. They
tried to confirm it but couldn't set it up on a calculator.

I had memorized reciprocals to substitute for division and knew 1/24 =
0.0416666. 4166.7 seconds = 3600 + 540 + 26.7

jsw