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Date: Sun, 14 Nov 2004 16:14:00 -0700
From: Bob White <bob@bob-white.com>
To: FlyRotary <flyrotary@lancaironline.net>
Subject: EWP Testing - More Food For Thought
Message-Id: <20041114161400.01bac5ae.bob@bob-white.com>
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This morning I wanted to do the test where the two hoses were connected
end to end to see if the current would drop as I was expecting.  It was
41 F on the patio when I went out.  I hooked everything up and
immediately noticed the current was reading almost 10 amps.  My first
thought was that maybe it was because the water was colder.  It was
probably 55 - 60 when I did the tests a couple of days ago, And I've
just been looking at viscosity numbers for water at different temps.

OK, so I took a slight detour.  This is the second pump. I only did
current measurements on the other pump before.  This pump is
pulling about one more amp in warm water than the first pump, and I
don't know what the temperature was.  Flow measurements were made
without the ammeter in the circuit because the kludge wiring for the
meter lowered the pump output from 16 GPH to 14 GPH.

Here are the current vs water temp measurements for this pump:

Temp (F)     Current (A)     Viscosity (cp)
40           9.2 to 9.4         1.55
60           8.6 to 8.7         1.13
75           8.2 to 8.4         0.92 (interpolated)

With the output blocked, current was 5.5A.

Finnaly I did the closed circuit hose test.  I started with the input
and output hoses submerged in the big container.  Then I held the two
ends together.  As you might expect, the current went UP 0.4A from 8.2A
to 8.6A.  (You should expect that because I predicted it would go down.)

I didn't make any flow measurements, but water temperature makes a
difference. It looks like the performance at 200 F where viscosity is
0.305 could be quite different from low temperature tests.  Then let's
throw in some anti-freeze to complicate it a little.  Looking back at
Bill Shertz' data (page 15 in the pdf file), I see that 50% ethlene
glycol/water has a viscosity 2.3 times higher than water, both at 200 F
or about the same as water at 95 F.   

(Bill's units are Ft^2/HR, and the units I found on the web are in
centipoises.  These units are not directly interchangable.  Bill's units
are kinematic viscosity, the ratio of absolute or dynamic viscosity to
the density. The cp is the absolute viscosity, and is measured as a
force per unit area.  Water density is about 4 % less at 200 F than it
is at 40 F.  Then 50 % ethylene glycol is about 4 % denser at 200 F than
pure water, so looking at ratio's between absolute and kinematic won't
create a very large error. Bill has a unit Cp on his chart which is not
related to cp. The Cp unit has to do with heat capacity. BTW, I picked
up most of this information at http://www.engineeringtoolbox.com/ which
has a lot of neat info.  There is an interesting article on centrifugal
pumps.)


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