On Tue, 02 Apr 2013 12:47:06 -0400, clare wrote:

On Tue, 02 Apr 2013 07:37:34 -0500, Richard
wrote:

On 4/2/2013 12:38 AM, Ed Huntress wrote:

What Clare is saying, I believe, is that the length of the hose makes
no difference in the value of suction -- if there is no airflow
through the hose. If you hook up a vacuum pump to, say, a bell jar or
to a perfectly sealed vacuum-cure rig for composites, it doesn't
matter how long the hose is. The vacuum will be the same.

What reduces the vacuum is the drag experienced by airflow through the
hose. Longer hoses will measure the same vacuum at their ends, no
matter how long the hose, if there is no flow. But if there is flow,
the more there is, and the greater the inherent drag in the hose, the
lower will be the measured vacuum at the end.

Rules of physics.


Same with electricity.

Vacuum = Voltage.

Length of hose = resistance.

Air mass flow = Current.

But if the resistance per circular mil foot is relatively low, and the
conductor guage(number of circular mils) is high enough, the number of
feet becomes a very small part of the equation. IE, if you have a 5
amp load on a 12 ga conductor 5 feet or 500 feet in length, it's going
to take a sensitive instrument to read the difference. Put 30 amps on
a 22 ga wire and the difference between 5 feet and 15 will be readily
discernable (if the wire lasts long enough to measure it )

Bad example! It doesn't take a particularly sensitive voltmeter to
distinguish a four-volt drop from 40 millivolts. 12 ga copper has about
1.6 ohms per thousand feet; 5A * .8 ohms gives 4 volts drop.

Even with gage 0 wire, at 0.1 ohms per thousand feet, there would be a
quarter-volt difference for 5A in a 500' run vs 5' run.

--
jiw