nuckolls.bob(at)aeroelect
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 Posted: Mon Oct 01, 2012 7:20 am Post subject: Renewing old data . . . |
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A few days ago there was some discussion about heated
pitot tubes and some ideas about directly heated tubes
as opposed to assemblies with embedded heating elements.
I was trying to recall some things I explored in
years past . . . a bit fuzzy so I went to the bench and
did the following exploration.
[img]cid:.0[/img]
Dug out my Weller 8400, a descendant of the very first soldering
tool I ever purchased (from Burnstein-Applebee in KC for about
$5). The snap-on ammeter showed just over 300 amps in the 'tip wire'
while the trusty John Fluke multimeter gave me 250 millivolts as
the potential delivered to the tip.
The snap-on confirmed a 120VAC current draw of 1.2A or right
at 140 watts power consumed.
Measurement of the tip material confirmed that the cross-section
was on a par with 10AWG wire . . . I used to make replacement
tips from 10AWG scrap provided by a journeyman electrician up
the street from my house.
The experiment confirmed my recollection that soldering guns
are horribly inefficient tools. The actual heating power
delivered to the working tip is tiny compared to the total
power consumed. My Metcal irons deliver a much greater proportion
of their power to the tip . . . a 30W Metcal will install terminals
on 2AWG wire! The Weller 8400 puts about .250 x 310 or 75 watts
of total consumption into the tip wire . . . and that's spread
all along the tip's length. Local heating at the tip plus conduction
from behind the tip might put half the total heat to good use?
In the case of a directly heated pitot tube, the feature that
gives a soldering gun such poor marks turns out to be exactly
what the DHPT needs . . . hot on every surface prone to collection
of ice that would render IAS inoperable.
Food for thought . . .
Bob . . .
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