Today's Message Index:
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1. 08:18 AM - Re: Extending thermocouple leads (Robert L. Nuckolls, III)
2. 09:34 AM - Re: PM Alternator filter Capacitor - best practices (Robert L. Nuckolls, III)
3. 11:52 AM - Re: Extending thermocouple leads (Radioflyer)
Message 1
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Subject: | Re: Extending thermocouple leads |
At 11:27 PM 8/2/2012, you wrote:
I think I understand how to do things right with thermocouples, but
want to make sure. I have a set of (GRT/EIS) J- type, CHT bayonet
thermocouples. The 2 foot leads are terminated from the factory in
what I believe to be standard spade terminals. I need to lengthen
these leads about 12 feet to reach my MGL Avionics quad CHT display
instrument. So I plan to get some J-type wire, crimp on some mating
spade terminals and similarly terminate the other end very close to
the instrument. (One set of spades will be in the hot engine
compartment and the spades on the other end of the extension will be
in the cabin.)
My understanding is that the parasitic couples at the spade terminals
will be cancelled out, so I should get accurate temp readings at the
instrument. Correct?
Sort of . . .
What do the 'spade' terminals attach to?
The "wrong+wrong=right" thing applies only
when pairs of wrongness are applied to EACH
lead . . . not opposite leads. When thermocouple
gages of yore were brought to an instrument on
the panel, they too were terminated in ordinary
terminals onto studs at the back of the instrument
in the cockpit environment. These too introduced
small errors that were acceptable in the grand
scheme of things. A few degrees of error in hundreds
of degrees of reading.
Putting ring terminals onto a thermocouple at
the 'cold junction' or 'reference junction' end
of the wire introduces little error . . . the
hotter the environment of the polluting junctions,
the greater the absolute error . . . but the
percentage of error vis-a-vis local temperature
is about the same.
Ring terminals used to splice a t/c wire run
with a nut-n-screw would be an example of many
wrongs adding up to not-too-bad. But the single
spade on one end of each side will induce some
small errors.
Having offered all that, the maximum error you
might encounter is insignificant in this situation.
You're not going to make decisions based on a few
degrees of reading. It's departures from 'normal' and
trends that deserve your attention.
So don't fuss over the details too much . . .
Is it worth the trouble to search for stranded J-wire or is solid good enough?
From a measurement perspective, solid and stranded
can be interchanged and spliced in the same system.
You can get small quantities of a variety of t/c
wires from Omega at
http://tinyurl.com/cwnomt3
Their part number builder says they'll sell you
25' of TT-J-24S for $30.00
eBay may offer additional opportunities at
http://tinyurl.com/cr35ff4
I've got a bunch of thermocouple wire stashed in
boxes yet unpacked from the move . . . else I'd
offer to send you a hunk of something suited to
your task. But in the 105 weather I'm not inclined
to rummage around out in an un-conditioned part
of the shop!
Bob . . .
Message 2
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Subject: | PM Alternator filter Capacitor - best practices |
>And thank you for Villard's Memoir. I loved reading every bit of it!
>
>Like every self-respecting EE I have kept my copy of Terman.
>
>[]
>
I lost my copy during some move over the years. Thought
I might replace it from a used book store someday . . . have
you seen what they sell for now? Hang onto the copy you
have and be sure to include it in the accounting of
your retirement portfolio.
And in every other class you'd get one 'ringer'. A kid fresh out of
engineering school who would nail you to the chalk board at the
slightest slip-up!!! You either got those guys on your side right
away or you were in for some tough sledding!
I don't recall having encountered that from the
run-of-the-mill students. Most of my guys were fresh
out of boot-camp. I did have a chief who was re-upping
and wanting to expand his technological horizons. He
and I had an arm-wrestling match over my dismal record
for Friday High-Class honors . . . but that's another
story. He did very well in his studies and received
benefits befitting his due diligence.
>[]
>
>I have a couple of these first generation DI-148Us.
>It will run at 11 KHz sampling rate on one channel.
Cool! Let's do some good numbers gathering off your
ship's electrical system when it's appropriate.
>
>Tell me more about your dynamic load bank. Sounds like just the ticket.
Here's one I started for my shop some years ago but
when the project-of-need went away, it's been waiting
for a reason to finish it.
The one I built for HBC might still be there but
since it never got a corporate acquisition sticker
or tool number put on it, it's probably been pitched
by now. There's nobody left there who saw what it does
or how to use it.
[]
The thing was used to study the effects of rapid
load dump and onset for some led lighting supplies
in the Hawker 800. When the cabin lights 'flicker'
in a $14M airplane, the boss can only sit there an
fume that this crap doesn't happen in his $60K
automobile.
The notion was to excite the bus with repetitive
square-wave loads either manually by external
controls while gathering responses and/or trying
fixes elsewhere.
For our purposes, a 555 timer, a boss-hog fet
and a power resistor could be quickly brass-boarded
into a similarly useful fixture.
[]
This test fixture would apply/release a load
at some periodic rate (in this case about 0.5Hz)
to let you observe system response after
the event.
Your Dataq DAS in the single channel, fast
acquisition mode would do nicely to record
the event.
Bob . . .
Message 3
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Subject: | Re: Extending thermocouple leads |
Bob, thanks for your reply. You asked what the spade terminals were attached to.
They are crimped on to the J wires of the thermocouple sensor. The wires are
red and white and one of them was magnetic.
You are right that a few degrees of error would not much matter for CHT readings.
I don't have a sense for how much error my "J-spade-J-spade-instrument" setup
would introduce, but if within say ten degrees then it would be ok. I just
want to be sure I can reliably tell that my cylinders are not exceeding max allowable
temperature.
--Jose
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