Today's Message Index:
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1. 09:57 AM - Re: Re: Thermocouple leads (Paul Millner)
2. 01:20 PM - Re: Re: Thermocouple leads (Robert L. Nuckolls, III)
3. 01:20 PM - Re: Re: Can Xfeed damage smaller battery. (Robert L. Nuckolls, III)
Message 1
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Subject: | Re: Thermocouple leads |
As Bob has shared, some really old systems (through, say, the 1950's)
heavily loaded the junction, so the apparent voltage was much less than
the theoretical voltage. But, as long as those systems were calibrated,
they were repeatable and accurate. Splicing a wire, even with J or K
wire, could increase resistance, and due to the relatively heavy current
flow, could affect the voltage and hence calibration. And at least in
my industry, we were still using those dinosaurs into the 1980's, so ya
gotta train to work with what you have, not what's being built today.
The predominance int he industry today, though, is high impedance
devices that don't load the T/C junction... which has the side benefit
of allowing multiple devices to share a junction. A good thing. A
splice resistance is unimportant, as current flow is nearly nil.
Dissimilar wire, especially with junctions at dissimilar temperatures,
really plays havoc with even the open circuit voltage, and there's no
way to calibrate that out, as the temperature of those splice junctions
into the future typically can't be known.
Paul
Message 2
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Subject: | Re: Thermocouple leads |
At 09:39 PM 2/2/2012, you wrote:
>
>Hmmm, Bob... the physics doesn't support your assertion, unless you
>(mistakenly I'd think) assume that all those junctions are somehow
>held at the same temperature. And how would you accomplish that?
The PAIRS have to be adjacent. For example, ordinary
gold over brass pins used to bring a t/c pair through a
connector adds negligible effect to calibration because
equal and opposite effects are inserted into each lead
and in close thermal proximity to each other and they
cancel each other out.
A tech told me of a case where he found an overheated
wire going through a connector because it was adjacent
to a non-compensated thermocouple lead . . . the other
lead in the thermocouple pair was not adjacent in the
connector housing and, therefor, was running cooler.
The warm-wire caused significant difference in temperature
reading for the afflicted pair as compared to other
t/c pairs in the same connector.
So putting 'foreign materials' into the t/c system is
not without risk . . . but easily managed with some
planning and attention to details.
>With the relatively common high impedance measuring devices,
>resistance in the leads isn't really the issue... but adding
>uncontrolled junctions at different temperatures can sure play havoc
>with accuracy.
Absolutely. For example, a silver soldered joint adds
two foreign junctions in immediate proximity. T/C-alloy
to-solder offset by a solder-to-T/C-alloy right next
to it. Two equal but opposite effects that cancel because
while not 'controlled' they are at the same temperature.
So my condition (2) was not completely explained. The
foreign junctions must be in close proximity as in the
soldered joint described above, or the t/c selector switch
box described in Figure 14-7B.
Splicing into t/c pairs to extend them will add loop
resistance. UNPOWERED temperature readout instruments
are still offered by manufacturers like ALCOR, Westach,
and others in the automotive world (see Figure 14-10C in the
'Connection). These meters must be capable of reading
thermocouple voltages directly . . . on the order
of 10 to 20 millivolts FULL SCALE. Hence they are wound
with few turns of heavier wire and draw considerable
current (like 10-50 milliamps). In these cases, the
termocouple wire tends to be heavy gauge just to get
low resistance. If you check out the range of sizes
available from Omega on type K wire:
http://www.omega.com/ppt/pptsc.asp?ref=XC_K_TC_WIRE&Nav=temh06
. . . we see everything from 14AWG to 36AWG. To be sure,
any gauge wire would work with a modern engine management
system with internally conditioned inputs. But if you
were driving an unpowered instrument over some distance
in a refinery or smelting operation, the fatter wires would
offer some options.
When unpowered gauges were used on large piston aircraft,
identical gauges and CHT/EGT thermocouples had to be individually
compensated using an external resistor so that effects
of lead length between instrument and engine could be
calibrated out.
This presupposes that the actual temperature value is of
interest. In many airplanes, one uses such an instrument
only to find "peak" on the mixture and then adjust in
25F incremental departure from peak. So whether the peak
occurs at 1/3rd scale or 3/4ths scale is not critical.
Hence, no calibration needed.
But if you've got a TIT limit to be observed, THAT system begs
for calibration. When such instruments are supplied without
provisions for exernal calibration, installation instructions
will generally include an admonition not to change the length
of the thermocouple lead.
Bob . . .
Message 3
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Subject: | Re: Can Xfeed damage smaller battery. |
At 10:50 PM 2/2/2012, you wrote:
Bob, thank you very much for your assistance. I now feel that I am up
to speed on this issue. The dark gray clouds of doubt have been lifted. :)
Cheers Bob Barrow
Pleased to hear it. Minimizing risk
begins with understanding.
Bob . . .
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