---------------------------------------------------------- AeroElectric-List Digest Archive --- Total Messages Posted Fri 02/03/12: 3 ---------------------------------------------------------- Today's Message Index: ---------------------- 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 _____________________________________ Time: 09:57:08 AM PST US From: Paul Millner Subject: Re: AeroElectric-List: 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 _____________________________________ Time: 01:20:22 PM PST US From: "Robert L. Nuckolls, III" Subject: Re: AeroElectric-List: 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 _____________________________________ Time: 01:20:49 PM PST US From: "Robert L. Nuckolls, III" Subject: Re: AeroElectric-List: 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. 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