AeroElectric-List Digest Archive

Fri 08/03/12


Total Messages Posted: 3



Today's Message Index:
----------------------
 
     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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    Time: 08:18:34 AM PST US
    From: "Robert L. Nuckolls, III" <nuckolls.bob@aeroelectric.com>
    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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    Time: 09:34:52 AM PST US
    From: "Robert L. Nuckolls, III" <nuckolls.bob@aeroelectric.com>
    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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    Time: 11:52:44 AM PST US
    Subject: Re: Extending thermocouple leads
    From: "Radioflyer" <skyeyecorp@airpost.net>
    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 Read this topic online here: http://forums.matronics.com/viewtopic.php?p=379950#379950




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