Return-Path: Sender: To: lml@lancaironline.net Date: Sat, 12 Nov 2005 17:03:15 -0500 Message-ID: X-Original-Return-Path: Received: from imo-d05.mx.aol.com ([205.188.157.37] verified) by logan.com (CommuniGate Pro SMTP 5.0.1) with ESMTP id 828459 for lml@lancaironline.net; Sat, 12 Nov 2005 00:01:36 -0500 Received-SPF: pass receiver=logan.com; client-ip=205.188.157.37; envelope-from=RWolf99@aol.com Received: from RWolf99@aol.com by imo-d05.mx.aol.com (mail_out_v38_r6.3.) id q.1f8.16c5299e (4254) for ; Sat, 12 Nov 2005 00:00:49 -0500 (EST) From: RWolf99@aol.com X-Original-Message-ID: <1f8.16c5299e.30a6d100@aol.com> X-Original-Date: Sat, 12 Nov 2005 00:00:48 EST Subject: Re: [LML] OAT Probe Wire Extension X-Original-To: lml@lancaironline.net MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="-----------------------------1131771648" X-Mailer: 9.0 SE for Windows sub 5022 X-Spam-Flag: NO -------------------------------1131771648 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit The short answer is that you can use regular copper wire to extend your thermocouples, but you'd have to make sure that the adjacent splices were at the same temperature. You can buy thermocouple wire at _www.omega.com_ (http://www.omega.com) and I'd recommend that. That way any temperature differences won't matter to your measurement of OAT. You'll find that fewer than one in a thousand people actually understand thermocouple physics. Now I'm not saying that I'm a genius -- hardly -- but I did have the opportunity to work in a research lab using thermocouples. Thermocouples work by having two junctions between different materials. A Type K thermocouple uses chromel and alumel (whatever the heck they are) as the two materials. A Type J thermocouple uses iron (at least I know what that is) and constantan (huh?). At least I think it's that way but I may have it backwards. Anyway, for the purposes of this discussion, it doesn't matter. So we have three sections of wire. Material A at each end with material B in the middle. AAAAAAAAA - BBBBBBBB - AAAAAAAA The hyphens represent the joints (junctions) where the wires are connected. Okay. Now we put one junction in ice water, and the other one on what we're trying to measure the temperature of. You will get a voltage difference -- usually in millivolts. That's one of those laws of nature, called the "thermoelectric effect". Note that if the junctions are at the same temperature then you'll get zero volts. You could make a thermocouple out of aluminum and steel, or lead and gold, or uranium and plutonium if you wanted to, but the voltages you'd generate would be too small to measure. Now, you've probably noticed that the electronic gizmos we use in our airplanes don't actually use a bath of ice water like we do in the laboratory. They somehow synthesize a cold junction. (It's BFM to me -- that's "black f**king magic" -- and maybe Brent Regan or Greyhawk can educate us on this topic. After all, Scott seems to have a lot of time on his hands, for which I am insanely envious, and Brent Regan IS a genius...but I digress) Also note that you can run these backwards -- if you apply a voltage difference then you will create a temperature difference. Put the cold junction on your beer can, blow air over the hot junction to take the heat away, and shazam! You have a thermoelectric refrigerator and cold beer. Nothing esoteric about this -- you can buy them at Wal-Mart. Put the power in one way and you get a cooler. Put it in the other way and you get an inefficient heater. Before you get excited and decide to turn your VMS 1000 into a beer cooler for your hangar, you should know that they don't do this with a single thermocouple junction; they make up little plates with hundreds of junctions on them. But I digress again... What you were proposing is like this, where C is copper. AAAAA - CCCCC - AAAAA - BBBBB - CCCCC - BBBBB (Note that the AAA and BBB at the ends may very well be the terminals on your electric gizmo.) You can definitely make this work. If the junctions between the copper and the thermocouple materials are at the same temperature then you'll get the proper readings. That doesn't mean both junctions need to be at the same temperature -- but where you splice the copper into the circuit should be the same temperature on both sides. Putting it yet a third way, the splices to copper near your gizmo should be the same temperature as each other, and the splices from copper to the thermocouple wires should be the same temperature as each other, but the splices near your gizmo do not have to be the same temperature as splices by the thermocouple. You could, for example, connect the copper wire to your electronic gizmo in the cabin, and the copper wire to the thermocouple in the engine bay. You can probably make this work by wrapping the two splices in the engine bay together, or by using a connector where the parts are basically all at the same temperature. But if you want to remove all doubt, use thermocouple wire for your extension cord. It's not expensive like airplane parts.... - Rob Wolf Lancair 360 81% complete Parker Colorado -------------------------------1131771648 Content-Type: text/html; charset="US-ASCII" Content-Transfer-Encoding: quoted-printable
The short answer is that you can use regular copper wire to extend your= =20 thermocouples, but you'd have to make sure that the adjacent splices were at= the=20 same temperature.  You can buy thermocouple wire at www.omega.com and I'd recommend that. = ; That=20 way any temperature differences won't matter to your measurement of OAT.
 
You'll find that fewer than one in a thousand people actually understan= d=20 thermocouple physics.  Now I'm not saying that I'm a genius -- hardly -= -=20 but I did have the opportunity to work in a research lab using=20 thermocouples.
 
Thermocouples work by having two junctions between different=20 materials.  A Type K thermocouple uses chromel and alumel (whatever the= =20 heck they are) as the two materials.  A Type J thermocouple uses iron (= at=20 least I know what that is) and constantan (huh?).  At least I think it'= s=20 that way but I may have it backwards.  Anyway, for the purposes of this= =20 discussion, it doesn't matter. 
 
So we have three sections of wire.  Material A at each end with=20 material B in the middle.
 
AAAAAAAAA - BBBBBBBB - AAAAAAAA
 
The hyphens represent the joints (junctions) where the wires are=20 connected.
 
Okay.  Now we put one junction in ice water, and the other one on=20= what=20 we're trying to measure the temperature of.  You will get a voltage=20 difference -- usually in millivolts.  That's one of those laws of natur= e,=20 called the "thermoelectric effect".  Note that if the junctions are at=20= the=20 same temperature then you'll get zero volts.  You could make a thermoco= uple=20 out of aluminum and steel, or lead and gold, or uranium and plutonium if you= =20 wanted to, but the voltages you'd generate would be too small to measure.
 
Now, you've probably noticed that the electronic gizmos we use in our=20 airplanes don't actually use a bath of ice water like we do in the=20 laboratory.  They somehow synthesize a cold junction.  (It's BFM t= o me=20 -- that's "black f**king magic" -- and maybe Brent Regan or Greyhawk ca= n=20 educate us on this topic.  After all, Scott seems to have a lot of time= on=20 his hands, for which I am insanely envious, and Brent Regan IS a genius...bu= t I=20 digress)
 
Also note that you can run these backwards -- if you apply a voltage=20 difference then you will create a temperature difference.  Put the cold= =20 junction on your beer can, blow air over the hot junction to take the heat a= way,=20 and shazam!  You have a thermoelectric refrigerator and cold beer. = ;=20 Nothing esoteric about this -- you can buy them at Wal-Mart.  Put the p= ower=20 in one way and you get a cooler.  Put it in the other way and you get a= n=20 inefficient heater.  Before you get excited and decide to turn your VMS= =20 1000 into a beer cooler for your hangar, you should know that they don't do=20= this=20 with a single thermocouple junction; they make up little plates with hundred= s of=20 junctions on them.
 
But I digress again...
 
What you were proposing is like this, where C is copper.
 
AAAAA - CCCCC - AAAAA - BBBBB  - CCCCC - BBBBB
 
(Note that the AAA and BBB at the ends may very well be the terminals o= n=20 your electric gizmo.)
 
You can definitely make this work.  If the junctions between the=20 copper and the thermocouple materials are at the same temperature then you'l= l=20 get the proper readings.  That doesn't mean both junctions need to be a= t=20 the same temperature -- but where you splice the copper into the circuit sho= uld=20 be the same temperature on both sides.  Putting it yet a third way, the= =20 splices to copper near your gizmo should be the same temperature as each oth= er,=20 and the splices from copper to the thermocouple wires should be the same=20 temperature as each other, but the splices near your gizmo do not have=20= to=20 be the same temperature as splices by the thermocouple.  You could, for= =20 example, connect the copper wire to your electronic gizmo in the cabin, and=20= the=20 copper wire to the thermocouple in the engine bay.  You can probably ma= ke=20 this work by wrapping the two splices in the engine bay together, or by= =20 using a connector where the parts are basically all at the same=20 temperature.  But if you want to remove all doubt, use thermocouple wir= e=20 for your extension cord.  It's not expensive like airplane parts....
 
- Rob Wolf
Lancair 360
81% complete
Parker Colorado
 -------------------------------1131771648--