Author Topic: egt gauge (Read 3980 times)

dieselpower · « **on:** October 24, 2004, 03:35:45 pm »

ok,
this is kinda off topic but i have a vdo egt gauge and the pyrometer. i tried to order the lead wire but i just got another pyrometer. anyway i've looked all over but can't find a source for the conector. does anyone know of a place that i can find one, or a way to hook up the gauge without one?
thaks for the help

mark

Rat407 · « **Reply #1 on:** October 24, 2004, 08:52:27 pm »

Did you try contacting VDO? or go to egauges.com they have a nice selection of hardware and gauges.

Otis2 · « **Reply #2 on:** October 25, 2004, 12:32:11 am »

I have a VDO pyrometer in my diesel vanagon - and about 15' of copper speaker wire connecting the probe to the dashboard...

The VDO pyrometer does not need the fancy red/yellow connector wire. You can just use regular copper speaker wire. Someone on the Vanagon e-mail list routed both types of wire to their gauge and tried test runs, and there was no difference in readings at all.

VWRacer · « **Reply #3 on:** October 25, 2004, 08:44:36 am »

Sweet, Otis...that's good info to have!

Otis2 · « **Reply #4 on:** October 25, 2004, 03:40:05 pm »

Apparently the fancy wire is required if the gauge is not independently powered by 12 volts (ie. the feeble current of the thermocouple powers the gauge movement). The VDO gauges *are* independently powered by 12 v (or at least mine is), so copper wire works fine.

I'm not sure these things are ever going to be "pin-point" accurate anyway, I'm just watching the analogue needle's sweep speed to get an idea of the temperature trends. I wouldn't bet my engine that 1450 as indicated on the gauge is safe, while 1550 is melt-down territory, regardless of what kind of wire I had connected to the probe!

dieselpower · « **Reply #5 on:** October 25, 2004, 05:11:03 pm »

hey,
thanks for the help but mine doesn't use external power.it has a conector that looks something like a square on one end and a triangle type thing on the other...its just a plug that can only go in in one direction.

i've looked all over...i dont have the contact info for vdo but like i said i tried to order one but they just sent another pyrometer.

QuickTD · « **Reply #6 on:** October 25, 2004, 05:12:11 pm »

Quote

Apparently the fancy wire is required if the gauge is not independently powered by 12 volts (ie. the feeble current of the thermocouple powers the gauge movement).

A thermocouple is just 2 dissimilar metals bonded together, chromel/alumel alloys in the case of a type K pyrometer. When heated this junction produces a very small voltage (only 31.6 millivolts @1400ºF) that is directly proportional to the junction temperature. Normally a thermocouple probe is supplied with a lead wire made of the same material as the junction. This avoids creating additional junctions at the connections to copper lead wire. If a thermocouple is connected with copper lead wire all of the copper connections must be kept at the same temperature (isothermal). If they aren't, the copper/chromel and copper/alumel junctions will produce voltages just like the main chromel/ alumel junction. These stray voltages will cause serious errors in the readings. Otis2 has been fortunate enough to be able to achieve reasonable temperature equilibrium between all of the copper/thermocouple junctions but I wouldn't assume that it will work properly in every case. Whether or not the pyrometer is powered or unpowered will not make a difference in the thermocouple cabling requirements. The powered pyrometer just uses an amplifier to drive the meter movement rather than driving a very sensitive (and more expensive) meter directly, as is the case in the unpowered unit. Often the powered units also use a 300º sweep movement as well, these are air core meter movements and require drive power greater than the thermocouple can provide.

Otis2 · « **Reply #7 on:** October 25, 2004, 05:49:49 pm »

Interesting, QuickTD, and an excellent explanation of the physics involved.

It's quite possible that my EGT readings are all in error, since there is obviously no good temperature equilibrium between the engine compartment of a diesel vanagon and its dashboard! However, I'm not sure that this VDO gauge was designed to be as sensitive to voltage differences as some other EGT gauges - perhaps those found in aircraft pyrometers, for example. I'm not seeing any really odd changes in the readings as the temperature in the engine compartment heats up relative to the dashboard. I would expect the readings to become more erratic with engine compartment temps if this gauge was very sensitive, which doesn't seem to be happening. Perhaps the sensitivity has been "dulled" somehow.

I don't pretend to have the evidence behind the suggestion to use copper wire. I picked up the bit about separate 12 volt power to the gauge being the determining factor from a message posted on the TDi conversion e-mail list, here:

http://autos.groups.yahoo.com/group/TDI-conversion/message/2916

I'd be curious to read your comment on Mr. Brubaker's assessment.

Otis2 · « **Reply #8 on:** October 25, 2004, 05:50:54 pm »

I'll post Brubaker's missive from the TDi Conversion list so you can avoid signing up to yet another Yahoo e-mail list...

David,

Sorry for the late response, but I didn't see that you got a good
answer. I hope can offer one. I have not looked at a book on this for
a long time, but this is what I remember. I work as an aircraft
piston engine mechanic where these instruments are regularly used. We
call them EGT gauges for 'Exhaust Gas Temperature'. It sounds like
the pyrometer you got is a traditional type that does not use 12v
power to run the instrument. This type is powered by the very small
current created by the thermocouple. To be accurate the resistance of
the wire between the instrument and the thermocouple has to be an
exact value. This value is often printed on the back of the
instrument. Since the resistance of a wire changes with length, when
you needed a longer wire it had to be made of a different material to
have the same resistance as the standard shorter wire. They may look
like the same material but they are not. Both the long wire and the
short wire will have the same resistance. This special material is
why it is so expensive. If the wire is to long it should not be cut.
Simply roll up the excess in some out of the way place.

Newer type EGT gauges that are electronic and use 12v to power the
instrument do not need this special wire because they only read the
voltage of the thermocouple, they don't use the current. These can
use any good copper wire and length is not critical. Many of these
instruments give a digital readout, although I think I have seen some
that swing a needle. This newer type may be more expensive for the
instrument, but you might save more than the difference by not having
to buy the special wire.

I to have asked what the redline temperature is for our turbo and
have not gotten a good answer. On aircraft turbos it can be 1600dg F
on the inlet to the turbo. On a long climb I have seen close to
1400dg F on my TDI with the Upsolute chip, measured before the turbo.
For a while I had thermocouple before and after the turbo to
determine the drop across it. At high temp readings I saw about 250dg
F difference from inlet to outlet.

Hope this helps.

Ray

QuickTD · « **Reply #9 on:** October 25, 2004, 07:25:09 pm »

As long as the 2 connections at the probe end and the 2 connections at the guage end are held at the same temperature the parasitic junction voltages should, in theory, cancel each other out. Temperature differences from one pair of junctions (one end of the extension cable) to the other should have minimal impact. The use of dissimilar metals for the connections can have an influence as well. If you use a copper terminal on the red wire you must use a copper terminal on the yellow. Plating on the terminals also has an effect and soldering is a no-no. With due care your guage can very likely be accurate to +/-25ºF. I might be splitting hairs. I learned about thermocouples and temperature control when working in the machine/process control industry. Quite often a 25º error was significant, not really so with an EGT guage. We always used thermocouple wire and connectors made from thermocouple alloy on any temperature measurement instrument or control, amplified or direct acting.

Mr. Brubaker is correct. The cable length, and therefore the resistance, can have a strong influence on the readings of a direct acting pyrometer. This type of instrument usually has a lead wire made entirely from the junction material which has far more resistance than copper wire. This makes it very sensitive to changes in length. He is also correct in that a powered unit is not as sensitive to this because it reads only junction voltage and not current. Both types of pyrometer are equally sensitive to the effects of parasitic junctions, however.