Date:         Sun, 28 Dec 2003 22:42:45 -0700
Reply-To:     Keith Hughes <keithahughes@QWEST.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Keith Hughes <keithahughes@QWEST.NET>
Subject:      What makes a pyrometer's cable so special?
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Well David, it depends;

If it's a thermocouple, then you need to use thermocouple
extension wire (of the same type, "T", "J", "K", "R", "S", etc.)
and of sufficient gauge to minimize the resistance. The affect of
a few feet of, say, 16-18ga extension wire is negligible on the
calibration. However, if you use regular copper wire, then you're
creating a secondary thermoelectric junction. The thermoelectric
potential generated would then result from the temperature
differential between the TC tip and the connection point to the
extension wire - *not* between the "meter" (where the zero
junction compensation is measured) and the tip. The resulting
error is the difference between the ambient temperature at the
meter, and that at the connection to the extension cable (plus
some minor thermoelectric offset caused by the additional junction
at the meter terminals).

If it's a thermistor (very likely for inexpensive units), the
measurement element is a semiconductor whose resistance decreases
(semilogarithmically) with temperature (i.e. very high - many
kohms typically - at ambient, decreasing to tens of ohms, or less,
at operating temp). For these units, any extension wire will
simply result in a nonlinear error that increases exponentially
with temperature. The error is basically zero at ambient temp, and
increases dramatically at higher temperatures. This type of sensor
is used frequently for low accuracy, low cost applications, since
unlike a thermocouple, it is a direct measurement (no cold
junction compensation is required).

If it's an RTD (resistance temperature detector), it will almost
certainly be a 2-wire 1000ohm nickel RTD. For this type of sensor,
addition of extension wire is of no consequence at all, as the
basic sensor resistance is so high, and the change in resistance
per unit temperature change (per unit basic probe resistance -
i.e. Ohm/Ohm/°C) is quite small.

Just measure the disconnected probe. If there's basically no
resistance, it's a TC. If it's a nice round number like 100, 200,
or 1000ohms, it's likely an RTD, and if its very high 4.5Kohms for
example, it's a thermistor.

Keith Hughes


> Date:    Sun, 28 Dec 2003 12:37:51 -0800
> From:    David Marshall <vanagon@VOLKSWAGEN.ORG>
> Subject: What makes a pyrometer's cable so special?
>
> I just bought a VDO Pyrometer for a customers 1.9TD installation.  VDO
> stressed to me that the cable can not be lengthened and if you need a longer
> cable than what comes with the kit I must order one that length.
>
> So, I ordered the gauge and a 14' cable that goes between the sensor on the
> exhaust manifold and the gauge.
>
> The cable that came with the gauge and the additional 14' long cable is two
> conductor, looks like aluminium 16 ga wire for each conductor.  There is
> nothing [to me] that appears to be 'special' about this cable other than the
> freaking price that I paid for it.  There is no shielding and the plastic
> covering that covers the two wires seems to be a little extra tough.
>
> Can someone enlighten me about what I am missing here!
>
> Also, on a semi-related note, how hot is too hot for a Turbo Diesel motor?
>
> Thanks in advance!
>
> David Marshall