terrarium heat cable

[Spehro Pefhany]

Measurement wire is used only for a few cm where the heat gradient is the
highest. Its alloy is accurately controlled to produce the exact seebeck
coefficient. Extension wire can be of much poorer quality, since there is
very little voltage produced across it.

Not just "quality", it's actually a totally different alloy in the
case of precious-metal T/C's. But note that there can be 50 or 100°C
difference between the "head" temperature and the isothermal block
temperature at the instrument, and the compensating leadwire must not
cause undue inaccuracy in the temperature reading due to that
temperature delta. Obviously if you eliminate the T/C itself and short
at the head terminal block the extension wire will not add any
additional error (i.e. will read the head temperature with the same
accuracy).


Best regards,
Spehro Pefhany

 

[John Fields]

They keep saying "wire" where *cable assembly* or *insulation*
would better reflect what they were trying to say.
The metal alloy is specified by the thermocouple type
and the range of deviation allowed is narrow.

---
The insulation really has nothing to do with it; it's the accuracy
of the junction at higher temperatures which differentiates the two,
AIUI. That is, there is thermocouple grade wire which is used to
make the working junction and there is thermocouple extension grade
wire which is used to make the run from the thermocouple to the
instrumentation/cold junction reference.

 

[John Fields]

They *may* be the same except for insulation. They certainly are not
the same for precious-metal thermocouples. They are not guaranteed to
be the same. For temperatures that lizards can survive at, it's
unlikely to make a lick of difference.

---
LOL, in this application the only thing that matters is the
resistance per unit length (and, possibly, the thermal and
moisture-absorbing characteristics of the insulation) since it'll be
used as a heater!

What I was being nit-picky about was JeffM's statement that
""Extension cable" is the same as thermocouple wire.", which it
_may_ be, but certainly doesn't _have_ to be.

 

[Roger]

A simple solution that reduces the long length of resistance wire
required for 120V is to just use a series of low value resistors
wrapped in heat shrink tubing. To achieve 15W out of 120V would require
960R. Depending on the surface area to cover you would have to work out
the number of resistors to make this value. Say 10 x 100R for
discussion sake, then each resistor would be required to dissipate 1.5W
so 2W devices could be used (These would end up quite chunky) The more
resistors the lower the power rating required for each.

You have not mentioned any sort of thermal control or shutoff
requirements so I would definately hope that you do not have anything
rare/expensive in your terrarium. Thermal switches are available but
may be hard to locate for the temp range you would probably be looking
at. Otherwise some simple circuitry would do the trick.

Cheers