Rick Hamelin on sat 1 feb 03
I have wondered about this for a while now. Is it possible to wire 3
thermocouples (6 leads) onto one rotary (if such a switch as a two pole, 6
line switch exists) or a pair of rotary switches (so to isolate both leads
3 "red" and 3 "yellow" if a single switch is not awailable) and then run a
pair to the pyrometer so that you can keep the probes inplace and simple
adjust the switches to read the zone temperature? Since I don't find this
available, is it because a temperature variation would result from the
switch? Of course the total length of wire must be cut to the pyrometer
calibration. I can only find rotary switches that are a single line neutral
with a single hot lead in and a number of optional hot leads feeding out.
Does anyone have any idea where to find such a switch and what it is called?
Heck, we could become single pyrometer owners with this idea!
Mike on sat 1 feb 03
Rick,
This would most likely effect the accuracy. Following is The Basic Principle
of the Thermocouple.
In 1821, the German Physicist Thomas Johann Seeback discovered that an
electric current flows continuously in a closed circuit of two dissimilar
metals when their junctions are at different temperatures. If the end if a
copper wire and the end of an iron wire first are joined together and
heated, and then the other ends if the wire are connected to an ammeter of
some type, current will flow through the ammeter from the iron wire to the
copper wire.
In practice if you need to extend thermocouple leads you have to use
thermocouple extension wire and where you splice the leads to the wire the
connections should also match.
A type K thermocouple uses Chromel for the positive wire which is yellow and
Alumel for the negative wire which is red. To extend this you would use Type
K extension wire made from the same material and a splice block made from
the same materials. By using a switch you would introduce other types of
metals into the circuit and at the high temperatures associated with kilns
this would effect the accuracy.
Thermocouple outputs are read in millivolts, the recording device then
converts the millivolt reading to temperature, and the conversion is not
linear.
Mike
>I have wondered about this for a while now. Is it possible to wire 3
>thermocouples (6 leads) onto one rotary (if such a switch as a two pole, 6
>line switch exists) or a pair of rotary switches
Sabine Wolf on sat 1 feb 03
> Thermocouple outputs are read in millivolts, the recording device then
> converts the millivolt reading to temperature, and the conversion is not
> linear.
You can switch thermocouples.If you bring in the line other metals you have
a small current on the connection to the new metal and the same current,
just negative, on the connection from the new metal to the metal of the
cable, if both connections have the same temperature, so the sum of both
currents would be zero and don't disturb the voltage of the thermocouple.
From Omega you can get special switches for thermocouples, they have an
isothermal design.
Tschau,
Sabine
Pink Boy on sat 1 feb 03
Mike sez,
> This would most likely effect the accuracy.
> In practice if you need to extend thermocouple leads
> you have to use thermocouple extension wire and
> where you splice the leads to the wire the
> connections should also match.
Pretty much true, because its the temperature
gradient along the thermocouple leads that is
important. AKA the voltage is generated along the
thermocouple leads not at the tip which is really
a dead short.
> A type K thermocouple uses Chromel for the positive
> wire which is yellow and Alumel for the negative
> wire which is red. To extend this you would use Type
> K extension wire made from the same material and a
> splice block made from the same materials.
Standard practice.
> By using a switch you would introduce other types of
> metals into the circuit and at the high temperatures
> associated with kilns this would effect the
> accuracy.
But if there is no temperature gradient in the switch
then there is no voltage generated and no effect
on accuracy. In practice I'd expect the error to
be small.
> Thermocouple outputs are read in millivolts, the
> recording device then converts the millivolt
> reading to temperature, and the conversion is not
> linear.
Actually micro-volts. Millionths of a volt. Very
tiny votages.
What has me worried is that if you use a switch to
slect which thermocouple the controller is using
then you risk forgeting to select the right
thermocouple, or someone absentmindedly flipping
the switch. What happens then is the controller
runns away and over heats the kiln.
Me, I'd use two controllers...
Mr Foo
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