Thermocouple query

[N_Cook]

Well assuming thermocouple. Ancient small kiln with simple T/C feeding
analogue ammeter. Ammeter seems ok , 1.6mA reads 600 deg C on the scale,
2.7mA reads 900 degC. The original T/C broke at the weld. Control of the
heater is just by timer.
Got some new T/Cs which seem to work ok in themselves, rising to about
8mV on DVM-V for 300 deg C hot air. But try them with the ammeter and no
observable movement of the needle, either way round . The metal of the
conductors on the original is much larger diameter of 1.5mm each, than
the new ones both contained within 1.5mm stainless sheath, that type is
Nc/Na, not matching original type? which would have been ?
Original has different patination on each conductor so dissimilar metal
is as far as I know.
The ammeter, should anyone have any info is HOYT D4735P-1 measuring
about 10.3 ohms across the terminals with nothing connected to it.
Furnace is Jelenco or Jelrus, New Hyde Park, model LFC or LFCSP

 

[N_Cook]

as one conductor is ferrous to some extent then probably J-type .
Probably makes more sense to go to K-type and cross-calibrate a DVM. The
original meter is pretty knackered anyway.

 

[Ralph Mowery]

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:mdf4mm$g4p$1@dont-email.me...

as one conductor is ferrous to some extent then probably J-type .
Probably makes more sense to go to K-type and cross-calibrate a DVM. The
original meter is pretty knackered anyway.

It is probably not a J type as the temperature ranges seems high for that
type. Ohm meter readings are almost useless except to determine if the TC
is open or not. I don't know why you are referring to a millamp as they are
usually rated for milliVolts. The diameter of the material should not have
any real effect on the measurment.

They probably shipped you the wrong type of TC. As you should know the
differant types put out differant voltages for the same temperature.

Company I worked for was shipped in about 25 J type for a lagre building
project. About half of them had the red and white coloring reversed. Took
a short period of time to determin why they would not work.

 

[N_Cook]

On 07/03/2015 16:15, Ralph Mowery wrote:

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:mdf4mm$g4p$1@dont-email.me...
as one conductor is ferrous to some extent then probably J-type .
Probably makes more sense to go to K-type and cross-calibrate a DVM. The
original meter is pretty knackered anyway.

It is probably not a J type as the temperature ranges seems high for that
type. Ohm meter readings are almost useless except to determine if the TC
is open or not. I don't know why you are referring to a millamp as they are
usually rated for milliVolts. The diameter of the material should not have
any real effect on the measurment.

They probably shipped you the wrong type of TC. As you should know the
differant types put out differant voltages for the same temperature.

Company I worked for was shipped in about 25 J type for a lagre building
project. About half of them had the red and white coloring reversed. Took
a short period of time to determin why they would not work.

Looking at a comparison plot of types, J type output is a bit less than
K so I don't see why I was getting no needle movement with the K type
one to 300 deg C. I was expecting to just replace the T/C with some sort
of T/C and rescale the meter. From that plot the linearity is better for
K type than J, but the ammeter scale is thoroughly linear, perhaps only
used as a rough guide.
I could only find J in the type listings that used iron as one conductor.

 

[Fred McKenzie]

In article <mdf316$9k4$1@dont-email.me>, N_Cook <diverse@tcp.co.uk>
wrote:

> The original T/C broke at the weld.

Re-Weld the original T/C. I found several discussions on the web
including
<http://reprap.org/wiki/WeldYourOwnThermocouple>.

Fred

 

[N_Cook]

On 07/03/2015 17:26, Fred McKenzie wrote:

In article <mdf316$9k4$1@dont-email.me>, N_Cook <diverse@tcp.co.uk
wrote:

The original T/C broke at the weld.

Re-Weld the original T/C. I found several discussions on the web
including
http://reprap.org/wiki/WeldYourOwnThermocouple>.

Fred

I'll look there but I'm assuming you need >50 amps from other than a car
battery.
I repeated with the type-K and problem must have been from poor contact
from corrossion on the nuts of the meter terminals, different materials
to match the different wires. Scale reading about .75 what is should be
for touching with a temp controlled soldering iron, which is about the J
to K type difference.

 

[gyro_john]

When you say that 'it broke at the weld', do you mean the point where the two dis-similar metals are welded together to form the thermocouple junction?

Because you can re-weld that with oxy-acetylene. With K-type and J-type anyhow. Here's how:
- Clean the ends of the two dis-similar metal wires.
- Twist or fold the ends over each other.
- Set up a hammer and a piece of flat metal to use for an anvil.
- Get a small flame going and gently heat up the joint of the two wires. You want them to glow dull to medium red. If you get them too hot and one of the wires melts off, start over.
- When you've got them glowing nicely, quickly put the torch down, position the hot joint close to the anvil and hit it good and hard with the hammer. You'll make a flattened lump with two wires sticking out of it, and there's your new thermocouple, freshly forge-welded.

I've done this several times. It works!