At my first real job with Tyco (AMP) we used thermocouples till they
failed at the weld (junction). Then we made a new weld just to use them
again. Thermocouples do not work properly if they fail at the weld
(junction). However, they are made from 2 dissimilar semi-conductors.
The heat causes the electrons to move however, they move at different
rates in each conductor wire. The measuring device is calibrated to work
with the specific semi-conductor wires to read the temperature. The
properties of the semi-conductor wires do not change by time or reflow
process temp.
The following is important:
- The connectors must be made from the same material as the thermocouple
wires.
- There must be only one weld, junction for each thermocouple cable. If
the wires overlap in 2 or 3 places then the measurement is being made at
the location closest to the connector.
- Thermocouples with different semi-conductors have different accuracy.
The T-type thermocouple cables are more accurate than the K-type
thermocouples.
- The same type thermocouples vary in accuracy. For example, the same
reading from 2 K-type thermocouples can differ by 2C!
- The measurement equipment accuracy can vary by the temperature range.
The system should be calibrated for the best accuracy at the critical
temperature readings.
Hope this helps,
Paymon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: October 2, 2007 3:00 PM
To: [log in to unmask]
Subject: Re: [TN] Thermocouple Longevity?
I wrote a small treatise on this earlier. Look in the archives. To
summarize; profile your new circuit boards with a new TC the first time
you set the board up.
For daily/weekly process control, make a fixture that clamps the TC
between two metal straps and measure the repeatability of the
temperature. You don't care what the actual temp is, just that it is the
same as yesterday (or last week). Expect your measurements do drop
slightly over time, and when it is out of the process window, time for
recalibration.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Carl Proescholdt
Sent: Tuesday, October 02, 2007 1:43 PM
To: [log in to unmask]
Subject: Re: [TN] Thermocouple Longevity?
Also, consider where the same thermocouple technology is used in other
processes for precise control: Solder Pots, Multilayer presses, etc..
These are expected to be in service for years without changing.
Carl
dca manufacturing
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Tuesday, October 02, 2007 1:37 PM
To: [log in to unmask]
Subject: Re: [TN] Thermocouple Longevity?
We are using the wire insulated with high temp cloth, Kevlar I believe.
We have used them many times. The voltage / temperature values are
unchanged.
This is a fundamental property of the materials. I would not expect to
see subtle drifting.
If the weld is good and the connection at the plug is good, the wires
insulated one from the other up to the weld, the voltage measured at the
ends of the wire should be reliable and temperature dependent.
I have seen failures of the insulation, cracks in the weld, and loose
connections inside the plug produce inaccurate measurements.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Tuesday, October 02, 2007 2:04 PM
To: [log in to unmask]
Subject: [TN] Thermocouple Longevity?
Hi Everyone!
How many reflow profiles do ya'll put the same thermocouples through
before you think about replacing them?
I've typically used them until they quit working or the insulation
deteriorates so bad that I don't want to use them anymore. I was told
that was not a good practice. That your typical K thermocouple starts
drifting after only 4 or 5 thermal cycles. I said; "Really?" I didn't
want to debate a whole lot because it was a customer who said this, so I
just agreed and went on.
So, do any of you know this to be a fact, and do any of you trash a
thermocouple after only 4 or 5 profiles and replace them with new ones?
Kind regards,
Steve Gregory
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