Nordson 303 779B, Руководство пользователя по изделию

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Profile Verification with Thermocouples
C 5-4
E
1998 Nordson Corporation
All rights reserved
303 779B
Issued 11/98
Manual 12-44
A thermally conductive and electrically insulated bond between the
thermocouple junction and the board surface is required. If the
thermocouple tip is not in contact with the surface at all points along its
junction length, convective cooling of the tip will occur, and it is possible
that temperature readings will be false.
The elevated temperatures of the oven promote accelerated oxidation of
thermocouple wire materials. Exposed wire is quickly deteriorated, which
results in a short life for the thermocouples. One way to prolong the life
of thermocouples is to eliminate exposure of bare wire to the air. To
achieve this, strip the thermocouple leads so there is minimal bare wire
showing. Cover any bare wire with the epoxy used to bond the junction
to the board.
The adhesive recommended for bonding thermocouples to the board is a
one-component thermally conductive, electrically insulating epoxy
available from Master Bond Inc. in Hackensack, New Jersey, USA. It is
specified by formula number EP3HTAOSM.
The following are common sources of error when working with
thermocouples.
The easiest malfunction to detect and correct is a poor junction
connection. Replace any junction if it appears to be open or damaged. If
the junction appears to be in good shape, though, further investigation
will be required.
A much more difficult malfunction to detect is de-calibration. Diffusion of
atmospheric particles into the metal, or annealing caused either by high
temperatures or cold working from bending the wires causes a shift in the
voltage output of the thermocouple. The portion of the wire that is most
susceptible to annealing is the area just behind the thermocouple tip. To
solve the problem, a section of the wires leading up to the thermocouple
junction should be removed with the junction.
After many tests with the same thermocouple wires, the insulation will
become brittle and start deteriorating. Since the wire is relatively long
with a small diameter, the series resistance of the wire is quite high.
When the insulation is degraded, the leakage resistance may be low
enough to complete the circuit path, and an incorrect voltage output will
be recorded. The best way to prevent this is to replace the thermocouple
wires periodically when they look worn.
6. Thermocouple Mounting
7. Sources of Error
Poor Junction
De-calibration
Shunt Impedance