Nordson 303 779B, Руководство пользователя по изделию

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Profile Verification with Thermocouples
C 5-2
E
1998 Nordson Corporation
All rights reserved
303 779B
Issued 11/98
Manual 12-44
Probably the most common and most obvious source of error in
temperature measurements is a result of a poor thermocouple joint. A
thermocouple generates voltage at the junction of two different types of
wire. This voltage changes with the temperature of the junction. If there
is more than one junction, a voltage is generated at each one. Or, if the
junction is contaminated, false readings may be obtained.
Three methods are widely used to join thermocouple wires. These are
soldering, welding, and twisting.
The problem with soldering is that you must use a solder with a high
melting point. Otherwise the solder will melt during the process and the
thermocouple junction will separate. Also, the introduction of a solder
alloy may create a new voltage function or cause leaching of the
thermocouple wires. Do not solder the thermocouple junction.
Welding of the thermocouple tip must be done in an inert atmosphere to
prevent oxidation. Also, overheating can degrade the thermocouple wire.
Commercially manufactured thermocouples have a good strong junction
created using an expensive capacitive discharge technique.
A quick and easy method to make a good thermocouple junction is
twisting the thermocouple leads. The main objective when twisting the
thermocouple wires is to have a good tight junction, while leaving as few
twists as possible. One way to ensure a tight junction is to place a tiny
object, such as a pin between the two wires and twist the wires tightly
together, and then remove the pin. Trim excess wire twists for a smaller
joint. Since temperature is measured all along the junction, the entire
junction must be in contact with the board surface, and it is much easier
to achieve full contact with as few twists as possible.
Zero the thermocouple before it can be used for testing, unless you are
using commercially manufactured thermocouples which are in matched
sets. There should be a zeroing adjustment for each channel of the
recording instrument. Zero the thermocouple junction by placing it in an
ice bath made up of mostly ice and very little water. Then the zeroing
control can be adjusted until the output reads 0
°
C (32
_
F). Ice point
calibrators are available commercially; however, they are quite
expensive.
Calibration can be done only if a known elevated temperature source,
such as an oil or sand bath is available. Most modern recording
instruments have pre-calibrated inputs which match a certain type of
thermocouple. This eliminates the need to calibrate the instrument.
3. Thermocouple Junction
Soldering
Welding
Twisting
4. Calibration and Zeroing