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FIGURE 2. Simple Temperature Control or Alarm System
4.
Adjust the zero pot until the output voltage is zero ±0.001
Volt.
5.
Return the amplifier to normal state.
Cold Junction Temperature Adjustment
1.
Remove the input thermocouple and replace with a tin
plated copper wire short.
2.
Measure the cold junction temperature with an auxiliary
thermometer accurate to ±0.5°F.
3.
Observe the output voltage with a 4 ½ digit DVM.
4.
Adjust the cold junction pot until the output reads the cold
junction temperature within ±0.5°F.
5.
Return the amplifier to normal state.
Cold Junction Compensation
The temperature of the cold junction is measured with a solid
state sensor that is typically linear to 1°C. This sensor is in
close thermal contact with the amplifier input terminal block
which is the cold junction. The cold junction signal is scaled to
10 mV/°F (or 1 mV/°F) and is added to the amplifier output and
is not effected by changing the gain of the amplifier. This
requires the gain of the amplifier to be adjusted for the same
scale factor as the ambient temperature output when changing
thermocouple types.
A separate ambient temperature output is provided. Note that
this is the temperature of the 470 module and will be a few
degrees above the actual ambient temperature due to its own
heat dissipation.
Span Adjustment
The span, i.e. gain, of the amplifier must be set for the type of
thermocouple being used. Either a known temperature source
and the desired thermocouple, or a low voltage source such
as a thermocouple calibrator may be used. It is generally best
to set the gain near the temperature of most interest to obtain
the best accuracy around that temperature.
First adjust the zero and leave the ambient temperature
output terminal jumper to common. Look in a table on the
thermocouple to be used for the thermocouple voltage at the
temperature of interest. A type K has an output of 11.289 mV
at 532°F, i.e. 500°F above 32°F. The gain required is 5.000V/
.011289 = 442.9. You can then apply .010 Volts to the
amplifier inputs and adjust the gain for an output between pins
1 and 2 of 4.429 Volts, or apply 11.289 mV and adjust for an
output of 5.000 Volts. Remove the ambient output jumper and
connect up the thermocouple.
Shielding Considerations
The Question of Shielding
Several types of electromagnetic interference can be
encountered in industrial plants. Table 1 lists the most common
sources. With un-shielded thermocouple wires the usual
problem is a common mode voltage caused by the electric
field from power distribution lines. The CMR (Common Mode
Rejection) of the amplifier will help to minimize this effect. For
severe cases, caused by very long leads or high electric
fields, it may be necessary to shield the thermocouple wires
with copper braid.
If the thermocouple must be grounded, then a common mode
voltage may be induced because of stray magnetic fields
acting on the large loop formed by the thermocouple wires and
the ground plane. The CMR of the amplifier will be adequate
to prevent measurement errors in most cases. For severe
cases, caused by very long runs or large magnetic fields,
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OMNI-AMP IV
