"
Specifications
Low Pass Filter
The output of the amplifier is sent through a 2-pole low pass
active filter. Since temperature changes are usually quite
slow, it is advantageous to filter out any high frequency noise.
This is valuable if the output signal is used to operate
comparators for alarm or control purposes. If the output noise
is kept low, the alarm or control comparators can be set very
precisely without fear of set point chatter or having to rely on
an excessive amount of comparator hysteresis. The Model
470 pole frequency is approximately 1.5 Hz.
Power Supply
The power supply operates from 115 VAC 50-60 Hz (a 230
VAC version is available) and utilizes a split bobbin isolated
transformer. Thus the common of the amplifier may be floated
above earth ground if desired. The power supply is
conservatively designed and provides regulated power for the
amplifier and filter plus a very stable reference voltage for the
cold junction compensation circuit. An external line fuse of 1/
8 Amp may be used for protection.
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.
Application Information
General Information
As delivered, the zero and cold junction adjustments have
been set. The span adjustment has been set up for a type K
thermocouple and a scale factor of 10 mV/°F (unless 1 mV/ °F
has been ordered). The thermocouple to be used is connected
to the + and - terminals using either bare wire or a lug
termination. Polarity should be observed. The table below
lists thermocouple wire color coding.
Zero Adjustment
In case the zero or cold junction calibrations are disturbed,
follow these adjustment steps after allowing 30 to 45 minutes
for warm-up.
1.
Remove the input thermocouple and replace with a tin
plated copper wire short.
2.
Jumper the ambient temperature output terminal to
common.
3.
Observe the output voltage with a 4½ digit DVM.
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