VTI Instruments EX1000A, User Manual

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VTI Instruments Corp.
32 EX10xxA Introduction
temperature at a slightly faster rate than the thermal mass (and thus cold junction measurement). In
this case, the channel measurements will show a transient error of negative polarity. Conversely, if
the ambient temperature drastically falls, the channel measurements will show a transient error of
positive polarity. In either case, as the rate of ambient temperature change decreases, thermal
equilibrium is regained, and the error is eliminated. This error mechanism is not affected by
calibration and is avoided by allowing for an appropriate thermal equilibrium delay.
NOTE Significant ambient temperature changes should be followed by a thermal equilibrium delay before
the initiation of measurements.
Initial insertion of a jack / screw terminal connection
A similar error mechanism is introduced when a male thermocouple jack / wire connected to screw
terminal is first inserted into an EX10xxA/RX10xx input channel. A transient error is generated
because the male thermocouple jack and mating female jack on the EX10xxA are at different
temperatures when they make first contact. If both the unit and male jack are at the same ambient
temperature, this error typically dissipates to less than 0.1 ºC within 3 minutes. However, a jack at
room temperature plugged into a unit that is at a substantially different ambient temperature will
generate a significantly larger initial error that takes longer to decay to a level of insignificance. This
error mechanism is not affected by calibration and is avoided by allowing for a thermal equilibrium
delay after initial jack insertion.
NOTE Initial jack insertion should be followed by a thermal equilibrium delay before the initiation of
measurements.
Insertion of jacks / screw terminal connection into neighboring channels
A less obvious source of thermal destabilization is the insertion of jacks into neighboring channels.
Because each CJC sensor is shared among four input channels, there is a thermal disturbance seen
by it when jacks are inserted into any of the four channels that it is monitoring. The disturbance
occurs because the newly inserted jack represents a thermal mass that is at a different temperature
than the internal thermal mass. This instantly lowers the measured CJC temperature to a small
extent. Since the same CJC temperature is used for three other channels, they exhibit a small
transient error of negative polarity as a result. The magnitude of the error is proportional to the
quantity of jacks that are inserted at a time, with three additional jacks being the worst-case scenario.
The error decreases with time, as thermal equilibrium is gradually restored. Empirical testing has
shown that the worst-case error typically dissipates to less than 0.1 ºC within 3 minutes. This error
mechanism is only present on channels that share a CJC sensor. Moreover, it is essentially not
present when jacks are removed, as no thermal disturbance is created. This error mechanism is not
affected by calibration and is avoided by allowing for a thermal equilibrium delay after jack
population changes. For reference, the association between CJC channels and input channels is
shown in Table 1-3.
NOTE Jack population changes should be followed by a thermal equilibrium delay before the initiation of
measurements.
CJC # Channel
Input
Channel
CJC0 48 0-3
CJC1 49 4-7
CJC2 50 8-11
CJC3 51 12-15
CJC4 52 16-19
CJC5 53 20-23
CJC6 54 24-27