Data Translation DT9871, User Manual

Page: 93 / 134

Principles of Operation
93
Out of Range Data for Thermocouple Channels
Each thermocouple type corresponds to an allowable voltage range. If a voltage is measured
on the input that is outside of the legal range for the selected thermocouple type, the channel
may be configured for the wrong type of thermocouple or something other than a
thermocouple may be connected to the channel.
For channels configured with a thermocouple type of None (voltage), the Limit (LMT) LED on
the rear panel of the instrument lights to alert you when the voltage is out of range; see
Figure
10 on page 37 for the location of this LED.
For channels configured with a thermocouple type other than None (voltage), the LMT LED
lights when the temperature limit is out of range for the specified thermocouple type.
In addition, if the input voltage is less than the legal voltage range for the selected
thermocouple type, the software returns the value TEMP_OUT_OF_RANGE_LOW (–88888
decimal). If the input voltage is greater than the legal voltage range for the selected
thermocouple type, the software returns the value TEMP_OUT_OF_RANGE_HIGH (88888
decimal).
Note: If you are continuously measuring from a properly configured thermocouple input
channel and the thermocouple opens or becomes disconnected, the open thermocouple
pull-up circuit causes the input voltage to rise to approximately 0.7 V over a few seconds.
In this case, the temperature value rises very quickly, and you will receive the
TEMP_OUT_OF_RANGE_HIGH (88888 decimal) value followed by the OPEN_SENSOR
(99999 decimal) value. In this case, the OPN LED lights when the open thermocouple is
detected and the LMT LED lights when the temperature limit is out of range for the
thermocouple type.
Table 4: Supported Input Ranges
Instrument
Type Models Input Range
TEMPpoint DT9871U ±0.75 V for all channels
DT9871 ±1.25 V for all channels
DT9872 ±1.25 V for all channels
VOLTpoint DT9873 ±10 V or ±60 V
(software-selectable for each channel)
a
a. Note that the maximum input voltage for the ±60 V range cannot be more than 30 Vrms, 42.4 Vpk,
60 VDC.
MEASURpoint DT9874-16T-16R-16V ±0.75 V for channels 0 to 15;
±1.25 V for channels 15 to 31;
±10 V or ±60 V for channels 32 to 47
(software-selectable for each channel)
a