Keithley 3720 User Manual page 111

Series 3700 System Switch/Multimeter User's Manual
Open thermocouple detection
Long lengths of thermocouple wire can have a large amount of capacitance that is seen at the
input of the DMM. If an intermittent open occurs in the thermocouple circuit, the capacitance can
cause an erroneous on-scale reading. The Series 3700 has an open thermocouple detection
circuit. When enabled, a 100PA pulse of current is applied to the thermocouple before the start
of each temperature measurement.
NOTE
The Series 3700 open thermocouple detection works in similar fashion to the open lead
detection. Refer to
5-30). The open thermocouple detection performs as follows:
x
x
x
x
Document Number: 3700S-900-01 Rev. A / August 2007
Default condition is dmm.opendetector = dmm.ON.
Effects of open inputs on ohms readings
VMEAS open voltage: If Input HI V
and return an overflow reading.
A calculated measurement, outside of the ranges contained in the following table, will
cause the "Overflow" message to be displayed.
Type
J
K
N
T
E
R
S
B
If during a measurement cycle, with dmm.opendetector = dmm.ON, the ohm's function
100PA I is pulsed on and off before the start of each measurement. The A/D will
SOUR
monitor V for 0.8msec. During the I
MEAS
input voltage is greater than 120mV, the A/D will abort in <100Psec and return an
overflow reading. If <1.15k: is detected and the input voltage is in the range of r120mV,
the open lead detection current is turned off and a normal thermocouple temperature
measurement is performed (see
I open voltage with dmm.opendetector. A hardware detector is used to continuously
SOUR
detect for open input lead. The hardware detector uses a comparator circuit to monitor
the voltage on the ohm I
voltage on I V
SOUR OPEN-HI-LEAD
and return an overflow reading. The following table notes timing with dmm.opendetector
= dmm.ON.
Section 5: Basic Digital Multimeter (DMM) Operation
is outside r120mV, the A/D will abort in <100Psec
MEAS
Range
-200°C to +760°C
-200°C to +1372°C
-200°C to +1300°C
-200°C to +400°C
-150°C to +1000°C
0°C to +1768°C
0°C to +1786°C
+350°C to +1820°C
, if the resistance >1.15k: is detected, or the
ONPHASE
Thermocouple connections
V terminal. If during a measurement cycle, the input
SOUR OPEN-HI-LEAD
terminal is greater than 7.1V, the A/D will abort in <100Psec
(see "Open lead detection" on page
(on page 5-39)).
5-37