Keithley 2750, User Manual

Page: 118 / 452

Model 2750 Multimeter/Switch System User’s Manual Basic DMM Operation 3-27
External reference junction
For switching modules that do not have built-in sensors to measure temperature, each
module can use a thermistor or 4-wire RTD to acquire the reference temperature. Connect
a thermistor to channel 1 or connect a 4-wire RTD to channel 1 and its paired channel.
Position the temperature transducer near the terminals for the channel(s) being used to
measure temperature. Be sure to electrically insulate the transducer leads to keep them
from making contact with other conductors.
When you close channel 1 to measure the cold junction temperature, that temperature
reading will be used to calculate the temperature when you close a thermocouple channel.
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 could cause an erroneous on-scale reading.
The Model 2750 has an open thermocouple detection circuit. When enabled, a 10µA pulse
of current is applied to the thermocouple before the start of each temperature measure-
ment. If >12k
Ω
is detected (open thermocouple), the OVRFLW message will be displayed.
If <12k
Ω
is detected, the current is turned off and a normal thermocouple temperature
measurement is performed.
NOTE
Channel average cannot be used with thermocouple temperature measurements
if open thermocouple detection is enabled.
Thermistors
For thermistors, the temperature measurement range is -80°C to 150°C (0.01°C resolu-
tion). Thermistor types that are supported include the 2.2k
Ω
, 5k
Ω
, and 10k
Ω
types.
The thermistor is a temperature sensitive resistor. Its resistance changes non-linearly with
changes in temperature. Most thermistors have a negative temperature coefficient. As tem-
perature increases, the resistance decreases. The Model 2750 measures the resistance of
the thermistor and calculates the temperature reading.
Of all the temperature transducers, the thermistor is the most sensitive. It can quickly
detect minute changes in temperature. It is a good choice when measuring very small
changes in temperature. The downside for this increased sensitivity is the loss of linearity.
Since they are especially non-linear at high temperatures, it is best to use them for mea-
surements below 100°C.
NOTE Curve fitting constants are used in the equation to calculate thermistor tempera-
ture. The thermistor manufacturer’s specified curve fitting constants may not be
exactly the same as the ones used by the Model 2750.
“Thermistor equation,”
page F-6 , provides the equation and the constants used by the Model 2750. It
also explains how to select a thermistor when the manufacturer’s constants and
the ones used by the Model 2750 do not match.
2750-900-01.book Page 27 Wednesday, August 3, 2011 7:56 AM