Voltage Excitation (Current Autorange); Thermal Emf Compensation With Voltage Excitation; Special Sensor Type Configuration - Lakeshore 340 User Manual

5.1.2

Voltage Excitation (Current Autorange)

Several RTD sensor types have a nonlinear temperature response. These sensors require special excitation
so that their signal can be read with good resolution and at the same time not too much power is dissipated in
the sensor. When these sensors are being read by the Model 340, the current excitation is automatically
ranged to keep the voltage across the sensor below that listed in the table above. To illustrate the effect,
power can be expressed by either:
Using these negative temperature coefficient resistors with resistance (R) and a constant current (I)
excitation, the power in the sensor will increase as temperature drops (resistance increases), but with
constant voltage (V) excitation, the power reduces as temperature drops.
Voltage excitation is indicated for several sensors in the table above that should be read using low power.
Other than using an autoranging current source, voltage excitation often requires compensation for thermal
EMF that is described below. Note that thermal compensation will reduce the update rate of these inputs from
20 readings per second to 10 readings per second.
5.1.3

Thermal EMF Compensation with Voltage Excitation

Sensors that require voltage excitation must be operated with little power dissipated in the sensor. To keep
power low, the voltage across the sensor is kept low. There are two major problems that occur when
measuring small DC voltages. The first is external noise entering the measurement through the sensor leads
which is discussed with sensor setup. The second problem is the presence of thermal EMF voltages,
sometimes called thermocouple voltages, in the lead wiring. Thermal EMF voltages appear whenever there is
a temperature gradient across a piece of voltage lead. They can be canceled in the measurement with a
similar temperature gradient in the other voltage lead. Thermal EMF voltages must exist because the sensor
is almost never the same temperature as the instrument. They can be minimized by careful wiring, making
sure the voltage leads are symmetrical in the type of metal used and how they are joined, and by keeping
unnecessary heat sources away from the leads. Even in a well designed system thermal EMF voltages can
be an appreciable part of a low voltage sensor measurement.
The Model 340 can help with a thermal correction algorithm. The instrument will automatically reverse the
polarity of the current source every other reading. The average of the positive and negative sensor readings
will cancel the thermal EMF voltage which is present in the same polarity, regardless of current direction. The
settling time associated with current reversal will reduce the update rate of a corrected input from 20 readings
per second to 10 readings per second.
Thermal correction can be turned off, but it is not recommended because reading errors will increase. The
default setting is ON. Thermal correction can also be paused for a short time. This feature is useful when the
temperature system is stable and the user wants to make delicate experimental measurements with little
disturbance entering the system. The Reverse setting is for diagnostic purposes only. Thermal correction
parameter field will appear on the input setup screen when a sensor type using voltage excitation is selected.
To change the thermal correction parameter to on, off or pause, press the Input Setup key. The input setup
setting screen will appear with the Input letter in the top left hand corner. Use the s or t key to select an
input. Press the Enter key or the Next Setting key to display the input parameters for that input. Press the
Next Setting key until the thermal compensation field is highlighted and then use the s or t key to select on,
off or pause. Press the Next Setting key to continue with more settings or press the Save Screen key to
store the changes in the Model 340. The default setting is On.
5.1.4

Special Sensor Type Configuration

The Model 340 allows sensors that do not match a pre-programmed type. Configure input excitation and input
range selections to meet specific needs. Users may set the temperature coefficient to allow a specially
configured input as a control channel, display units in volts or ohms, and create and select a response curve
for the special input or use a linear equation to compensate for the raw reading.
The Model 340 pre-programmed settings do not include every combination of excitation and input range.
Setting combinations not included in pre-programmed conditions do not have published specifications. The
user must determine input configuration suitability for their application.
5-2
Lake Shore Model 340 Temperature Controller User's Manual
P = I
2
V
2
=
× R or P .
R
Measurement Operation