Oxford Instruments Mercury iTC, Handbook

Page: 66 / 146

Handbook
November 2011 / 59-UMC0071 / Issue 01
Mercury iTC
Page 66
FUNCTIONAL DESCRIPTION
Original Instructions
4.4 Overview of temperature measurement
4.4.1 Measurement principles
The iTC uses a highly accurate measurement circuit to measure the voltage produced
by a sensor.
4.5 Types of temperature sensor
The iTC can use several types of temperature sensor:

Thermocouples
A thermocouple comprises two junctions of dissimilar metals held at different
temperatures. This acts as a voltage source, based on the Seebeck effect, the output
voltage of which increases with increasing temperature difference. The iTC contains
cold-junction compensation, so a reference junction is not required.

Metallic resistance thermometers
The resistance of metallic resistors increases with increasing temperature due to the
increase of scattering events of the conduction electrons. Over much of the
temperature range the relationship is approximately linear until, at low temperatures,
impurity scattering becomes significant and the sensor sensitivity flattens off. The iTC
passes a constant current through the sensor and measures the voltage produced,
using a 4-wire measurement. Resolution to 1 mK is possible at low temperatures, if a
suitable resistor and energisation current are used.

Semiconductor resistance thermometers
The resistance of semiconductor resistors decreases with increasing temperature.
The relationship is non-linear. The iTC uses a 4-wire connection, applying a constant
voltage across the resistor. The iTC then measures the current, which is used to
calculate the temperature. The current through the resistor increases with
temperature and its relationship to the temperature is more linear than the resistance
of the resistor. Using a constant voltage ensures that power dissipation in the resistor
decreases as the temperature decreases. Resolution to 0.1mK is possible at low
temperatures.

Semiconductor diodes
These devices produce a voltage that decreases as the temperature increases. The
iTC passes a constant current through the diode and measures the voltage
produced. Resolution to 1mK is possible at low temperatures.
The iTC includes calibration curves for the following sensor types.
Table 4-6 Sensor range data
Code Sensor Range
Lin
1
Linear range 0-1677.7
Null Ref:[
1]
Centre zero -838.9 to +838.9
Con 1 Conductance 0 to 20
TG 5 AuFe 0.03/Chr 2 to 500K
TG 5 AuFe 0.03/Chr 2 to 500K