Capacitance Temperature Sensors
2Pages

Capacitance Sensors Capacitance Temperature Sensors* Capacitance features DD Virtually no magnetic field-induced errors DD Capable of mK control stability in the presence of strong magnetic fields DD Monotonic in C versus T to nearly room temperature * Patent #3,649,891, exclusively assigned to Lake Shore Cryotronics, Inc. Temperature reproducibility Over a period of days, thermal cycling of capacitance sensors can provide variations in their capacitance/temperature values equivalent to several tenths of a degree at 4.2 K, 77 K, and room temperature. Over longer periods of time, variations can reach one degree or more. However, any reduced capacitance, C(T)/C(4.2 K), is generally stable to within ±0.5 K. These variations, or shifts, in the temperature response curve have no effect on the sensor’s stability when operating at a given temperature and, therefore, do not impair the sensor’s primary function as a control element. Capacitance sensors (CS) are ideally suited for use as temperature control sensors in strong magnetic fields because they exhibit virtually no magnetic field dependence. Displacement current is not affected by magnetic fields. Consequently, temperature control fluctuations are kept to a minimum when sweeping magnetic field or when changing field values under constant temperature operation. Because small variations in the capacitance/ temperature curves occur upon thermal cycling, calibrations must be transferred to the capacitor from another sensor after cooling for the best accuracy. It is recommended that temperature in zero field be measured with another temperature sensor and that the capacitance sensor be employed as a control element only. Temperature stability/ temperature transfer accuracy Capacitance sensors will provide very stable control conditions for long periods of time at operating temperature, but because an operational “aging” phenomenon exists, care must be taken to account for this occurrence in their use. The variation in capacitance/temperature characteristics is likely the result of the time dependence of the dielectric constant and the dielectric loss, or “aging”, that all ferroelectric dielectrics exhibit. This time dependence, which occurs as a short term drift (minutes to hours) in capacitance/temperature value, is initiated by disturbing the sensor thermally or by changing the voltage or frequency of excitation. To compensate for this, the sensor should be stabilized for one hour after initial cool-down to desired operating temperature and whenever significant adjustments in control temperature are made. After the one hour stabilization, this short-term drift is on the order of a few tenths of a millikelvin per minute at 4.2 K, and several millikelvin per minute at 305 K. The drift is always in the direction of decreasing capacitance; consequently, it corresponds to decreasing temperature below 290 K. Typical CS dimensionless sensitivity Lake Shore Cryotronics, Inc. | t. 614.891.2244 | f. 614.818.1600 | e. info@lakeshore.com | www.lak

Capacitance Sensors Packaging options Standard curve Not applicable Minimum limit For more information on sensor packages and mounting adapters, see page 21. Accuracy (interchangeability) Not applicable Accuracy (calibrated) Calibration should be performed in situ Recommended excitation 1 to 5 kHz, 0 to 7 V peak to peak or any other acceptable capacitance measuring method Typical magnetic field-dependent temperature errors1 ΔT/T (%) at B (magnetic induction) Package parallel to field B Dissipation at recommended excitation Not applicable Expected long-term stability ±1.0 K/yr Thermal...