Paul R. Malmberg

Thermistor Temperature Control

A temperature controller using small bead thermistors as temperature sensing elements has been developed. The controller has an over‐all range of +20 to +300°C, or −60 to +190°C, using 51A2 or 32A12 thermistors, respectively, in an ac bridge. Extension of the operating temperature range to other limits is possible using other sensing elements. Linear proportional action is achieved with power relay output by comparing the amplified and rectified bridge signal with a stable and linear sawtooth wave form. This comparison controls the fractional ``on time of the power relay during a periodic ``on‐off cycle. The controller offers stable control under all conditions of unbalance, and is stable to 0.1°C or better over an eight‐day period.

Electronegative Gas Detector

An electronegative gas detector has been developed which is capable of measuring one part of SF6 in 107 parts of air. The operation of the instrument is based on the differences between various electronegative gases in the rates of electron attachment at low electron energies, and on the differences of mobilities of the resulting negative ions in an electric field. The instrument operates at atmospheric pressure (E/p∼1 v/cm‐mm Hg) and uses a modulated ultraviolet light source to produce at a photocathode pulses of photoelectrons which are attached to produce negative ions. The differences in cathode‐anode transit times of the negative ions result in different phase shifts of the observed cathode current. The measured phase shifts serve to indicate the amount of electronegative gas present.