Design, development, and qualification tests of prototype two-channel cryogenic temperature transmitter

Abstract

A two-channel prototype cryogenic temperature transmitter is developed using an ARM Cortex-M3 series precision analog microcontroller using a rapid prototyping method for use in indigenous developments. The developed prototype utilizes an Arduino compatible baseboard equipped with interfaces for programming/debugging. Additional circuits are fabricated, and embedded application software is developed and tested. The input circuit consists of a low-value high accuracy precision current source to excite the cryogenic temperature sensors of Resistance Temperature Detector (RTD) type and employs a standard four-wire ratiometric measurement technique for accurate resistance measurement. The ratiometric measurement eliminates measurement errors due to current uncertainty. The precision microcontroller is equipped with internal programmable gain amplifiers to accurately scale low-level analog signals from cryogenic temperature sensors. The developed transmitter can interface with two cryogenic RTDs (Cernox® and PT-100 types) and can accurately measure resistance over its calibrated range (300–4 K). The cubic spline interpolation method is employed in application software for converting the measured resistance to temperature. The measured temperature is transmitted to a programmable logic device (via 4–20 mA signals) using the pulse width modulation technique. The developed transmitter is tested for its performance against commercially available transmitters at the liquid nitrogen temperature, liquid helium temperature, and over the entire measurement range using Gifford–McMahon type cryocoolers. The developed transmitter was utilized to assess the impact of the thermal resistance of the cryogenic sensors at the lowest temperature of the cryocooler (∼2.6 K). This paper outlines design details, application software development, experimental setup, measurement uncertainties, and test results.