In situ temperature calibration for critical applications near ambient

Abstract

Many agricultural, medical and food storage applications require both accurate and reliable knowledge of the temperature to guarantee quality and enable sound decision making. For example, grape growers incur large costs when they are not able to accurately predict frosts; vaccine and blood products need to be stored within a known temperature range, and the longevity of many food goods depends on stable storage temperatures. Although many systems offer accurate temperature measurements for these applications, these systems are often exposed to conditions that may compromise either the sensor or the measurement equipment, often without the knowledge of the user. To provide on-going confidence in automated temperature logging systems, a low-cost, robust and easy-to-use after-market solution is needed. One such solution is based on organic phase-change materials, which are inexpensive, have low toxicity and are demonstrated to provide highly repeatable transition temperatures. By attaching a small vial of the chosen material to an existing temperature sensor, a brief plateau can be induced as the temperature changes, signalling a known temperature to the control system or user. This repeatable temperature 'marker' provides ongoing assurance that the temperature monitoring system is operating correctly. In this study, several promising materials are trialled in the range of 1 °C to 7 °C. Initial results show the method to be a practicable and affordable addition to critical monitoring and control systems.

Progress towards in-situ traceability and digitalization of temperature measurements

“…This can be monitored in situ by using a miniature phase-change cell (fixedpoint) in close proximity to the measurement junction (tip) of the thermocouple [2]. The fixed point is a very small crucible containing an ingot of metal (or metal-carbon alloy [3] or organic material [4]) with a known temperature. The latest devices developed by NPL are able to accommodate the entire thermocouple and fixed-point assembly within a protective sheath of outer diameter 7 mm; the cell is typically about 4 mm in diameter and 10 mm in length.…”

“…where the integer 𝑚 represents the resonance mode, 𝑛 eff is an index characteristic of the waveguide and 𝐿 is the round-trip length of the loop. The temperature dependence of the refractive index and physical dimensions of the ring enable the use of the device as a thermometer by measuring the temperaturedependent shift in the wavelength given in (4). In practice, the change in refractive index per unit temperature is a factor of approximately 100 larger than the thermal expansion coefficients of the materials involved, so the latter may be ignored.…”

Towards Digitalization of Temperature Measurements

“…This can be monitored in situ by using a miniature phase-change cell (fixed-point) in close proximity to the measurement junction (tip) of the thermocouple [2]. The fixed point is a crucible containing an ingot of metal (or metal-carbon alloy [3] or organic material [4]) with a known temperature. The latest devices developed by NPL are able to accommodate the entire thermocouple and fixed-point assembly within a protective sheath of outer diameter 7 mm.…”

A triple point of water cell-based fixed-point blackbody for radiation thermometry

“…Different materials such as, single-component Ga and its bimetallic eutectic alloys Ga-In (∼15.7 • C), Ga-Sn (∼20.5 • C), Ga-Zn (∼25.2 • C) and Ga-Al (∼27 • C), were investigated in [12] and used as inflight blackbody calibration sources for space-borne radiometers. More recently, several low-cost eutectic organic materials with the phase change temperatures between 1 • C and 7 • C for the in situ calibration of thermometers have been reported in [13].…”