An Intelligent Approach to Eliminating Thermal Disturbances in Non-contact Temperature Measurement

October 1, 2019 | 09:00
"hearables" - devices worn inside the ear
"hearables" - devices worn inside the ear
Author: Joris Roels – Marketing Manager Temperature Sensors, Melexis
 
As we become more conscious of our health and our environment, temperature sensing is becoming increasingly relevant. As a result, it is a function now being added to many devices, including health monitoring devices in the form of medical body thermometers and smart wearables.
 
Non-contact temperature sensing relies on detecting energy emitted in the infrared wavelength region. Every object emits energy in this way, which can be measured to calculate its temperature. However, as the sensing devices behind this get ever smaller, they become more susceptible to the impact of thermal shocks, which can induce measurement error and thermal noise.
 
In this technical article, Melexis discusses some of the principles behind non-contact temperature sensing as well as the approaches used to minimize the effects of thermal shock. The article then looks at a new and intelligent approach to eliminating the effects of external thermal disturbances in micro-miniature sensors.
 

Integrated MEMS Thermopile Technology

Thermopile temperature sensing technology is increasingly being used in medical (including home healthcare) and industrial applications, as it is robust, accurate and reliable. A thermopile is simply an electronic transducer that converts thermal energy into an electrical signal and works on the principle that everything emits thermal far-infrared (FIR) radiation.
 
Electrically speaking, a thermopile is comprised of several thermocouples connected in series. Together, they generate a voltage that is proportional to the temperature difference between two points; this difference gives a relative temperature measurement.
 
A MEMS thermopile sensor uses a thin, thermally isolated membrane.  As this has a low thermal mass, it is rapidly heated by incoming heat flux, consequently creating a temperature differential that the thermopile can report as a temperature difference. By incorporating a reference thermistor into the MEMS system, an absolute temperature measurement can be generated.


Figure 1: Basic construction of a MEMS thermopile-based sensor

At the heart of this measurement technique is the Stefan-Boltzmann Law that states the energy radiated per unit surface area of a black body is proportional to the fourth power of its temperature. This is generally expressed as:-
 
J = η σ T4

Where:
J = radiant emittance [W/m2]
η = emissivity (surface property)
σ = 5.67e-8 [W/m2/K4]
T = absolute surface temperature [K]
 
Making the reasonable assumption that for non-metallic materials the emissivity (η) is approximately 1, the surface temperature can be tied to emitted power.
 
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