Feature: T&M
Ensuring the accuracy of non-contact temperature measurements
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By Joris Rols, Marketing Manager Temperature Sensors, Melexis J = ησT4
emperature sensing is found in an ever-broadening range of electronic systems – from consumer, portable and wearable devices, to medical equipment and industrial
instruments. For non-contact measurement, far-infrared (FIR) technology is particularly popular, wherever there is a risk of the sensor elements being damaged by touching extremely hot surfaces or a risk of cross- contamination. Covid-19 has brought on even more
demands for FIR-based non-contact measurement, especially in setups where handheld units are used for temperature checks of patients, but also pupils in schools, shoppers in some commercial centres, and staff in work environments. With the growing demand for
temperature sensing in ever-smaller spaces, there’s been an impact on the sensors’ operational performance, such as accuracy, connectivity and power draw. Also, the effect of thermal shock has become more apparent, which has the potential to impinge on the sensor’s accuracy levels, if appropriate steps are not taken to prevent it.
Non-contact temperature measurement Non-contact temperature measurement is achieved through the application of the Stefan-Boltzmann Law. Te law states that energy radiated per unit surface area of a black body emitter (referred to as the radiant emittance) is proportional to the fourth power of its surface temperature, as shown in Equation 1. Tis allows the temperature of an object to be calculated from the radiation that it gives off.
(1) where J = radiant emittance [W/m2 ], η is
emissivity – a surface property of the sample being measured (for non-metallic materials this value is assumed to be approximately 1), σ is the Stefan-Boltzmann constant – which is 5.67 x10-8
[W/m2 /K4 ], and T is absolute
surface temperature [K]. One of the most effective ways to conduct
non-contact temperature measurement is with a thermopile, which typically consists of several series-connected thermocouples. Combined, they generate a voltage proportional to the temperature difference between two points – thereby allowing the relative temperature to be determined. Melexis thermopile sensors are based
on micro-electro-mechanical system (MEMS) structures, which helps with miniaturisation. Tey consist of a series of hot junctions on a thin, thermally-isolated membrane. Because of the low thermal mass of the membranes, they take very little time to respond to the incident FIR radiation. A temperature differential is measured and a corresponding output generated. Inclusion of a reference thermistor to monitor the ambient temperature means that an absolute temperature figure can be derived.
PCR tests One of the ways non-contact thermometry helps in the fight against Covid-19 is through enabling effective temperature control in the labs performing analysis on polymerase chain reaction (PCR) tests, which detect viruses in biological samples. Te process allows specific DNA to be replicated many times, amplifying it, in order to detect virus DNA; see Figure 1.
22 December/January 2021/22
www.electronicsworld.co.uk Te PCR process consists of three stages.
Te first is denaturation, where exposure to high temperatures (approximately 96°C for about 30 seconds) breaks the sample- DNA’s hydrogen bonds. Tis causes the distinctive double-helix structure to split into two separate DNA strands, which then act as foundation for the amplification. Next, the temperature is reduced and the DNA template strands are annealed, with DNA primer molecules being bound to the complementary areas on the two DNA templates. Precise temperature control is essential here, to ensure that the primers attach properly. Te next step involves elongation, where
polymerase enzymes add nucleotides to the templates/primers to create new double-stranded molecules, resulting in the formation of two identical replicas of the original. Te process is then repeated, with the number of molecules doubling each cycle. In a relatively short time, over a billion DNA molecules can be generated. A thermal cycler unit is used to deliver
the specific temperatures needed for each stage of the PCR process. Tis exposes the specimen tubes (which are held in a thermal block) to a well-defined heat profile, where the temperature at each stage is reached and remains there for a certain duration. Tight control loops are needed to keep the temperature stable throughout the process and ensure unwavering repeatability on each new cycle. Tis calls for meticulous temperature monitoring in real time. When the specimens are replaced
frequently, since logistical pressures mean the PCR processes need to be executed quickly, measurement of the specimen tubes’ temperatures via conventional
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