Validity of contact skin temperature sensors under different environmental conditions with and without fabric coverage: characterisation and correction

Abstract

Contact skin temperature (T) sensors are calibrated under uniform thermal conditions but used in the presence of a skin-to-environment temperature gradient. We aimed to characterise the validity of contact T sensors when measuring surface temperature under a range of environmental and fabric coverage conditions, to estimate practical temperature limits for a given measurement bias and to explore correcting for bias. Using two types of contact T sensors (thermistors, n = 5; iButtons, n = 5), we performed experiments in three phases: (1) conventional calibration (uniform thermal environment) over 15-40 °C in 5 °C steps (at t = 0, and 24 h, 12 weeks later), (2) surface temperature measurements of a purpose-made aluminium plate (also 15-40 °C) at different environmental temperatures (15, 25, 35 °C) with different sensor attachments and fabric coverings to assess measurement bias and calculate correction factors that account for the next-to-surface microclimate temperature and (3) surface measurements (33.1 °C in 20 °C environment) for assessing generated corrections. The main results were as follows: (1) after initial calibration, T sensors were valid under uniform thermal conditions [mean bias < 0.05 °C, typical error of the estimate < 0.1 °C]. (2) For the surface measurements, bias increased with increasing surface-to-microclimate temperature difference for both sensor types. The range of surface temperatures possible to remain within given bias limits could be estimated for the various conditions. (3) For a given measurement, using corrections encompassing the microclimate temperature (mean difference - 0.1 to 0.5 °C) performed better than conventional calibration alone (mean difference - 2.1 to - 0.3 °C). In conclusion, the bias of T sensors is influenced by the microclimate temperature and, therefore, body coverings. Where excessive bias is expected, the validity can be improved through sensor and attachment selection and by applying corrections that account for the local temperature gradient.

Development of Body-Tissue Temperature-Control Transducer

“…For skin temperature measurement, various studies used thermocouples [6,33,37], infrared sensors [27,32], thermistors [29], with the number of types being 147, as specified in a review article [30]. Other studies [29,55,56,57] showed that the measurement of the skin surface temperature with contact sensor was influenced by the ambient temperature, the sensor mounting method, and the method and quality of the thermal insulation of the sensor from the environment.…”

A Thermal Skin Model for Comparing Contact Skin Temperature Sensors and Assessing Measurement Errors

“…Temperature control of the skin model was achieved using an aluminum plate immediately below the skin simulant; the aluminum plate had internal channels, which were perfused using a 25 L water bath and circulation thermostat (RM 25 S; Lauda GmbH & Co. KG, Lauda-Königshofen, Germany). The setup was capable of a steady state stability of SD <0.01 °C [ 22 ].…”

“…Temperature control of the skin model was achieved using an aluminum plate immediately below the skin simulant; the aluminum plate had internal channels, which were perfused using a 25 L water bath and circulation thermostat (RM 25 S; Lauda GmbH & Co. KG, Lauda-Königshofen, Germany). The setup was capable of a steady state stability of SD <0.01 • C [22]. Cross-section of the skin model at the measurement site, showing the surface position of the skin temperature sensor and/or tape when used (left) and the same section of the model in the state 'unmeasured' by the skin temperature sensor (right).…”

“…However, measurement limitations of these contemporary (and comparatively bulkier) T sk sensors are incompletely characterized. Interrelated challenges include that the measurement system may modify the local T sk via effects on heat and mass transfer [ 18 ] and that the sensor is in contact with other mediums, such as the ambient environment and attachments (e.g., tape), in addition to the skin itself [ 19 , 20 , 21 , 22 ]. The interplay between the physical properties of a sensor system, the skin, and the measurement conditions may contribute to the mixed agreement between sensor systems reported in more recent studies [ 21 , 23 , 24 , 25 , 26 ].…”

“…Similarly, laboratory tools that facilitate relatively easy and ‘standardized’ intra- or inter-sensor comparisons are required, given the proliferation of monitoring technologies and associated sensor designs and the emerging need for investigating or tracking relative measurement performance within or among those sensors. Laboratory models with metallic surfaces have been used in the past [ 21 , 22 , 29 , 30 , 31 ], typically to capitalize on heat transfer within the material itself, although models that more realistically reflect the thermal properties of human skin would be advantageous.…”