Therminus-K2: A Thermal Conductivity Measurement Device for Textile Layers at a Crime Scene

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Abstract

Each year, many people die due to non-natural causes of death, such as accidents, suicide, and murder and manslaughter. In these cases it is often necessary to investigate the cause, manner and time of death, which are investigated by forensic pathologists and the Police. To determine the time of death, the post mortem interval (PMI), the time passed shortly after death, should then be determined.
Wilk et al. (2020) developed a new method to determine the early PMI (3 – 72 hours). An input parameter in this method is the Thermal Conductivity Coefficient (k-value) of the textile layers surrounding the deceased. A method to determine the k-value of textile layers at, or around, a crime scene is needed. Therefore, this study aimed to design and develop a Thermal Conductivity (TC) measurement device for textile layers at a crime scene.

A functional decomposition chart of the device and a morphologic overview were created to design a suitable concept which was used for further development of the final prototype. SolidWorks was used to perform heat transfer simulations that were required to make the right design decisions and subsequently a final design and prototype were developed. Tests were performed to assess the performance of the prototype in terms of device configuration, accuracy, precision and measurement time. Furthermore, the effects of moisture content of the sample and sample compression during measurement on the calculated k-value were investigated.

A Guarded Hot Plate (GHP) based prototype was developed: Therminus-K2. This prototype was equipped with a back heater and four guard heaters to ensure one dimensional heat flow from the main heater through the sample towards the cold plate to eliminate other heat flows. Therminus-K2 obtains a precision of < 5% in TC measurements within 30 minutes. A steady-state measurement is achieved within a maximum of 7 minutes. Increasing the moisture content (2 states: dried and wetted) in the sample resulted in an increase in determined k-value (205 – 415 %). However, the uncertainty in sample thickness measurement was high (up to 16.7%) and complicated measurement of the sample compression effect on k-values.

Therminus-K2 delivers precise and fast TC measurements of textile samples: impressive results considering the simple components that were used. The future development of Therminus-K2 should focus on improving sample thickness measurement and improve user friendliness in order to make the device suitable to use in PMI estimation at actual crime scenes.