We are using heat flux sensors developed and manufactured by Thermonetics Corporation (La Jolla, CA) to measure conductive and convective heat loss in swimming Steller sea lions.

Heat flux sensors (HFRs) are used to measure heat flux. HFRs are typically comprised of miniature thermopiles. Thermopiles are arrays of thermocouples, arranged in series to boost the output voltage from the microvolt range typical for thermocouples, to milli-volts. All junctions of one type (e.g. copper - constantan) are on one side of the flat sensor disk, and all junctions of the other type (constantan - copper) are on the other side of the disk. Any temperature gradient across the sensor disk therefore produces an output voltage <> 0.

metal1 to metal2 junctions on the lower hot side of the sensor disc alternate with metal2 to metal1 junctions on the upper cold side of the sensor disc. There are nine junctions of each type in this schematic. The resulting heat flux from the hot to the cold side is reflected in an output voltage of the two leads of the sensor.

Thus, HFRs consist of serial dissimilar-metal junctions (thermocouples), or of serial semiconductor junctions (Peltier elements). It is important to realize that heat flux sensors invariably affect the very parameter measured. For starters, since HFRs are solid, heat flow is locally converted to a conductive flow. HFRs may also create local deflections of heat flux, as a result of changing thermal conductivity near the flux boundary of interest (resistance error, e.g. if the sensor decreases the thermal conductivity at the boundary, more energy will flow through the sensor-covered boundary than surrounding areas). In addition, the presence of the sensor may affect heat flow within the body or medium on one side of the boundary of interest, and may as a result change the isothermes in that body (deflection error). As a result, for heat flux determinations using HFRs it is important to quantify the effects of the sensors on the thermal properties of the areas of interest, as well as on the heat flow itself. Frequently (but certainly not always), the effects of small, thin sensors can be neglected.

See these publications on our development and use of HFR devices on swimming sea lions:

Willis K, Horning M (2005) A novel approach to measuring heat flux in swimming animals. Journal of Experimental Marine Biology and Ecology 315(2): 147-162.

Willis K, Horning M, Rosen DAS, Trites AW (2005) Spatial variation of heat flux in Steller sea lions: evidence for consistent avenues of heat exchange along the body trunk. Journal of Experimental Marine Biology and Ecology 315(2): 163-175.

The following image shows how we mount the HFSs to a sea lion. The actual attachment is performed under anesthesia. A small patch of fur is shaved, and a sensor holder is glued to the surrounding fur using cyanacrylate glue. The holder consists of a small section of PVC tubing glued to a donut-shaped neoprene ring. The outer rim of this neoprene ring is glued to fur. The HF sensor can then simply be slipped underneath this holder, and is pressed against the skin by the elastic force of the neoprene donut.

Each of these heat flux sensors by Thermonetics Corporation also includes a thermistor for the accurate measurement of skin temperature.

A heat flux sensor is held to the skin of a sea lion through a holder made of a PVC ring underneath a neoprene donut. The donut is glued to the fur of the sea lion.

In our application on Steller sea lions, four HF sensors are glued to a sea lion, and connected to an 8-channel data logger via waterproof connectors, as illustrated in the following image:

Two heat flux sensors are attached to the skin of a sea lion, and are connected to a multichannel data logger, which in turn is attached to a small neoprene saddle glued to the fur of the animal.

The recorder is bolted to a baseplate, which is firmly attache to a neoprene "saddle". The saddle is in turn glued to the dorsal fur of the sea lion. This way, we can quickly remove the sensors and the recorder on a daily basis.

For more on this application of HFRs, check out our project “Thermoregulation in Steller sea lion” in the Former Projects section of our website.