Near-Infrared Temperature Measurement Technique for Water Surrounding an Induction-heated Small Magnetic Sphere

The overall goal of this experiment is to measure temperature distributions near a heated small sphere or a point heat source located inside water or non-turbid acquiesce media. This method will be very helpful for researching local heating inside media, such as hyperthermia research using magnetic particles. The main advantage of this technique is that it's easy to set up and implement.

Demonstrating the procedure will be Keisuke Nishijima and Van Cuong Han, who are grad students from my laboratory. to prepare a water or aqueous liquid sample, first use the minimum necessary amount of quick-drying, water resistant glue to fix a two millimeter diameter steel sphere to the end of a thin, non-metallic string or rod. Ensure that the glue minimally affects the shape of the sphere and the heat transfer rate, then thread the string through the center hole of a PTFE cuvette cap.

Tape the string in place with 22 millimeters of the string hanging below the cap. Equip a water filled syringe with a 0.22 micrometer syringe filter and a plastic dispensing needle. Carefully fill a glass cell with an optical path length of 10 millimeters, and a height of 45 millimeters with filtered water, being careful to avoid the formation of air bubbles.

Air bubbles and suspended particulates should be removed as much as possible in advance to obtain correct measures and successfully estimate temperatures. Place the cap on the cell so that the sphere hangs in the center of the rectangular cell. To prepare an aqueous gel sample, first heat the aqueous gel until its viscosity is sufficiently reduced to be poured smoothly.

Use a syringe to half-fill a rectangular glass cell with an optical path length of two millimeters with the heated gel. Allow the gel to cool. Then place a 0.5 millimeter diameter steel sphere at the center of the gel surface.

Fill the rest of the cell with heated aqueous gel in the same way and allow it to cool. Once the aqueous liquid or gel sample has been prepared, place the cell in a plastic holder on the optical rail of the Near-IR imaging system. To begin preparing the Near-IR imaging system, equip a halogen lamp with a fiber light guide.

Place a narrow bandpass filter with the transmittance peak at either 1, 150 nanometers or 1, 412 nanometers between the fiber light guide in the cell. at 1, 150 nanometers and 1, 412 nanometers, the water absorbance increases as the water temperature increases. Place a bandpass filter with the wider transmission range around the chosen wavelength between the halogen lamp fiber light guide and the narrow bandpass filter.

Mount an iris diaphragm between the narrow bandpass filter and the cell holder. Then set up a Near-IR camera in line with the sample cell. Fix an object-space telecentric lens between the cell and the camera.

Turn on the Near-IR camera, ensure that the camera is connected to a computer, and open the image acquisition software. Turn on the halogen lamp, check the image displayed on the monitor, and adjust the lamp output power as needed. Adjust the axis, position, and focus of the telecentric lens to obtain a finely-detailed image of the steel sphere.

The optical asset adjustment is important for achieving the telecentric optical system as to be carefully made to look at the reflected images. Then prepare an induction heating system consisting of a high frequency generator, a water-cooled coil, and a water chiller. Mount the coil on an XYZ moveable stage.

Position the coil over the sample cell, so that the distance between the center of the coil and the steel sphere is approximately 15 millimeters. Ensure that no other metal components are near the coil. Turn on the water chiller for the induction heater and start the water circulation.

Ensure that the frame rate and the camera integration time are set to the maximum available values. Once the induction heater is ready, set the maximum number of frames to record, name the image data file, and start recording images in the image acquisition software. Run the induction heater for the desired length of time.

Either allow the image acquisition to run for the set duration, or manually stop recording after the desired length of time. Save the images in a non-compressed format, such as a TIFF sequence. Open the sequence in image processing software, and convert the saved images of transmitted light intensity to the absorbance difference images.

Colorize the images with the desired color range. The temperature can then be estimated from these images using additional command scripts. The increase in water temperature around an induction-heated two millimeter diameter steel sphere was observed as a circularly symmetric change in absorbance with respect to the Near-IR image prior to heating.

Free convection was observed after 1.2 seconds of heating, suggesting a transition from a pure thermal conduction regime to a free convection regime. Similar effects were observed around an induction-heated 0.5 millimeter diameter steel sphere in maltose syrup at three different induction heating powers. The radius and magnitude of the change in absorbance increased with increasing power.

Free convection was not observed after 1.2 seconds. In the water sample, the magnitude of the change in absorbance over time was greatest close to the sphere. A combination of two or three Gaussian functions can achieve a good fit for the relationship between absorbance and the radius of the image plane.

Similar results are observed in the gel sample. The fitted functions can then be transformed into the change in temperature in the water and the gel, assuming that thermal conduction occurs in the radial direction, and the temperature profiles are spherically symmetric. The key devices for the temperature measurement are a near-infrared camera, a narrow bandpass filter.

Once they are prepared, the procedure from setup to image acquisition is easily done by any researcher. This imaging technique will be applied not only to induction heating, but also to various heating method. Temperature and heat generation inside media which have never been directly measured so far can be revealed.