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Temperature sensor for measuring human body temperature
Hi. I'm doing a project in body sensor networks. In this i'm measuring the body temperature. In previous forums you have suggested to use SA1-RTD from OMEGA company for measuring the temperature. Will this give the exact temperature of the body? Can you suggest me an alternative way to measure the temperature of a human body? Waiting for your reply.
A properly-selected thermistor can give you good accuracy in the temperature range that you need and they have a far greater output than an RTD.
Regards, Neil P. Albaugh ex-Burr-Brown
Satish,An RTD or thermistor will do a good job of measuring the temperature of whichever surface it is in contact with. If that surface is the skin on a human's body, then that temperature will be accurately reported (within the accuracy and linearity limitations of the sensor, of course). If skin temperature is what you want to measure in your body sensor network, then your solution is very simple. However, if you're attempting to measure the internal/core temperature of the human body, then that becomes more complex.I recommend you read through this article: Body Temperature Variability (Part 1): A Review of the History of Body Temperature and its Variability Due to Site Selection, Biological Rhythms, Fitness, and Aging. Of most interest to your application will be the section covering temperature variability due to site selection. This section discusses the range of temperature values one should expect when measuring internally and externally, and since the human body uses the skin as a cooling mechanism the temperatures measured on the skin will generally be lower than the core body temperature. Axillary (under the arm) measurements are a good option for a convenient external site, but some sort of correction would be required in order to approximate the internal temperature.Best regards,Ian WilliamsLinear Applications EngineerHigh Performance Linear
There are tiny thermistors mounted in the point of a hypodermic needle that will measure the temperature of a body wherever you like-- this is an invasive method, of course.
A non-invasive (and non-trivial) method of measuring internal body temperatures is to use a microwave radiometer. In the low GHz range, the body has sufficient microwave attenuation to be a decent blackbody radiator. In this area of the spectrum, the Rayleigh-Jeans Approximation holds true and the RF power emission is proportional to absolute temperature. This method is complex and expensive, but-- depending on the frequency choice-- temperature measurement from virtually any tissue depth is possible.
Can you elaborate on the use of microwave radiometer in core body temperature? I'm interested in learning more about this technique. Is there any papers that you know of that you could direct me to ? I would love to talk more about this with you if you sometime.
Any object above 0 degrees Kelven radiates energy; very hot objects, such as the sun, radiate at mainly the visible and infrared wavelengths but cooler objects have their peak energy radiated at longer wavelengths. Look up the "Wein Displacement Law".
A microwave radiometer measures energy in the short radio wavelengths that are radiated by an object. In the case of a human body, the choice of wavelength determines how far into the body the energy is measured. By chosing a wavelength that has sufficient attenuation in the depth that the measurement is desired, the internal temperature can be determined. Very high attenuation prevents looking deep into the body-- think of the IR sensors, the IR attenuation is so high that it only barely penetrates the skin. This is why IR thermometers measure skin temperatures. At too long a wavelength, the body tissues become rather transparent to RF and a measurement cannot be made. I'm sure a bit of research can uncover some wavelength vs attenuation in human tissue data. Tissue is a high percentage of water so it may be around 2.4GHz.
Microwave radiometers are not simple or cheap, but they are far more feasible these days with low noise solid-state amplifiers. Another factor is the antenna used to focus on the area to be measured. At millimeter wavelengths, pyramidal horns become small enough to be practical and generate a reasonably small beamwidth. These wavelengths raise the radiometer complexity and cost, however.
Read about radio astromony-- measuring temperature with a radiometer is the fundamental principle.
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