Class: BE209
Group: W6
Members: CHEN, KAREN; HEIL, ERIC M; PATEL, ANAND S; PUIG, ANDREA; SAHNI, NIKHIL; THIEU, KHANH

Date: December, 2002

Full Text

Background

In clinical settings, obtaining vital signs for patients is crucial in making a diagnosis. Temperature telemetry is convenient because it allows wireless temperature measurements by encoding the measurement in light signals.  A typical telemetry device consists of two sections: the back-end and the front-end.  Often, the back-end unit is placed in a patient or object of interest where it continuously obtains temperature readings.  These readings are then transmitted to a front-end unit through proper light signal encoding which is then converted by a computer into temperature values.

Telemetry devices are widely used for continuous and automatic logging of an object’s temperature during important physiologic activities.  For example, female ovulation can be characterized by the patient’s vaginal temperature1.  Another popular use of telemetric devices has been to measure animals’ deep body temperature changes due to environmental stressors (e.g. ambient temperature, ambient pressure, etc.)2.  Lastly, temperature telemetry has been used in clinical studies of prosthetic implants.  For instance, biotelemetry devices have been implanted to gauge the frictional heat generated on the acetabular head of a patient with a hip implant3.  This provides useful information on whether the implant is capable of functioning in daily activities without generating excessive frictional heat.

The goal of this project is to create and understand a basic temperature telemetry device.  Using a LM555 timer chip and a thermistor, the temperature values are encoded as pulse frequencies.  With the telemetry circuit it is hypothesized that:
1.  A pulse frequency generated by the timer chip can accurately be received and then decoded into temperature values.
2.  A narrow range (18 – 28 °C) calibration curve will yield more accurate decoding of temperature readings than a wide range (8 – 40 °C) calibration.
3.  Increasing the distance between the transmitting and the receiving units of the telemetry device will decrease the accuracy of the temperature decoding.

The constraints of the circuit are the distance between the LED and the phototransistor, the calibration ranges, and the response time. The distance remains constant throughout the experiment until hypothesis three is tested.  The initial wide and narrow range calibration curves are used as references for determining the temperature. The linearity of the calibration curves will be accepted if the linear regression model renders a fitting above 95%. Due to the limitation on the calibration ranges, accuracy of the temperature reading may be affected once the temperature falls either below or beyond the range. The response time of the system is another factor that determines the practical applications of the device. Commonly used electronic thermometers4 have a response time of 4-15 seconds, thus it is expected a similar performance for our device.