Biomedical Electronics & Bioinstrumentation

1. Biomedical Electronics & Bioinstrumentation Basic Sensor Principles (Temperature)

2. Contents • Temperature Measurements • Thermocouples • Thermistors • Fiber-Optic Sensor

3. Temperature Measurements • Significance of temperature parameter: • Patient’s body temperature indicates physiological state of human body. • External body temperature used to evaluate patients in shock. How does this relate? • On the other hand, what happens to patients with infection? • How would you relate these conditions with change in body temperature?

4. Temperature Measurements • Significance of temperature parameter: • You have already learned about enzymes and proteins. Explain their reactions to changing temperature. • How does inflammation relates to temperature change? • How does a temperature sensor play a role in an infant incubator?

5. Temperature Measurements • Why do we need specific body site for body? • Types of thermally-sensitive measurement methods: • Thermocouples • Thermistors • Radiation • Fiber-optic detector

6. Thermocouples • Used as means of converting thermal potential difference to electrical potential difference • Thermoelectric thermometry first discovered by Seebeck in 1821. • He observed that emf exists across a junction of two dissimilar metals.

7. Thermocouples • Phenomenon is the sum of two independent effects: • Emf generated solely by contact of two dissimilar metals and the junction temperature. • Emf due to the temperature gradients along each single conductor. • These effects were discovered by Peltier and Thomson (Lord Kelvin).

8. Thermocouples • The Seebeck voltage is given by the following power series expansion: • The temperature is in °C and the reference junction maintained at 0°C. • The equation is curve-fitted from the obtained calibration data.

9. Thermocouples • Thermocouple uses 3 empirical laws. • Homogenous circuit States that in a circuit composed of a single homogeneous metal, one cannot maintain an electric current by the application of heat alone.

10. Thermocouples • Thermocouple uses 3 empirical laws. • Intermediate metals States that the net emf in a circuit consisting of an interconnection of a number of unlike metals, maintained at the same temperature is zero.

11. Thermocouples • Thermocouple uses 3 empirical laws. • Successive or intermediate temperatures States that an emf E1 + E2 are generated when the junctions are at T1 and T2.

12. Thermocouples • The thermoelectric sensitivity,  are given by: •  is not a constant but varies with temperature. • Sensitivities range from 6.5 to 80µV/°C at 20°C with accuracies from 0.25% to 1%. • For accurate reading, reference junction must be kept at a triple-point-of-water temperature. • Usually achievable through construction of properly constructed ice bath.

13. Thermocouples • What about modern days thermocouples? How do we implement ice baths at the cold junction? • Thermopiles are combination of thermocouples. • Connected in series to increase accuracy. • What happens if we connect it in parallel? • The thermocouple voltage can be obtained using digital multimeter.

14. Thermocouples • Chart recordings can be achieved through use of self-balancing potentiometer system. • What about system linearity? • Advantages: • Fast response time • Small size • Ease of fabrication • Long-term stability

15. Thermocouples • Disadvantages: • Small output voltage • Low sensitivity • Need of reference temperature • Application: Thermocouples can be made small and inserted in catheters and hypodermic needles.

16. Thermistors • Thermistors are semiconductors made from ceramic materials that are thermal resistors with a high negative temperature coefficient. • T increase, R decrease • T decrease, R increase • The resistivity of thermistor semiconductors used for biomedical applications is between 0.1 to 100Ωm.

17. Thermistors

18. Thermistors • Advantages • Small in size • Relatively large sensitivity to temperature change • Excellent long-term stability • The empirical relationship between Rt and absolute temperature given by:

19. Thermistors •  is the material constant while T0 is the standard reference temperature in K. •  also known as the characteristic temperature increases slightly with temperature. • Thus, can you tell me why do we still use a thermistor as a sensor? •  ranges from 2500K to 5000K with typical value of 4000K.

20. Thermistors • Temperature coefficient,  is given by: • This is found by differentiating the empirical relationship with respect to T and further dividing it by Rt. •  is a nonlinear function of temperature.

21. Thermistors • Typical thermistor zero-power resistance ratio-temperature characteristic for various material. • At zero-power resistance, the thermistor is operated at a very small amount of power such that there is negligible self-heating.

22. Thermistors • The V versus I characteristics are linear up to the point where self-heating becomes a problem. • When there is excessive heating, the thermistor voltage decreases as the current increases.

23. Thermistors • Which electrical law applies to the linear region of the V-I characteristic graph? • Another important feature is the current-time characteristics. • Time-delay for current reaching maximal value is determined by: • Voltage applied • Mass of thermistor • Value of series-circuit resistance

24. Thermistors • Linearization of R-T characteristics: • Circuit schemes • Microcomputers • Circuitry for readout is the same as conductive sensors. • What advantage does the application of bridge circuits bring? • Very small differences can be found using a differential temperature bridge.

25. Thermistors • Various shapes of thermistors: • Beads • Chips • Rods • Washers • Most common is glass-encapsulated bead thermistor. • Why is this packaging desirable?

26. Thermistors • Glass-coating protects from hazardous environment from the body without affecting the thermal response time. • Application: • Small-sized thermistor allows application on catheters and hypodermic needle. • An example seen in thermodilution-catheter system. • Also used to obtain oral temperature.

27. Fiber-Optic Temperature Sensor • A small prism-shaped sample of single-crystal undoped GaAs is epoxied at the ends of two side-by-side optical fibers. • The sensors and fibers can be quite small, compatible with biological implantation after being sheathed.

28. Fiber-Optic Temperature Sensor • Principle of operation: • One fiber transmits light from a light-emitting diode source to the sensor, where it is passed through GaAs and collected by the other fiber for detection in the readout instrument. • Some of the optical power travelling through the semiconductor is absorbed, by the process of raising valence-band electrons, across the forbidden energy gap into the conduction band. • Because the forbidden energy gap is a sensitive function of material’s temperature, the amount of power absorbed increases with temperature.

29. Fiber-Optic Temperature Sensor • Application: The non-metallic probe is particularly suited for temperature measurement in the strong electromagnetic heating fields used in heating tissue for cancer therapy or in patient rewarming.

30. Further Reading… • Webster, J.G. (2009). Medical Instrumentation: Application and Design. 4th Ed., Wiley. • Chapter 2 • Carr, J.J. (2000). Introduction to Biomedical Equipment Technology. 4th Ed. Prentice Hall. • Chapter 6