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Temperature sensors

Temperature sensors. Normal body temperature is 37°C. Varies by: Age Time of day Location. High and low temperatures are dangerous and may indicate disease . Range of only 10°C. Sleep apnea. Polysomnography – “sleep study”.

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Temperature sensors

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  1. Temperature sensors 414-8

  2. Normal body temperature is 37°C Varies by: Age Time of day Location 414-8

  3. High and low temperatures are dangerous and may indicate disease. Range of only 10°C 414-8

  4. Sleep apnea 414-8

  5. Polysomnography – “sleep study” 414-8

  6. Polysomnography – “sleep study”Movement of eyes, EEG, pressure in airway, blood oxygen may also be measured Apnea event Movement of ribcage Movement of abdomen [CO2] in airway Temperature in airway Exhaled air is warmer than inhaled air 414-8 From “Comparison of Apnea Identified by Respiratory Inductance Plethysmography with the Detected by End-tidal CO2 or Thermistor,” by Weese-Mayer, et al., Am. J. Respir. Crit. Care Med., Vol. 162(2), pp 471-480.

  7. Respiratory sinus arrhythmia – Heart rate increases during inspiration and decreases during expiration From Yasuma and Hayano, “Respiratory Sinus Arrhythmia: Why Does the Heartbeat Synchronize with Respiratory Rhythm?” in Chest, vol 125(2) pp 683-690. 414-8

  8. Clinical use of RSA measurement • Lower RSA amplitude (difference between shortest RR interval and longest interval) in • Older people (10% decline per decade) • Non-athletes • CPR survivors • Those with coronary artery disease (clogged arteries) 414-8

  9. Many sensors exist that can measure temperature in this medical range 414-8

  10. Silicon bandgap – Turn-on voltage of a base-emitter junction varies with temperature 414-8

  11. The difference in voltage for two BE junctions with different currents is proportional to absolute temperature. k = Boltzmann’s constant = 8.617x10-5 eV/K T = temperature in Kelvin Q = charge of electron = 1.602x10-19 C I = current in Amperes A = area of p-n junction in m2 (areas should be the same) 414-8

  12. Use an integrated circuit sensor Amplify the output to achieve some specific number of V/K 10 mV/K 19.5 mV/K 6.25 mV/K User selectable Transistors wired this way act like diodes. 414-8

  13. The LM335 integrated temperature sensor is modeled as a Zener diode. 414-8

  14. The “Zener voltage” of the LM335 is dependent on temperature. 414-8

  15. Choose Vddand R so that the current is in the operating range for all V in temperature range. • T ↑, Vout↑, VR ↓, I ↓ • T ↓, Vout ↓, VR ↑, I ↑ • When the temperature changes, it is possible that the device is no longer operating properly. 414-8

  16. Thermistors – resistance of semiconductor decreases with increases temperature An equation that fits the observed relationship • Ro = resistance at temperature To = 25°C • T = temperature in K • β = property of material 414-8

  17. Thermistor circuit • Use the op amp and negative feedback to keep the current through the thermistor constant (Measure it) • Keep the power dissipated by the thermistor low • Measure the voltage across RT and use Ohm’s Law to find RT • Don’t assume v-=0V 414-8

  18. Compare the performance of the two sensors • Accuracy • Ease of use • Response time • Sensitivity 414-8

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