Temperature Processing - Nellcor OxiMax N 5500 Service Manual

System Processing Description
pair of electrodes is determined by dividing the voltage difference between the two
electrodes by the current that passes between them. When the electrodes are placed on
the actual structure, respective structures change.
A low-level excitation signal is applied to these leads, and the variation of the thoracic
impedance caused by the breathing is sensed and processed for display and
measurement. This variation is processed to the voltage value for the measurement.
In order to transfer the thoracic impedance by a transformer, it is used a minimum
constant current of the sine wave carrier signal. The transferred thoracic impedance is
changed to the voltage signal by using bridge circuit and differential amplifier. Then,
ECG signal is removed by filter, and carrier frequency is removed by full wave rectifier
and filter in order to extract only thoracic impedance in amplifying at the definite level
of signal. This extracted thoracic impedance signal is used to measure the respiration by
digital signal processing.

10.7 Temperature Processing

Measurement of patient temperature is accomplished by processing the signal from a
probe containing a resistor whose resistance is temperature dependent. The class of such
components is called thermistor.
Temperature measurement used by N5500 patient monitoring system is based on a
thermistor whose resistance is inversely proportional to its temperature. By measuring
the thermistor's resistance, its temperature can be calculated. The resistance of the
thermistor is measured by passing a current through it and measuring the voltage
developed across it.
The N5500 patient monitor is designed to accept the signals from electrically isolated a
range of temperature probes from the Monotherm™ range. The probes may be used for
skin or rectal temperature measurement. Probes are furnished with a standard 10-feet
lead; extension leads are available. The signal from the probe is conditioned by the
monitor input circuitry, processed, and used to drive the numeric display.
92 N5500 Service Manual