Welch Allyn SureTemp Plus 692 Series Service Manual page 30

5.1.1 Temperature Probes
Model 690 and 692 Thermometer probes are configured at the factory to be Oral/Axillary or
Rectal probe types.
The probe contains a thermistor, a heater resistor, and an EEPROM
Table 5- 1: Probe Parts
Probe
Part
Thermistor
Heat Resis-
tor
EEPROM
5.1.2 Probe Switch
The probe switch is an optical LED switch. It is arranged as an open circuit when the probe is
in the well, to save power. This causes a High-to-Low transition when the probe is pulled.
5.1.3 Power Supply
Power for the Model 690 and 692 is drawn from the three AA cells directly to the circuit
electronics, providing a peak voltage of 4.8V and an average voltage of 3.60 V. Under normal
use, battery life is expected to provide approximately 5,000 temperature measurements for
Model 692, and 6,000 temperature measurements for Model 690. This number is based on a
22.2 ºC (72.0 F) ambient temperature, with the security features turned off. Colder ambient
temperatures, excessive security alarms, and other usage patterns can reduce battery life.
Instruments are shipped with fresh batteries, but we cannot guarantee full life from the first set
of batteries due to potential long storage times between shipping and actual use. Battery life is
also reduced by storage at elevated temperatures.
The battery holder is interlocked to prevent reverse polarity. This is an important aspect of safe
operation of the unit.
5.1.4 Low Battery Detection
The Model 690 and 692 Thermometers include a low battery detector circuit, which shuts the
device off when the battery degrades below 3 volts. This ensures that erroneous temperature
readings are not given due to a low battery.
5.1.5 Temperature Measurement and Display
The thermometer probes incorporate negative temperature coefficient thermistors. When the
temperature of the probe is increased, its electrical resistance decreases.
Model 690 and 692 Thermometers use 20K thermistors, providing a resistance of
approximately 20 K at room temperature. At 37°C (patient temperatures) they are near 12
K. The change in resistance is nonlinear with temperature and an equation describing this
curve is programmed into the thermometer.
26
Description
The Thermistor is used to sense patient temperature.
The heater is used to create a stable thermal environment at
the tip of the probe prior to applying the tip to the patient.
2
During calibration, the I
connector and SCL by a special probe connection that does not
connect to the thermometer. The thermometer reads the
EEPROM via its 2-wire serial-only (VESA DDC1) interface. The
interface is guaranteed to work with VCC of 2.7 to 5.25 volts.
C SDA line is accessed via the probe
Sure Temp® Plus Service Manual
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