User's Manual

24 Sure Temp® Plus Service Manual

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.

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 alkaline 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, use of non-alkaline batteries,

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.

Table 5- 1: Probe Parts

Probe Part Description

Thermistor The Thermistor is used to sense patient temperature.

Heat Resistor

The heater is used to create a stable thermal environment at the tip of the

probe prior to applying the tip to the patient.

EEPROM

During calibration, the I

C SDA line is accessed via the probe 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.