Welch Allyn SureTemp 670 Especificações descarregar pdf (Página 25)

Theory of Operation

SureTemp

Model 678/SureTemp

Model 679 4-3

The shape of the rising temperature curve is monitored and the best fit to a curve is

found. When the curve fit is stable, the final predicted temperature is displayed.

•

In the oral mode, if the prediction criteria is not met within 15 seconds of starting

the prediction process, it will automatically switch to Monitor mode.

•

In the axillary mode, if the prediction criteria is not met within 30 seconds of

starting the prediction process, the thermometer displays a final temperature but

also indicates that the probe is out of position.

•

In the oral or axillary mode, if the ambient temperature is above 33.9

C (93.0

the unit will automatically switch to Monitor mode.

•

Rectal probes are not prewarmed. Rectal temperature measurements will take

about 15 seconds.

•

Within 60 seconds after the probe is removed from the well, if the prediction

process has not started, the thermometer will switch to Monitor mode.

Power Supply

The Model 678 contains a battery pack made up of 3 AA size batteries. This provides

a maximum supply voltage of about 4.8 volts. Power is drawn from the three AA

alkaline cells directly to the circuit electronics. The voltage from the batteries is

unregulated but filtered by capacitor C25. The power supply voltage will range from

about 4.8 volts with new batteries to 3.0 volts at shut down.

The thermometer has two low battery voltage indications.

•

The first is a warning that batteries are getting low and is indicated by the battery

icon flashing in the display. This begins when the batteries fall to about 3.2 volts.

Accuracy is not affected during low battery warning indication.

•

When the batteries fall to approximately 3.0 volts, the low battery error condition is

defined to exist. Operation is halted and the E2.1 error message is stored in

memory. At this point, the batteries must be replaced and the thermometer

electronics reset. See

Reset Self/Tests

on page 4 and on page v.

Low Battery Detection

The Model 678 includes a low battery detector circuit which shuts the device off when

the battery degrades to 3 volts. This ensures that erroneous temperature readings

are not given due to a low battery. For this operation, the reference voltage (VREF) is

measured by software using channel 1 of the 8 bit A/D in the microprocessor. The

A/D is powered with the battery voltage (VCC) through Q12. A/D channel 1 is

compared to the full scale on the A/D. As the battery voltage gets lower, channel 1

readings get higher. A reading above a fixed limit indicates a weak battery.

Exceeding another limit indicates a dead battery and the device will shut off.

Microcontroller

A NEC UPD78064 or UPD78063 single chip microcontroller in a QFP package (U1) is

used for signal digitizing, data processing, program memory addressing and storage,

and I/O interfacing. The microcontroller also includes an LCD controller/driver which

allows internal conversion of CMOS logic levels to a data format capable of driving the

Model 678/679 LCD. In this application, the microcontroller is running at

approximately 2.5 MHz, which is achieved by using a 4.9152 MHz Crystal (X1).

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