Dräger 8000 IC Function Description page 38

into a safe condition. The over-temperature condition is stored by a bipolar relay on the
Motherboard PCB. The heating control can only be released by acknowledgement via a reset
button after the temperature has fallen below the comparator limit.
d. testing of over-temperature comparator
The function of the over-temperature comparator is checked software-controlled every 10
minutes. On the Analog PCB the relay (K2) or by FET transistors on newer devices as of 1996
substitutes for a short time the over-temperature sensor with a fixed resistor simulating a
temperature higher than 40.0 °C.
e. Skin temperature sensor
The skin temperature sensor is part of the feedback loop of an operational amplifier and
supplies an output voltage proportional to the temperature which is adapted to the input
voltage range of the A/D converter via a second amplification stage.
Software version 10.n and 20.n:
The temperature of 32.0 to 39.0 °C is represented by 0.0 to 5.0 V input voltage of the ADC.
Software version 11.n and 21.n (units with Thermomonitoring):
The temperature of 30.0 to 42.0 °C is represented by 0.0 to 5.0 V input voltage of the ADC.
f. Testing of the skin temperature sensor circuit
The accuracy of the skin temperature sensor circuitry is checked every 10 minutes as part of
the 10 minute test. On the Analog PCB the relay K1 or by FET transistors on newer devices as
of 1996 replaces software controlled the sensor for a short time with a fixed precision resistor
with the temperature equivalent of 36.0 °C. If this reference temperature is measured with an
error of more than 0.3 °C, a sensor alarm (Err) is caused.
In addition to the automatic accuracy test of the skin temperature sensor measurement, the
operator can manually test the circuit by pressing the "Check 36 °C" key. This procedure in the
same manner replaces the skin temperature sensor with the reference resistor by means of
relay K1 or FET transistors. The operator can look for the correct display of 36 °C.
In order to detect a defective relay K1 or FET transistors which could cause a complete
corrupted skin temperature measurement and control because of the fixed reference resistor a
software test routine is implemented. This routine analyzes the skin temperature sensor data
and makes a plausibility test. The basic assumption is that the reference resistor will lead to a
very stable temperature value with virtually no drift. If, in a 20 minute time interval, the
temperature drift is less than 0.003 °C it is assumed that the reference resistor is measured
instead of the skin temperature sensor. A sensor failure alarm (Err) will then be generated.
6141.22X Incubator 8000 IC/SC/NC
Function Description
09.99 Page 38 of 49