The SP-53A is a tin dioxide emiconductor gas sensor which

has a good sensitivity to highconcentration ranges ofammonia with improvedresponse speed compared withconventional models.

Structure

Gas sensitive semiconductormaterial is formed on the aluminasubstrate on which the goldelectrodes are printed. A thick filmheater of ruthenium-oxide (orplatinum) is printed on the reverseof the substrate and placed in thehousing (Fig 1).Operating conditionsFig 2 shows the standard operatingcircuit for this model. The change ofhe sensor resistance (RS) isobtained as the change of theoutput voltage across the fixed or

variable resistor (RL). In order toobtain the best performance andspecified characteristics, the values

of the heater voltage (VH) circuitvoltage (VC) and load resistance (RL)must be within the range of valuesgiven in the standard operatingconditions shown in theSpecification table on the nextpage.Sensitivity characteristics

Fig 3 shows the sensitivitycharacteristics curves of the SP-53A (typical data). Sensitivitycharacteristics of the FIS gassensors are expressed by therelationship between the sensorresistance and gas concentration.The sensor resistance decreaseswith an increase of gasconcentration based on alogarithmic function.The sensitivity characteristics ofhe SP-53A is specified by thefollowing parameters.• Sensor resistance level: atammonia 600 ppm

• Sensor resistance changeratio: between ammonia200 ppm and 600 ppm