Conversion Metric
The ratio of generated output voltage to the incident radiant power of an infrared detector defines the efficiency of thermal-to-electrical energy conversion. This thermopile responsivity is expressed in volts per watt and represents a primary performance specification for non-dispersive infrared gas sensors. Higher values indicate a stronger electrical signal for a given change in gas concentration.
Measurement Mechanism
Thermocouples connected in series generate a voltage proportional to the temperature difference between the active and reference junctions. The magnitude of the thermopile responsivity is determined by the number of thermocouple pairs, the thermoelectric coefficient of the materials and the thermal design of the sensor membrane. This sensor membrane must have low thermal conductivity to maintain a high temperature gradient across the active junctions.
Performance Degradation
Exposure to moisture or high temperatures can degrade the sensor materials, resulting in a reduction in responsivity. Solder joint deterioration and chemical contamination of the reflective surfaces can also affect the incoming radiant power.
Calibration Procedure
Test engineers measure the responsivity by exposing the sensor to a blackbody radiation source under controlled laboratory conditions. The resulting value is compared against the manufacturer specification to ensure it falls within the required tolerance. This measurement is critical because any deviation in responsivity will affect the accuracy and limit of detection of the gas analyser.
