Datasheet SGP40 (Sensirion) - 9

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3 Sensor Output Signal Description 3.1 Raw Signal Measurement
Calling the sgp40_measure_raw_signal command launches/continues the VOC measurement mode. It starts one measurement of the raw signal (SRAW_VOC) which is returned after 30 ms. Raw signal value in ticks is provided as one 16-bit word followed by one CRC byte (
Figure 6
). The raw signal in ticks is proportional to the logarithm of the sensor resistance. This signal is used as input for Sensirion’s VOC Algorithm to provide a processed VOC Index (see section 3.2 VOC Index Measurement). In case the VOC Algorithm is not used, discard the first measured value. Typically, a measurement is performed every second by calling the sgp40_measure_raw_signal command again every second without turning the heater off. To exit the VOC measurement mode, the master calls the sgp4x_turn_heater_off command which switches off the hotplate and puts the sensor in idle mode.
Figure 6
Process flow for performing a raw signal measurement with the SGP40. For enabling on-chip humidity compensation, current relative humidity and temperature values have to be supplied as part of the measurement command. The SGP40 features an automatic on-chip humidity compensation of the raw signal. The feature remains disabled as long as default values for both humidity and temperature are sent together with the sgp40_measure_raw_signal command. For enabling the compensation, the measurement command includes two words (+ respective CRC bytes) where the first word represents the relative humidity and the second one represents the temperature.
3.2 VOC Index Measurement Figure 7
Schematic of the VOC Index driver. For best performance in indoor-air-quality use cases, Sensirion recommends processing the (humidity-compensated) raw signal SRAW_VOC through Sensirion’s powerful VOC Algorithm. The VOC Algorithm (
Figure 7
) automatically adapts its output to any indoor environment and maps all VOC events to a VOC Index scale ranging from 1 to 500 VOC Index points (
Figure 8
). The value 100 refers to the average indoor gas composition over the past 24 h. While values between 100 and 500 indicate a deterioration, values between 1 and 100 inform about improvement of the air quality. The VOC Algorithm is included in the VOC Index driver package available on github (see SGP40 product page on Sensirion’s webpage). SGP40 raw signal needs to be provided to the VOC Algorithm at a sampling rate of 1 s. www.sensirion.com Version 1.1 – September 2021 9/19