Yes, SCD30 series are optical while SCD40 series are photoacoustic. STC series are thermal conductivity based. STC and SCD40 are smaller than SCD30 but less acc

Yes, SCD30 series are optical while SCD40 series are photoacoustic. STC series are thermal conductivity based. STC and SCD40 are smaller than SCD30 but less accurate if memory serves (check datasheet).