Digital Output Quantum Sensor SDI-12

In stock (can be backordered)


In stock (can be backordered)

SKU: SN-SQ-421-SS Categories: , , , Tags: , , , ,


The SN-SQ-421X-SS is a digital sensor that uses SDI-12 communication protocol. The sensor is calibrated for sunlight and has a command that automatically applies the immersion effect correction factor for underwater measurements. The sensor features a rugged, self-cleaning sensor housing design, and high-quality cable terminating in pre-tinned pigtail leads for easy connection to dataloggers and controllers. Typical applications include PPFD (Photosynthetic Photon Flux Density) measurement over plant canopies in outdoor environments, greenhouses, and growth chambers, and reflected or under-canopy (transmitted) PPFD measurements in the same environments. Quantum sensors are also used to measure PAR (Photosynthetically Active Radiation)/PPFD in aquatic environments, including salt water aquariums where corals are grown.

Spectral Response

The spectral range (denoted with an “X”), is 370 to 650 nm. The spectral response can be seen in the graph below.

Quantum Sensor Spectrum Response

Quantum Sensor Spectrum Response

Rugged, Self-cleaning Housing

Apogee’s Patented domed shaped sensor head (diffuser and body) facilitate runoff of dew and rain to keep the sensor clean and minimize errors caused by dust blocking the radiation path. Sensors are housed in a rugged anodized aluminum body and electronics are fully potted.

Accurate, Stable Measurements

Sensors are cosine-corrected to maintain their accuracy when radiation comes from low zenith angles. The sensors measure PPFD with a cosine response accurate within ± 5 % at 75° zenith angle. Long-term non-stability determined from multiple replicate quantum sensors in accelerated aging test and field conditions is less than 2 % per year.

Ideal for Measurements in Air and Underwater

Apogee quantum sensor are calibrated to make absolute PPFD measurements in air. The waterproof sensors can be used to make absolute PPFD measurements underwater by applying an immersion effect correction factor, the SQ-420X, SQ-520, MQ-210X, and MQ-510 already apply the correction factor. For details on making underwater measurements, please refer to Underwater PAR Measurements.


To ensure accuracy each sensor is carefully calibrated in controlled conditions and traceable to NIST reference standards. The Original sensors are pre-calibrated for either sunlight or electric light sources.

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