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Enabling better global research outcomes in soil, plant & environmental monitoring.

SQ-212 0-2.5Vt Sun Calibrated PAR Sensor

The SQ-212 is an amplified quantum sensor that measures photosynthetically active radiation and is calibrated for use in sunlight. The sensor housing design features a fully potted, domed-shaped head making the sensor fully weatherproof and self-cleaning. Photosynthetically active radiation (PAR), or photosynthetic photon flux (PPF), is the wavelength range from 400 to 700 nanometers and is strongly correlated with plant growth. Gardeners, greenhouse managers, growth chamber users and salt-water aquarists measure PAR to insure optimal specimen health.

Sensitivity: 1.0 mV per µmol m-2 s-1
Calibration Factor: 1.0 µmol m-2 s-1 per mV (reciprocal of sensitivity)
Calibration Uncertainty: ± 5%
Measurement Repeatability: < 1%
Non-stability (Long-term Drift): < 2% per year
Non-linearity: < 1% (up to 2500 µmol m-2 s-1; maximum PPF measurement is 2500 µmol m-2 s-1)
Response Time: < 1 ms
Field of View: 180°
Spectral Range: 410 nm to 655 nm (wavelengths where response is greater than 50% of maximum)
Directional (Cosine) Response: ± 5 % at 75° zenith angle
Temperature Response: 0.06 ± 0.06% per °C
Operating Environment: -40 to 70°C, 0 to 100% relative humidity, Can be submerged in water up to depths of 30 m
Dimensions: 2.4 cm diameter and 2.8 cm height
Mass: 90g (with 5m of lead wire)
Cable: 5m of shielded, twisted-pair wire. Santoprene rubber jacket
Warranty: 4 years

Radiation that drives photosynthesis is called photosynthetically active radiation (PAR) and is typically defined as total radiation across a range of 400 to 700 nm. PAR is often expressed as photosynthetic photon flux (PPF): photon flux in units of micromoles per square meter per second (µmol m-2 s-1, equal to microEinsteins per square meter per second) summed from 400 to 700 nm (total number of photons from 400 to 700 nm).

While Einsteins and micromoles are equal (one Einstein = one mole of photons), the Einstein is not an SI unit, so expressing PPF as µmol m-2 s-1 is preferred. Sensors that measure PPF are often called quantum sensors due to the quantised nature of radiation. A quantum refers to the minimum quantity of radiation, one photon, involved in physical interactions (e.g. absorption by photosynthetic pigments). In other words, one photon is a single quantum of radiation.

Typical applications of quantum sensors include incoming PPF measurement over plant canopies in outdoor environments or in greenhouses and growth chambers, and reflected or under-canopy (transmitted) PPF measurement in the same environments.

Apogee Instruments SQ series quantum sensors consist of a cast acrylic diffuser (filter), photodiode, and signal processing circuitry mounted in an anodised aluminium housing, and a cable to connect the sensor to a measurement device. Sensors are potted solid with no internal air space, and are designed for continuous PPF measurement in indoor or outdoor environments. SQ series sensors output an analogue voltage that is directly proportional to PPF under sunlight (e.g. model SQ-110) or electric lights (e.g. model SQ-120). The voltage signal from the sensor is directly proportional to radiation incident on a planar surface (does not have to be horizontal), where the radiation emanates from all angles of a hemisphere.

Apogee offer these amplified versions of PAR Sensors to allow for easy integration into systems that cannot measure small millivolt signals. There are two options: 2.5V and 5V. The voltage listed is the maximum output at the maximum light for which the sensor is calibrated. It is also the minimum input voltage required to power the sensor. For example: the SP-215 has an output of 5V when the solar radiation is at its maximum of 1250 W/m2. If the voltage supplied was only 4V the maximum output would be 4V which means that the sensor would output the same for any light level over 1000 W/m2. This is something to watch for if you are supplying the voltage with batteries.

These sensors have been designed to have all the circuitry inside the head, the same as Apogee’s non-amplified sensors. This means they are rugged and there is no worry about water damage to the circuit board. The circuitry is a basic non-inverting configuration of an operational amplifier (op-amp). The op-amp has been specifically chosen to have a low voltage offset and be physically small enough to fit inside of the current sensor body head. The op-amp has a max voltage specified at 5V which is why the maximum output of our amplified sensors is 5V. Included in the circuitry is a Zener diode that will short to ground if a voltage higher than 6V is applied. This keeps the main circuitry from getting destroyed if the sensor is wired incorrectly.