Apogee MQ-300 Series Owner's Manual - Page 13

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Underwater Measurements and Immersion Effect

When a quantum sensor that was calibrated in air is used to make underwater measurements, the sensor reads low. This

phenomenon is called the immersion effect and happens because the refractive index of water (1.33) is greater than air

(1.00). The higher refractive index of water causes more light to be backscattered (or reflected) out of the sensor in water

than in air (Smith,1969; Tyler and Smith,1970). As more light is reflected, less light is transmitted through the diffuser to the

detector, which causes the sensor to read low. Without correcting for this effect, underwater measurements are only relative,

which makes it difficult to compare light in different environments.

The original quantum sensors covered in this manual have an immersion effect correction factor of 1.08. If you wish to use

your meter to make measurements underwater, simply multiply the measured number by the immersion effect (1.08).

When making underwater measurements, only the sensor and cable can go in the water. The handheld meter is not

waterproof and must not get wet. If the meter might get wet from splashing, we recommend placing it in a plastic bag or

other container to help protect it from accidentally getting wet.

Further information on underwater measurements and the immersion effect can be found at

http://www.apogeeinstruments.com/underwater-par-measurements/.