Immersion Effect Correction Factor
When a radiation sensor is submerged in water, more of the incident radiation is backscattered out of the diffuser
than when the sensor is in air (Smith, 1969; Tyler and Smith, 1970). This phenomenon is caused by the difference
in the refractive index for air (1.00) and water (1.33), and is called the immersion effect. Without correction for the
immersion effect, radiation sensors calibrated in air can only provide relative values underwater (Smith, 1969;
Tyler and Smith, 1970). Immersion effect correction factors can be derived by making measurements in air and at
multiple water depths at a constant distance from a lamp in a controlled laboratory setting.
The MQ-510 sensor has an immersion effect correction factor of 1.25. The immersion effect correction factor is
already accounted for in the MQ-510 meter firmware so there is no need to apply the correction factor to your
measurements. If you wish to use your meter to make measurements in air, simply divide the measured number
by the immersion effect (1.25).
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. Getting the meter wet can lead to
corrosion that could void the warranty.
Further information on underwater measurements and the immersion effect can be found on the Apogee
webpage (http://www.apogeeinstruments.com/underwater-par-measurements/).
Smith, R.C., 1969. An underwater spectral irradiance collector. Journal of Marine Research 27:341-351.
Tyler, J.E., and R.C. Smith, 1970. Measurements of Spectral Irradiance Underwater. Gordon and Breach, New York,
New York. 103 pages
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