Use the picture below to determine which PAR Sensor (PAR-BTA) you have. Sensors sold from May 2023 have a Apogee SQ-100x sensor head that is attached to the main cable using a P68-rated stainless-steel marine grade (M8) connector (see red arrow in picture below). This connector is located 25 cm from the sensor head. If your sensor does not have a metal (M8) connector then it is the older PAR Sensor and has an apogee SQ-110 sensor head calibrated for use in sunlight.

The older sensor heads have different spectral response profiles than the new sensor heads (see graph below) and thus the newer PAR sensors use different correction factors when measuring PPFD under water, under certain artificial lights, and in certain field conditions. Please see the following link on when to use a correction factor with a PAR sensor

The graph above shows the spectral response profile of the older Apogee SQ-110 sensor head (gray line) compared to the new SQ-100X sensor head (green line). An idealized PPFD response (dotted black line) is presented for comparison.

If you need the manual for the older PAR sensor you can get it by clicking on the link below.