Reply To: Negative turbidity readings

The shield wire of the OBS3 sensor cable doesn’t actually connect to anything on the sensor, so it’s okay to leave that floating, especially if you’re just testing the sensor.  The green and black wires both definitely need to be connected to GND or the sensor won’t work properly.  Both the high range and low range should read around 0 NTU when the sensor is in air (assuming the sensor is clean and not pointing at any objects).  You should also not test it near sunlight (out of the water) because the light from the sun will interfere with the sensor’s IR input and you’ll get extremely inaccurate results.  Placing a dark object like a towel or a boot or a glove over the sensor will give inconsistent results since the IR reflectivity of different color objects will give different readings.  We found that using a piece of white paper or a bare hand about an inch away from the sensor is a good way to see if you get full-scale readings from the two sensor outputs.  Using a voltmeter, you should see around 2.5v for both sensor output wires when you do this.  With nothing near the sensor, you should see very close to 0v on both channels.  (Be sure to check that you’re getting 5v from the Mayfly on the V+ line of the screw terminal too).