the green line is the diagnostic wire, which I think is only used for calibration if needed.
I haven't changed vref yet.
What is the purpose of the green wire?
This is what it says in the instructions:
"Diagnostic Output Signal: Connect between the Blue or Green wire and the
Black wire. The diagnostics output signal is a DC Voltage, proportional to the
LED current level, and indicates when the sensor is in or out of calibration.
This voltage is typically in the 1.0 – 2.0 Volts range for all sensors and can
vary from sensor to sensor. The LED current is continuously adjusted to
compensate for the sensor electronics temperature fluctuations, fiber bending,
or aging of the opto-electronic components (Optrand’s patented autoreferencing technique). A diagnostic voltage greater than 2.5 Volts a faulty
sensor. In dynamic operation the diagnostic wire does not require any
connection unless the user wants to continuously monitor the sensor health.
The auto-referencing feature, uniquely available to Optrand sensors, is
particularly useful in long term monitoring and control applications. "
Might seem like a silly question, are you sure there is no pressure on the sensor's port?
You may express the magnitude of my silliness. 
There's no pressure for sure. The sensor is supposed to be installed into a combustion chamber but so far I have only tried to test it outside of any equipment.
So atmospheric pressure is the only pressure applied to it.
The sensor effectively replaces a spark plug on a gas engine or a glow plug on a Diesel engine. The S series outputs 0.5 to 5.0 to indicate the instantaneous pressure. But the output is 39.15 millivolts per bar. So until the pressure is enough to produce 0.5 volts, the effective output is 0.0 volts! Atmospheric pressure will never produce output.
The OP needs to produce a test environment for this sensor!
So I should install it and test it with pressure and see if the output is reasonable? Am I understanding it right?
Why are you spending all this time, effort, and money on this project if you are not going to test it and see if it was worth it?
I am going to test it.
Just thought about connecting it first to the arduino to see how it works and to avoid errors in the future when applying pressure to it during the experiment.
But the values without pressure are not being reasonable. That's why I asked for help in the forum.
So you think I should try it with pressure applied to see reasonable values?
Do you have any other way in mind?
No no.
2435c4fd-ac9c-41e6-9f39-d4cbc95e5f4a.pdf (620.7 KB)
Tom.... 
