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I have been experiment with the PPD42NJ SPI particle sensor,I can read the pulse width from the sensor output,but how can I convert the reading to a meaningful figure,at least it can give me some figure like the SPI figure publish by the environment department,the Datasheet didn't mention anything about the convertion
I had a quick look at the datasheet and the acual particle concentration is encoded in the dutycycle of the pulsed output, not just in the pulse width. So, you need to measure and calculate the dutycycle to begin with.
Once you have that the way forward depends on the what kind of accuracy and resolution you need.
The curve looks pretty linear from 0 to 4000 particles per cubic foot but then it tapers off slightly but lets say it didn't and that your dutycycle value ranges from 0 to 10000 representing 0-100% in 0.01% steps. At the 4000 particles Count the dutycycle for an average sensor is around 6.5% or the value 650. You need to multiply 650 by 6.1538 . Here's one way: ParticleCount=DutyCycleValue */1575.
If you need to take the slightly unlinear response into consideration then a lookup table of values (say at every 500 counts) and linear interpolation between those will probably work.
/Henrik.
Thank you,I can manage to get some meaningful figure now,thanks a lot