This afternoon I was working on a baby Stihl 170 chainsaw and I needed to test it, so carried it out of the shed and standing about 2m from the shed door fired it up. The saw had not been run for two plus years and even though the old fuel had been replaced and the tank rinsed out with fresh fuel, it still released an enormous cloud of of blue smoke. It stuttered and revved, back and forth, making lots more smoke and after about a minute of running I realised I had left the (1.2m wide) shed door open and the slight breeze heading straight for the doorway had filled the shed with smoke.

Inside the shed I quickly looked at the particle meter and saw the PM2.5 at 855 µg/m^3 and quickly turned on 2800 CFM of ventilation and monitored the PM2.5 in time and this is what I got.
The dotted blue line is an average exponential decay across the 20 odd minute I collected data.
The y=1477 exp(-0.266) is the exponential function that describes the dotted blue line.
The 0.266 exponent is the equivalent of the "inverse time taken for half the smoke to be ventilated" - this works ut to 3.75 minutes.

Screen Shot 2022-10-18 at 5.35.25 pm.jpg
The data does not fit the dotted (theoretical) blue line all that well - the reason for this was probably that there was slight breezes outside the shed blowing the remaining smoke remaining outside the shed in different directions . Some of the smoke vented from the shed might even have even blown back over the shed and sucked back inside the shed tru the door.
This may explain the existence of 3 distinct slopes, red, green and purple which better fit the data.
If the decay in smoke inside the shed had followed the green line the time taken for half the smoke to ben vented from the shed would have been closer to 2.5 mins.


My shed is about 100 m^3 and the 2800 cfm (74 m^3./min) of ventilation should vent an entire sheds worth of air in 1.35 minutes (room air change time)
BUT
In practice the best ventilation can usually achieve is vent about half the dust/smoke/air in those 1.35 minutes.
So while I should have got 1.35 minute,s In practice I got 3.75 minutes.


CAVEATS
My shed floor plan is “L” shape which is not that easy to vent - I have two exhaust fans 1200 CFM near the middle and 1600 CFM at the long end of the “L”.
Ideally the shed is being ventilated with fresh outside air and not residually smokey outside air,
I could have turned on my DC for an extra 1200 CFM of extraction.
The PM2.5 outside the front of the house ie well away from the smoke, was at that time about 4 µg/m^3
The PM1, PM2.5 and PM10 readings were close together suggesting the smoke was mostly PM1 ie a reading of PM2.5 = 855 gave a reading of PM10 of ~870 and a a PM1 of ~790.


This is something I would normally do deliberately but I thought I should take advantage of it one way or another.