On 8/16/2018 12:25 AM, Mikko OH2HVJ wrote:
mike writes:
We have a lot of wildfires in Oregon and the
TV is warning of seriously bad air quality
due to smoke.
OK, so I took the Nikken 1394 outside.
It registers 2000 particles/liter.
The scale is yellow indicating no big deal.
I dug out the Dylos DC1100 Pro Calibrated to
a particle size split at 2.5 microns.
It read 5322 total count particles per .01 cubic foot.
96% categorized as below 2.5 micron.
After accounting for the units difference, there's
an order of magnitude difference between the readings.
If I take both devices over to the sofa and bang on
the cushions, I can drive both off scale.
Symptoms suggest that the problem is relative to particle
size.
The Nikken (what passes for a manual) gives no
clue as to particle size.
The Nikken website has no info.
This is all I found on the web
http://802.11junk.com/jeffl/Nikken%2...ity%20Monitor/
I'd rather not take it apart if it's working as designed.
Anybody got any relevant info?
I've done some aerosol particle counters down to nm-level. Th Nikken
unit looks like it has simple LED-based optics and maybe
electrometer-based measurement, which typically measures total 'mass' of
particle down to nm-range. There's not a lot of mass in small particles,
so electometers are typically used with size selectors when measuring
nm-size particles.
Dylos has two detectors, one with a cut-off of 2.5um and other with
500nm. Cut-off is the diameter where 50% of the particles are counted.
Nikken has a cut-off probably in the 1-5um range. And it might not be
counting particles, but rather using 'photometric mode', where the total
intensity is used to estimate particle count. This is used also in
scientific particle counters when going over the capability of counting
single particles.
The count of smaller than 2.5um particles can be really high on forest
fires. I always remember when we got our condensation particle counter
prototype working in the lab, it indicated 5M particles/l. We thought
it was an error, but we had just reached way better cut-off of 7nm than
we thought and there was a big forest fire about 300km away!
The difference (2000 vs 19000/l) you're seeing is well explained by
different cut-offs.
That was my conclusion, but...If I understand correctly,
the Nikken measures reflections from particles.
The amplitude of the light reflection should be proportional
to the square of particle size. You could sort particle size
by reflection amplitude.
Dust on the optics could reduce the reflection amplitude and
make huge difference in measured amplitude. The detection threshold
would move up to larger particle size.
If the cutoff is correct as designed, there's nothing to be done.
If it's changed because of optical contamination, taking it apart
to clean the optics might be useful.
I have no idea of the history of the device. It may have been
used in an environment that led to dust coating the optics.
For a device where you can't filter the input, wouldn't you
expect optical contamination? Maybe it calibrates itself before
it starts the fan???
I've been using the Nikken as a survey tool.
In its current condition, it appears to be useless in finding
contaminants that are dangerous to health.
It does appear to be sensitive to household dust like you get
when you pound on the sofa cushions.
If the Dylos will run on 8V, I think I'll make a battry pack
for it.
__________________________________________________ _________
The Dylos has a real-time bar graph. It sits mostly at the
zero end of the scale, then flashes periodically to full scale.
There's a long averaging time in the numerical readout.
That suggests an infrequent hit in a small volume averaged
and scaled up by a huge amount.
It's been years, but I thought I'd read that the Dylos does
the same dark-field reflection method and differentiates
particle size by amplitude of the signal.
I have slightly better knowledge of the Dylos's history.
Bought it from someone who'd bought it new and used it in
controlled 'inside' conditions.
--
mikko