Notes taken from meeting with GRIMM @ ARSF Gloucester on 24 / 04 / 2012

Present at meeting:

ARSF ( Gary, Dave, Phil, James, Mark1)
FAAM (Jamie)
GRIMM (Wolfgang - from Germany, Vimal - from UK office)

[Missed the start of the meeting as couldn't get in building :). Also, presentation was mainly in German as it hadn't been translated yet. Vimal to forward it on after translation complete.]

Most interesting part of the GRIMM Sky OPC instrument is the "measuring cell". This uses it's aerodynamic properties to 'ensure' the particles go through the centre of it. The laser is focused at the centre of the measuring cell. Laser light is scattered off particle onto a mirror, then onto the detector.

Can measure up to 2 million (I think) particles per cubic litre of air. The detection volume is always 1 single particle, so it does not detect more than 1 particle at once.

There are 4 pressure sensors in the machine to monitor the pressure before/after critical orifices and pumps.

There are 32 channels (bins) of different particle size.

Can be operated at different time resolutions:

1 sec, 2 sec, 3 sec - Only 16 channels are used: either > 0.25 um to 2.5 um or >2.5um to 32 um. No multiplexing
6 sec - uses multiplexing: 1 second to measure first (smallest particles) 16 channels, 5 seconds to measure the last 16 channels. The time difference is because there are fewer particles at the larger sizes so you get better stats by measuring for longer.
1, 5, 10, 15, 30, 60 minutes

Measuring cell does not work well in low temperatures - 20 degrees C is good. We should put it in a insulated box to keep temperature up (especially for eg in Iceland).

Probe should ideally be straight, or with a gentle smooth curve - Wolfgang says our one is curved but looks good. Probe should be heated also so as to prevent condensation or other effects due to air temperature change.

If we want a scientific contact for this kind of data: Prof. W at Dusseldorf has done much work with GRIMM data.

Note that an optical measuring system (such as the Sky OPC) can not give you information regarding particle density (?).

Period of time showing the newer labview software - seems easy to use: set time intervals, specify file settings, start. If you get lots of 0's this probably means the pump isn't running or some other kind of flow problem.

When you start the machine you get a message called the "K line". This contains potentially important info regarding the quality of the calibration.

K line: DC/v DC_d DCc_h C0_h C0_d Ilalo Ilahi

DC_d dark current (laser off)
DC_h dark current (laser on)
C0_h / C0_d are count numbers and should be 0 (or less than 5ish) If higher contact GRIMM and ask what it may mean.

Taking the ratio of DC_d and DC_h should give a number similar (+/- 30%) to the calibration certificate value. If not it means the instrument need recalibrating.

Phil to set up system such that when it shuts down it actually shuts down, starts up, shuts down. This will give us a K line either end of the data collected.

Also get P-lines every x messages. This contains info such as date/time and also the 2 pressure values - these should be similar to each other. Jamie was talking about a correction you could apply to the data if it turns out the pressure values are different to correct for this effect.

Jamie also discussed the need to measure the inlet / probe behaviour. Test it with an aerosol generator straight into the Sky OPC, and then again by attaching the generator to the inlet. In perfect conditions these would give the same (similar?) results. Any attenuation in the numbers would be due to particles "getting lost" in the inlet. This will be temp/pressure dependent. FAAM do this and have a aerosol generator (discussions on ARSF using it at FAAM). Note number of particles identified is not as important as particle size identified (i.e. maybe all 25um are missing when using the inlet etc). It is important to keep the inlet at a constant temperature.