GRIMM U-DMA,
Universal Type Electrostatic Classifier

Our classifier, the Vienna type U-DMA was developed in collaboration with the Aerosol Physics Group at the University of Vienna.  This is an interchangeable DMA. and the the kit allows the user to work with either a 5-340nm or 10-1100nm DMA. This capability is achieved by the ability to interchange the cylinder and inner rod of the DMA.  It has a  unique universal adapter for inner (center) and outer cylindrical electrodes. With this design of the U-DMA, three different lengths of the middle parts (electrodes) can be attached or exchanged to cover 3 different measurement ranges (L-DMA/M-DMA/Nano- respectively S-DMA). The exchange of the middle part (electrodes) can be done easily; the DMA head and the bottom part stay always the same. (figure 2).

When the Vienna U-DMA electrostatic classifier is used with our Ultrafine Particle Counter, Model 5.402, it gives a maximum concentration of 10⁷.  If used with our portable battery operated Ultrafine Particle Counter, Model 5.403, it has a maximum concentration of 10¹⁰.  The complete system weighs less than 30Kg and can be ready to start taking measurements in only a few minutes from initial set-up.  This makes it ideal for field applications.

Remote operation can be via modem, but the built-in battery pack, data logging and auto-reboot facility means that totally independent operation is also possible.  Like many other particle counters, the Grimm CPC uses Butanol vapor to make the particles detectable.  Over time, particularly when measuring in wet conditions, condensed water vapor can contaminate the Butanol and reduce efficiency but our CPCs have an automatic moisture drain-off to prevent this happening. The Butanol container is also spill-proof which greatly adds to its portability. An automatic self test operation checks that the system is functioning correctly before measurements begin.

 

Figure  above left: Schematic setup of the "Vienna-Type“ DMA. Figure above right: The M-DMA and L-DMA models of the Vienna style U-DMA series. 1 - cylindrical outer electrode 6 - sheath air inlet  2 - pressure channel 7 - DMA HV-module and control  3 - sample inlet + impactor with differential pressure measurement 8 - excess air outlet  4 - DMA-head with laminator 9 - DMA base with integrated filter for DMA-exhaust air 5 - neutralizer 10 - sample air outlet (mono mobile aerosol flow).

Model	Range [nm] (Sheath Air)	Active Length [mm]	Total Height [mm]
L-DMA	10 - 1100 (3.0 l/min)	350	492
M-DMA	5 - 350 (3.0 l/min)	88	230
(Nano-) S-DMA	1 - 55 (10.0 l/min)	15	157

The table above shows the different "Vienna-Type“ U-DMA models which can be simply built up by changing the middle parts (figure 3). The Nano S-DMA model is normally operated with an electrometer (FCE) instead of a CPC and a high-precision voltage supply combined with higher sheath airflow rates. With this setup particles down to 1 nm size, or even below (ions), could be detected with highest resolution. The resolution of the Nano S-DMA is in the range of 1 Angström.

In the Vienna style DMA models the high voltage is fed to the bottom of the centre rod.  The advantage of this method is the lowered sensitivity to flashovers e.g. in combination with higher humidity in the sample airflow. The center rod is operated as a positive electrode. The reason is the slightly higher statistical probability of negative charged particles and a higher rate of classified particles in each mobility fraction.

Figure 2 Long electrode of a „Vienna-Type“ U-DMA 1 - outer cylindrical electrode 2 - centre rode 3- channel to pressure sensors 4- tube for sheath air  5 - impactor nozzle

The alteration of the U-DMA from model M-DMA to L-DMA  (range: 5-350 nm respectively 10-1100 nm at 3 l/min sheath air)

In this design a high value has been set on lowest particle loss. According to that dictum the sample inlet and outlet sections have been designed. Additionally unnecessary electrical fields have been avoided beyond the electrical classification field also to avoid again particle loss. Figure 3 shows that the L-DMA has relatively high transmission efficiencies, even down to the lowest particle sizes (10 nm). The losses of the L-DMA is largely identical with the M-DMA even in the range < 20 nm.

Figure 3

Figure 3 Transmission efficiency of "Vienna-Type“ L-DMA and M-DMA models sheath airflow of 3 l/min (e.g. in combination with GRIMM CPC #5.400). The U-DMA models by default are already featured with pressure and temperature monitoring inside the DMA classifying area. Changes in the ambient conditions could be now followed in a modified operation and data reduction algorithm. There is a pressure difference measurement over the impactor in the sample inlet. Corresponding pressure sensors are integrated in the DMA-controller unit attached to the U-DMA. The high voltage power supply and a microcontroller unit communicates bidirectional with the GRIMM CPC #5.400 (16 bit - 255 channels, with offset control for exact voltage settings) and sets the appropriate voltage.

It also includes pressure and temperature sensors. An autonomous voltage control monitors the correct settings at the DMA and lets the CPC as central control unit know if voltages have been set and if the DMA is properly operating.