4 System Overview
8
4
System Overview
4.1.
Dilution and Conditioning
4.1.1.
Principle
testo ASET15-1 Air Supply / Evaporation Tube is an accessory for the testo MD19 which is the
Rotating Disk Diluter with external diluter head for performing the primary dilution as close as
possible to the aerosol source.
This combination complies with the method of ThermoDilution according to the regulation for
nanoparticle measurement UN-ECE R83 and R49. ThermoDilution with testo MD19 and testo
ASET15-1 separates sampling, dilution and conditioning of the aerosol into the following steps:
Primary dilution of combustion engine emissions from tail pipe or CVS with the testo
MD19-diluter. testo ASET15-1 generates the primary dilution air for the testo MD19-
diluter with a calibrated and controlled flow of 1.5
l/min.
Removal of volatile particles in the Evaporation Tube where the temperature can be
adjusted up to 400°C (recommended heating temperature according to GRPE-
draft =
300
°C). No recondensation takes place in the cooling down zone assuming the
measuring gas is below the dew point after primary dilution.
Secondary dilution in an adjustable dilution factor range from 1 to 11 in a mixing
assembly whose construction minimizes thermophoretic losses. The primary diluted
measuring gas from the primary testo MD19-3E diluter with a flow of 1.5
l/min and the
evaporation tube is diluted with secondary dilution air generated in testo ASET15-1. Its
flow is adjustable in a calibrated range of 0...15
l/min corresponding to a dilution factor
range of 1...11. The total measuring gas flow, up to 16.5
l/min enables the user
furthermore to connect nanoparticle instrumentation, which consumes higher measuring
gas flows than can be drawn from the testo MD19-diluter, whose diluted measuring gas
flow is limited to 5
l/min.
4.1.2.
ThermoDilution
Fig. 4.1 shows a schematic plot of the mass concentration of a volatile compound against the
temperature of the surrounding gas. In a dilution tunnel both the concentration and the
temperature of the substance are reduced (path A
B). During dilution, the compound passes
its dew point and nucleates into nanodroplets (curve N). Subsequent secondary dilution (B
D)
will reduce the number concentration of the droplets, but is unable to evaporate them, because of
a hysteresis effect between nucleation and evaporation.
A strategy to avoid the mere formation of nanodroplets is direct sampling from the hot exhaust in
combination with hot dilution (A
C). Given a sufficient dilution factor, the volatiles will not
nucleate during subsequent cooling (C
D) even though the same final state is assumed as
through dilution tunnel and secondary dilution (A
B
D). However, in some applications e.g.
measurement on CVS tunnel, direct sampling is not possible, and nanodroplets already exist in
the gas sample (B). In those cases the diluted gas sample (D) has to be heated above the
evaporation point of the compound (C
D, crossing curve E). Like with hot dilution, the
compound remains in vapor phase upon subsequent cooling
(C
D). The combination of diluter and heater (B
D
C
D) is known as ThermoDiluter.
Hot dilution is realized in Testo rotating disk diluters. Together with Testo rotating disk diluter
testo MD19-3E the testo ASET15-1 forms a complete ThermoDiluter system.
Fig.4.1: volatile mass diagram
