Model 11-R
GRIMM AEROSOL TECHNIK GmbH & Co.KG Seite 70 von 83
6.4 K-line, function test of the optical measuring cell, laser diode and photo
diode
The condition of the optical measuring cell and the function of the optics, photo diode in the optical
measuring cell and laser diode are determined during the self-test. These values appear in what is
called the K-line when the device is controlled using HyperTerminal. The individual values in the K-line
are explained in the following.
DC/v DC_d DC_h c0_h c0_d La_i la_h
K 665 1292 1654 0 0 59 105
The value DC/v shows the DC voltage of the pre-amplifier without bias voltage (offset value). This
serves as a reference value and has no meaning for the customer.
DC/v DC_d DC_h c0_h c0_d La_i la_h
K 665 1292 1654 0 0 59 105
The photo diode generates an open-circuit voltage when the laser diode is off. This open-circuit voltage
is called DC_d where d stands for “dark”. A value of e.g. 1292 in the K-line corresponds to a voltage of
129.2 mV. When the laser is turned on, the voltage of the photo diode in the example shown above
increases to 165.4 mV. The h in DC_h stands for “high”. The increase in the DC voltage results from the
residual light of the laser, e.g. due to reflection in the measuring cell. Most of the laser light is normally
absorbed by the light trap.
The DC_d and DC_h values are variable, for example depending on the temperature. However, the
difference between DC_h and DC_d should always remain nearly constant. Approximately 36 mV in
the example above (165.4 mV – 129.2 mV). Therefore the difference is a good indicator for the condi-
tion of the measuring cell. If the difference between DC_d and DC_h changes a lot, the optics have to
be cleaned or repaired by a service technician. The nominal difference between DC_d and DC_h can
be obtained for each device from the most recent service record, or for new equipment from the ac-
ceptance record.
If the state of the measuring cell changes, e.g. because of dirt, this difference increases rapidly. For
example, lint caught in the aerosol intake nozzle and projecting into the laser beam can cause the DC_h
value to rise to 65000. A difference near zero, e.g. 2.3 mV is typical for a defective laser diode. A defec-
tive laser diode only emits weak light similar to a red LED.
DC/v DC_d DC_h c0_h c0_d La_i la_h
K 665 1292 1654 0 0 59 105
The values C0_h and C0_d always have to be 0.
C0_h stands for counts without particles or zero counts with the laser turned on, and h therefore means
“high”. Particle-free purge air flows through the measuring cell during the purge test. Therefore no parti-
cles can get into the measuring cell and the value has to be 0. A value not equal to zero indicates un-
wanted particles in the measuring cell, e.g. due to a leak in the sample air path.
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