Forward Scattering Spectrometer Probe (FSSP) Droplet Measurement Technologies Particle Simulator

The Droplet Measurement Technologies' (DMT) Forward Scattering Spectrometer Probe (FSSP) Particle Simulator was designed as an aid in measuring important electronic characteristics of Particle Measuring Systems' (PMS) FSSP and as a tool for locating problems in the electronic circuitry of this instrument.

The particle simulator uses red LEDs (635 nm wavelength) to produce light pulses of frequency, length, and amplitude similar to the pulses of scattered light produced by droplets passing through the FSSP's laser beam. The simulator circumvents the optical path of the FSSP and attaches directly to the signal and annulus photodetectors. Thus, the entire chain of electronics is exercised with this device. Although the amplitude of the light pulses are within the acceptable size range of the probe, the simulator should not be used as a Calibrator since the light levels have not been adjusted to correspond to any particular size category.

The particle simulator controls the pulsing of two LEDS htat are ae assembly (plastic cube) that is similar to the photodetector mounting block of the FSSP. The first step in the use of the simulator is to remove the mounting block carefully from the FSSP. Power should be removed from the probe at all times when attaching the simulator to the FSSP. Gently extract both photodetectors from this block and insert them in the plastic cube attached to the simulator. Make sure that the annulus detector is inserted in the receptacle marked with an A. Position the photodetector module to insure that it will not accidentally make electrical contact between any components or printed circuit board traces and the rest of the probe.

After applying power to the Probe and turning on the power switch of the simulator, the simulator will control the relative magnitude, frequency, and duration of the light pulses.

PULSE LEVEL SIGNAL: Turning this dial adjusts the amplitude of the light at the signal photodetector. If the FSSP is attached to a data system while being tested, counts will fall in increasingly higher size channels as the signal amplitude control is rotated clockwise (this happens only if the annulus pulse level is turned off or lower or is lower than the signal pulse level).

PULSE LEVEL ANNULUS: The light at the annulus detector is varied with this control in the same fashion as the signal pulse level control. The annulus and signal amplitudes are independent of one another.

PULSE FREQUENCY: Different droplet concentrations can be simulated with three fixed repetition rates selectable in the Low, Medium, and High position of the pulse frequency control. These three positions give frequencies of 33.3, 38.5, and 45.4 Khz, respectively. Note: The measured counts from the FSSP will be substantially less than these frequencies because of the velocity rejection circuitry. If velocity rejection is disabled, the number of counts out of the probe should be nearly equal to the number that are input.

PULSE WIDTH: Different airspeeds are simulated by selecting one of six pulse durations with the pulse width control knob.

Although the annulus and signal amplitudes can be controlled independently, the pulse frequency and duration controls produce the same pulse frequency and duration at both the annulus and signal detector.