Forward scatter probes use a laser light source to sample low level dust density from smaller ducts. The sensor can be mounted using a single-entry into the duct making installation more simple. Scattered light levels are proportional to the dust density in the light path.
Inside the forward scatter dust monitor
The measuring system works according to the scattered light measurement principle (forward dispersion). A laser diode beams the dust particles in the gas flow with modulated light in the visual range (wavelength approx. 650nm). A highly sensitive detector registers the light scattered by the particles, amplifies the light electrically and feeds it to he measuring channel of a microprocessor as central part of the measuring, control and evaluation electronics. The measuring volume in the gas duct is defined through the intersection of the sender beam sent and the receive aperture.
Continuous monitoring of the sender output registers the smallest changes in brightness of the light beam sent which then serves to determine the measurement signal.
Determining the dust concentration
Measure scattered light intensity is proportional to dust concentration. Scattered light intensity not only depends on the number and size of particles but also on the optical characteristics of the particles and therefore the measuring system must be calibrated using a gravimetric comparison measurement for exact dust concentration measurement. The calibration coefficients determined can be entered directly in the measuring system as numerical values.
A function check can be triggered at fixed intervals as from a definable starting timepoint for an automatic function check of the measuring system. Any unallowed deviations from normal behaviour that may occur are signalled as errors. A function check triggered manually can help localise possible error causes should a device malfunction occur.
The function check comprises:
- Approx. 45 s measurement of zero value, control and contamination of the optical interfaces. The measuring time depends on the increase in contamination value (change > 0.5% – measurement is repeated up to 2 times).
- Every 90s (standard value) output of values determined (duration parameter can be modified.
Zero value measurement
The sender diode is switched off for zero point control so that no signal is received. This means possible drifts or zero point deviations are detected reliably in the overall system (e.g. due to an electronic defect). A warning signal is generated when the “zero value” is outside the specified range.
Control value measurement (span text)
Sender beam intensity changes between 70 and 100% during control value determination. The light intensity received is compared against the standard value (70%). The measuring system generates an error signal for deviations greater than +2%. The error message is cleared again when the next function check runs successfully. The control value is determined with high precision through statistical evaluation of a high number of intensity changes.
For contamination measurement the receiver optics is slewed into a reference position and the scattered light intensity is measured. The measured value determined and the value defined as factory setting are used to calculate a correction factor. This fully compensations any contamination that occurs.
A value between live zero and 20 mA is output on the analog output for contamination values <40%; when this value is exceeded, the “Malfunction” status is output (on the analog output the set error current.
Measuring system Dusthunter SP100 comprises the components:
- Sender/receiver unit DHSP-T
- Connection line to connect the sender/receiver unit to the MCU control unit 9lenghts 5m, 10m)
- Flange and tube
- MCU control unit to control, evaluate and output the data of the sender/receiver unit connected via the RS485 interface
- With integrated purge air supply, for internal duct pressure -50 …+10 hPa
- Without purge air supply, therefore additionally required:
- Optional external purge air unit, for internal duct pressure -50 … +30 hPa
Communication between sender/receiver unit and MCU
As standard, each sender/receiver unit is connected to an MCU control unit via the connection line.
The sender/receiver unit comprises two main subassemblies:
- Electronics unit – It contains the optical and electronics subassemblies to send and receive the light beam as well as to process and evaluate the signals. In the version for use with high internal duct pressure, the electronics unit is in a pressure-proof enclosure.
- Measuring probe – the measuring probe is available in different versions and nominal lengths as well as for various gas temperate ranges and defines the device variant.
Data transfer to and power supply (24 V DC) from the MCU control unit run via a shielded line with 4 wires and plug connector. An RS485 interface is available for service purposes. Clean air to cool the probe and keep the optical surfaces clean is fed via a purge air connection.
The sender/receiver unit is fastened to the duct with a flange tube.
The standard technique for low-dust measurement on small process ducts
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