Particulate concentration,
read from a single beam
of scattered light.
The Saaphzone BDM Series mounts directly on the stack and fires a 650 nm laser into the flue. Dust scatters it back, the optics read the return, and the result is a continuous, calibrated mg/m³ reading — through wind, rain, and process heat up to 50°C.
Built for the stack, not the lab bench.
The BDM Series is a continuous, in-situ dust / SPM concentration monitor for stationary pollution sources — engineered to keep reading accurately through wind, rain, lightning, dust loading, and temperature extremes most lab-grade sensors won't tolerate.
The BDM detector head — all-metal housing, IP65.
Cement
Kiln and clinker-line stack monitoring.
Thermal power
Boiler and ESP outlet compliance.
Steel & metallurgy
Furnace and sinter-line dust load.
Oil refining
Process-heater and flare-stack stacks.
Petrochemical
Reactor off-gas particulate checks.
Aluminium
Pot-line and casting dust extraction.
Pulp & paper
Recovery-boiler emission monitoring.
Glass
Furnace stack particulate tracking.
Back-scatter, explained in one pass of light.
A stabilised 650 nm laser source inside the detector fires across the flue, through the dust-blocking window. Particles in the gas stream scatter a fraction of that light straight back toward the receiver — and the intensity of that return is what carries the concentration signal. A condenser lens focuses the scattered light onto the optical sensor, which converts it to a voltage the processing module can read.
Because the window itself collects dust over time, the math has to separate “signal lost to a dirty lens” from “signal lost to a genuinely cleaner stack.” That's what the K1 term below does — and it's also why the purge-air system in Installation isn't optional.
- Po Output power of the probe laser, proportional to excitation voltage Vt.
- D Flue dust / SPM concentration — the value you actually want.
- K1 Dust-blocking window attenuation. Degrades as the lens accumulates dust — adjustable, and the reason for routine cleaning.
- K2 Smoke / dust reflectance coefficient — depends on particle structure and size.
- K3 Lens convergence gain, treated as constant.
Fig. 1 — Optical path, redrawn from the BDM Series engineering reference.
Laser emission
Wavelength-stabilised 650nm source with adaptive optical-power control.
Optical receiving
Condenser lens + sensor converting backscattered light to voltage.
Central processing
Wide dynamic-range phase-locked amplification and zero-drift compensation.
Interface
4–20mA analog loop and RS-485 / Modbus RTU, live simultaneously.
Spec sheet.
Field-configurable across three measurement ranges — BDM-100, BDM-500, and BDM-1000 — sharing one mechanical and electrical platform.
| Working principle | Laser back-scattering |
| Test object | Industrial flue gas / dust (SPM) |
| Measurement range | 0–100 / 500 / 1000 mg/m³ |
| Indication error | ≤ ±1% F.S. |
| Zero drift | ≤ ±2% F.S. / 24h |
| Range drift | ≤ ±2% F.S. / 24h |
| Response time | ≤ 10 s |
| Applicable flue diameter | 0.7 – 20 m |
| Operating wavelength | 650 ± 20 nm |
| Calibration model | Y = KX + B |
| Outer shell | All-metal |
| Dimensions (H×W×D) | 245 × 160 × 160 mm |
| Weight | 4 kg |
| Protection rating | IP65 |
| Power supply | 24 V DC, 0.3 A |
| Operating temperature | −20°C to +50°C |
| Analog output | 4–20 mA |
| Digital interface | RS-485, Modbus RTU |
| Reference standard | HJ 76-2017 |
Where it goes, and how it's aimed.
Placement and aim affect accuracy as much as the instrument itself. The short version: stable flow, a properly proportioned optical path, and a flange that's perpendicular — or tilted very slightly down.
- Site selection. ≥8D downstream and ≥2D upstream of bends, valves, and reducers (D = flue diameter). Where that's not possible, pick the most stable cross-section, keep the upstream run longer than the downstream run, and ensure gas velocity exceeds 5 m/s.
- Optical path. The distance from the laser/receiver intersection to the inner flue wall should not be less than 30% of the flue diameter. Changing the flue diameter later means re-adjusting this path — contact us for support.
- Environment. Avoid corrosive or harmful-gas locations — they shorten connector and IC life and make routine maintenance difficult. Keep the mounting point accessible.
- Signal routing & grounding. Route signal cable away from electromagnetic equipment. Keep protective-ground resistance at or below 10 Ω.
- Mounting angle. Perpendicular to the flue, or tilted 3–5° downward at the front. Never tilt upward — condensation will pool on the window in humid flue gas.
- ⌀160Flange outer diameter, maximum.
- ⌀130Bolt-hole center distance (fixed).
- ⌀10Mounting hole diameter, minimum.
- M10Four connecting bolts & nuts, supplied.
Fig. 2 — Embedded-flange mounting, with signal and power cable routing.
Wiring — 7-pin aviation connector
| Pin | Function |
|---|---|
| 1 | 24V supply − |
| 2 | 24V supply + |
| 3 | Analog signal − |
| 4 | Analog signal + |
| 5 | RS-485 B |
| 6 | Ground (PE) |
| 7 | RS-485 A |
Purge / backflush system
Keeps the optical window from fouling. It is not optional — leave it running 24/7, including through plant power outages.
Compressed air (standard conditions)
200 Pa – 2000 Pa, clean, dry, oil-free. In harsh dust conditions, raise the supply to roughly 0.3 MPa and fit the high-pressure backflush (pagoda) connector.
High-pressure blower (positive-pressure stacks)
Output at least 200 Pa above the flue-gas pressure at the measuring point. Use 220V AC, fit a weatherproof cover outdoors, and keep an inlet filter ahead of the fan — air temperature into the fan should stay below 40°C.
Negative-pressure stacks
Below −100 Pa, the air chamber and main unit can be bolted together directly, with a simple air filter on the inlet — no fan required.
Staying accurate in the field.
The instrument checks its own zero point automatically every hour, with a fuller self-calibration cycle every 24 hours. That keeps it internally consistent — it doesn't, on its own, guarantee the output matches the true dust concentration. For that, a reference test against a known method establishes Y = KX + B, and those K/B values get written into the CEMS system. Skipping this step is one of the most common reasons installations fail acceptance testing.
In-situ calibrator
A protected knob on the unit selects between three states. Unscrew the cover only when calibrating; if it's stiff, the cover itself reverses to work as a wrench.
Record the range output the first time you unpack the unit — that becomes your baseline. Re-calibration error beyond 2% of range against that baseline signals it's time to recalibrate, not just re-zero.
Fig. 3 — Calibrator knob (Z / S / W) and back-cover section.
Optical window care
- Check at day 3, then day 15, then monthly if clean.
- Clean with 50% alcohol or distilled water — never oil-based alcohol — or a lens cloth.
- Six Phillips screws release the pressure plate for full removal of the window slats.
Moisture interference
Above roughly 100°C, flue-gas moisture stays gaseous and doesn't interfere with the reading — true for most power-plant stacks running 100–200°C. Below 100°C, moisture can condense into droplets, which do scatter the laser. If droplet content stays stable, reference testing absorbs the effect; if it swings significantly, treat the readings with caution.
Internal optics
Internal optical components are aligned at the factory with specialised tooling — don't adjust them in the field. If the back cover needs to come off to touch the optical path, power down first and bring in our technical team.
Talk to it directly — RS-485 / Modbus RTU.
For panels that want raw register access instead of (or alongside) the 4–20mA loop. The electrical layer is fixed and cannot be reconfigured — only the application-layer registers below are user-adjustable.
| Baud rate | 9600 |
| Parity | None |
| Data bits | 8 |
| Stop bits | 1 |
| Transport | Modbus RTU |
| Slave address | 0x02 (fixed default) |
| Read function | 0x64 |
| Write function | 0x65 |
| Check | CRC-16 |
Prefer not to poll registers at all? The 4–20mA analog loop runs in parallel and wires straight into most PLC and DCS analog cards.
| Addr | Description | Len | Mode | Default |
|---|---|---|---|---|
| 0x0001 | Dust concentration | 2 reg | R | — |
| 0x0009 | Concentration range | 1 reg | R/W | 100 |
| 0x000A | Calibration K ×100 | 1 reg | R/W | 100 |
| 0x000B | Calibration B ×100 | 1 reg | R/W | 0 |
| 0x0014 | 485 slave address | 1 reg | R/W | 2 |
| 0x0015 | Current-loop gain | 1 reg | W | 0 |
All registers are 2 bytes. Concentration and K/B values are integers — divide the raw register value by 100 to get the actual reading.
02 64 00 01 00 02 A0 30
// Response
02 64 02 00 00 27 10 53 18
// 0x00002710 = 10000 → ÷100 = 100.00 mg/m³
Common symptoms, fast fixes.
No current-loop output
Instrument unpowered, or the output cable is broken / loosely connected.
Check the power supply first, then the control-cable connections (see Installation → Wiring). Replace the cable if it's damaged.
Digital interface not responding
Instrument not running, a communication-module fault, or a broader instrument malfunction.
Confirm the unit is powered and operating, then check the RS-485 wiring (pins 5/7). If the module itself has failed, it will need repair.
Readings consistently too low
Contaminated optical window, or an optical path that's too short for the flue.
Clean the window (see Calibration → Optical window care). If the path geometry is wrong, contact us to re-adjust it.
Readings fluctuate heavily
Unstable on-site operating conditions — pressure swings, moisture, or turbulent flow — rather than the instrument itself.
Review process conditions at the measurement point; consider relocating per the site-selection rules in Installation.
Readings sit near zero
Heavily contaminated optical window, or a damaged laser source.
Clean the window first. If the reading stays near zero after cleaning, the laser source likely needs repair.
Built to pass acceptance testing — not just to run.
HJ 76-2017
Designed against the technical requirements and test methods for continuous emission monitoring systems on stationary pollution sources.
IP65 housing
All-metal enclosure rated for continuous outdoor duty — wind, rain, dust, and −20°C to +50°C ambient swings.
QEO-managed delivery
Quality, environmental, and occupational-health controls run end to end: acceptance, storage, installation, commissioning, and after-sales support.
Continuous Dust & SPM Monitoring
Ready to integrate the BDM series into your existing CEMS setup or require a site assessment for stack emission compliance?