The Silton 4750 Online SDI Analyser is a fully automated instrument for continuous monitoring of Silt Density Index (SDI) in reverse osmosis (RO) pretreatment systems. Designed and manufactured by Tech Inc. in Vaughan, Canada with manufacturing in Chennai, India. Tech Inc. is a Saudi Aramco approved vendor.
Overview
The Silt Density Index (SDI) is the most widely used fouling index in the membrane water treatment industry. SDI measures the fouling potential of feedwater by quantifying the rate of plugging of a standard 0.45 µm microfiltration membrane at a constant applied pressure of 207 kPa (30 psi). RO membrane manufacturers universally specify maximum SDI values (typically SDI₁₅ < 5, preferably < 3) as a prerequisite for membrane warranty coverage. Monitoring SDI is critical for protecting expensive RO membranes, optimizing pretreatment performance, and ensuring reliable long-term plant operation.
The Silton 4750 replaces manual SDI testing (which requires an operator to perform time-consuming measurements every few hours) with a fully automated, online instrument that provides continuous, real-time SDI data 24/7. This eliminates operator dependency, captures transient fouling events that manual testing misses, and provides trend data for predictive maintenance and pretreatment optimization.
What Is SDI (Silt Density Index)?
The Silt Density Index is a standardized empirical test defined by ASTM D4189. It measures the time required to filter a fixed volume (500 mL) of water through a standard 47 mm diameter, 0.45 µm pore size microfiltration membrane at a constant pressure of 207 kPa (30 psi). The test involves measuring the initial filtration time (t₀), then after a standard interval (typically 5, 10, or 15 minutes), measuring the filtration time again (t₁). The SDI is calculated as:
SDI = (1 − t₀/t₁) × 100 / T
Where T is the test interval in minutes. SDI₁₅ (15-minute interval) is the standard measurement. An SDI₁₅ value of 0 indicates perfectly clean water with no fouling potential, while values above 5 indicate high fouling potential requiring additional pretreatment before RO treatment.
Principle of Operation
The Silton 4750 automates the ASTM D4189 SDI test procedure using precision flow measurement, automated valve sequencing, and microprocessor-controlled timing. A sample of feedwater is drawn from the process line at the required pressure (regulated to 207 kPa). The instrument automatically loads a fresh 0.45 µm membrane filter disc from an internal magazine, measures the initial flow rate through the membrane, waits for the prescribed test interval (5, 10, or 15 minutes, selectable), then measures the flow rate again. The SDI value is calculated automatically and transmitted to the plant control system. After each test, the used membrane disc is automatically advanced and a fresh disc is loaded for the next test cycle. A magazine of membrane discs provides weeks of unattended operation.
The instrument performs continuous sequential SDI tests, providing a new SDI reading at each test interval. This creates a continuous trend of SDI values that reveals variations in feedwater quality due to seasonal changes, upstream process upsets, chemical dosing changes, or other transient events that would be missed by periodic manual testing.
Technical Specifications
Measurement range: SDI₅, SDI₁₀, SDI₁₅ (0–6.67)
Test interval: 5, 10, or 15 minutes (selectable)
Membrane filter: Standard 47 mm, 0.45 µm (ASTM D4189 compliant)
Filter magazine capacity: 25 membrane discs (replaceable magazine)
Sample pressure: 207 kPa (30 psi) ± 3%, internally regulated
Sample flow rate: 100–500 mL/min
Sample temperature: 5–45°C
Ambient temperature: 5–50°C
Power supply: 100–240 VAC, 50/60 Hz, 50W
Analog output: 4–20 mA (SDI value, configurable range)
Digital output: RS-485 Modbus RTU
Relay outputs: 2 × alarm relays (high SDI, system fault)
Display: 7-inch color touchscreen with trend graphs
Data logging: Internal storage for 10,000+ test results
USB port: Data export to USB flash drive
Enclosure: IP65 rated, wall-mount stainless steel
Dimensions: 600 × 400 × 250 mm (H × W × D)
Weight: 18 kg
Certifications: CE marked
Key Features
Fully automated ASTM D4189 SDI measurement — no operator intervention required
Continuous 24/7 monitoring captures transient fouling events
Automatic membrane disc loading from replaceable magazine
Precision pressure regulation ensures ASTM-compliant test conditions
High-resolution color touchscreen with real-time trends
Multiple output options (4–20 mA, Modbus, relays) for SCADA/DCS integration
Internal data logging with USB export capability
Configurable alarms for high SDI and system faults
Selectable test intervals (SDI₅, SDI₁₀, SDI₁₅)
Low sample consumption minimizes water waste
IP65 enclosure for harsh industrial environments
Simple maintenance — magazine replacement takes less than 2 minutes
Applications
RO desalination plant pretreatment monitoring
RO feedwater quality verification for membrane warranty compliance
Pretreatment system performance optimization (media filtration, UF, cartridge filters)
Municipal water treatment plant intake monitoring
Industrial process water RO system protection
Power plant boiler feedwater RO pretreatment
Pharmaceutical and semiconductor ultra-pure water RO systems
Oil and gas produced water treatment
Food and beverage process water treatment
Research and development laboratories
Frequently Asked Questions
Q: How often do I need to replace the membrane disc magazine?
A: With a 15-minute test interval running continuously, a 25-disc magazine lasts approximately 6.25 hours. For most applications, a daily magazine change during normal rounds is sufficient. Optional extended-capacity magazines are available.
Q: Can I use the Silton 4750 for MFI (Modified Fouling Index) measurement?
A: The standard Silton 4750 measures SDI per ASTM D4189. Contact Tech Inc. for MFI-capable configurations.
Q: What maintenance is required?
A: Routine maintenance includes membrane magazine replacement, periodic calibration verification (quarterly recommended), and annual service of pressure regulator and flow sensors.
Q: How does the Silton 4750 compare to manual SDI testing?
A: Manual SDI testing requires a trained operator, takes 15+ minutes per test, and is typically performed only 1-3 times per day. The Silton 4750 provides continuous automated measurement, eliminates operator variability, and captures transient events that manual testing misses entirely.
Q: Can I connect the Silton 4750 to my plant SCADA system?
A: Yes. The 4–20 mA analog output and RS-485 Modbus RTU digital output allow seamless integration with any SCADA/DCS system.
Related Products
Tech Inc. Lab RO Test Skid — Laboratory-scale reverse osmosis testing system
Tech Inc. UF/MF Crossflow Test Skid — Pretreatment membrane testing
Tech Inc. Clean-In-Place (CIP) System — Automated membrane cleaning
Tech Inc. Membrane Autopsy Kit — Post-mortem analysis of fouled membranes
For pricing, custom configurations, and technical consultation, contact Tech Inc. at techincresearch.com.