The GEOKON Model 1900-11-150M Reed Switch Probe with 150-Meter Measurement Tape, engineered and manufactured by GEOKON, INC. in Lebanon, New Hampshire, USA, is the primary sensing instrument of the Model 1900 Magnetic Extensometer System, recognized worldwide for reliable subsurface settlement and heave monitoring.

Operating Principle of the Model 1900 Magnetic Extensometer:
The system is engineered to quantify total vertical displacement as well as zone-specific settlement across multiple geological strata:
- A 1" Schedule 80 PVC access tube (with telescoping couplings) or a 70 mm OD grooved inclinometer casing is installed down the borehole or placed within structural fill.
- Permanent magnetic anchors (spider anchors in boreholes, plate anchors in fills) and a bottom datum magnet anchored into stable non-yielding bedrock are positioned at predetermined measurement elevations.
- The operator lowers the 1900-11-150M probe down the access tube. When the probe's internal reed switch enters the radial magnetic field of an anchor, the electrical circuit closes.
- Two internal electrical conductors within the steel tape conduct this signal to the surface reel, instantly activating a high-decibel audible buzzer and a bright indicator light.
- The operator then records the precise depth reading on the graduated steel tape against the precision index marker positioned at the top of the casing collar.

Key Engineering Features & Operational Advantages of GEOKON 1900-11-150M:
- Precision Passivated Stainless Steel Probe: CNC-machined from high-grade stainless steel (178 mm length by 19 mm outer diameter), ensuring corrosion resistance against brackish groundwater, aggressive leachates, and high hydrostatic pressures. The slender 19 mm profile ensures smooth, snag-free transit within standard 1" pipe tubing.
- Nylon-Coated High-Tensile Steel Tape: Features an internal high-tensile spring steel core that prevents physical stretching under heavy hanging loads, encapsulated in an extruded clear nylon sheath. Precision-graduated in millimeter intervals (1 mm resolution, ±3 mm repeatability), ensuring reliable long-term measurement fidelity.
- Rugged Transport Reel with Audio-Visual Signal Unit: Heavy-duty freestanding winding reel equipped with a friction brake, a standard 9V battery compartment, a battery test function, and a buzzer/light indicator panel.
- Independent Inter-Anchor Settlement Delineation: By tracking relative distance shifts between sequential magnetic anchors over time, geotechnical engineers can isolate the exact compressibility and consolidation rates of individual soil layers.

Target Industrial Sectors & Applications:
- Transportation Infrastructure & Highway Embankments: Settlement tracking of highway approach fills over soft clays, preload surcharge monitoring, and bridge abutment stability.
- Water Resources, Earth & Rockfill Dams: Profiling vertical consolidation within impermeable clay cores, rockfill shells, and downstream toe embankments.
- Deep Foundations & Commercial High-Rises: Multi-level subsoil settlement and heave monitoring beneath raft foundations and adjacent to deep basement excavations.
- Geotechnical Ground Improvement Skids: Settlement profiling during preloading with prefabricated vertical drains (PVD), vacuum consolidation, and deep soil mixing columns.
- Land Reclamation & Port Terminals: Tracking consolidation rates and primary settlement across offshore reclamation fills and coastal breakwater berms.

• Manufacturer: GEOKON (GEOKON, INC.)
• Country of Origin: 100% USA (Made in USA)
• Product Line: Model 1900 Magnetic Extensometer System Accessories
• Model Designation: 1900-11-150M (Part Code: 1900-11-150M)
• Unit of Measure: Piece / Unit (Unit: pc)
• SENSING PROBE SPECIFICATIONS:
  - Sensor Principle: High-sensitivity magnetic reed switch triggered by permanent magnetic fields
  - Probe Construction: Passivated stainless steel hermetically sealed probe body
  - Probe Dimensions: Length 178 mm x Diameter 19 mm (L 178 × Ø 19 mm)
  - Submersible Rating: Fully submersible, rated for deep downhole hydrostatic water pressure
• MEASUREMENT TAPE SPECIFICATIONS:
  - Total Tape Length: 150 meters (150m)
  - Tape Metallurgy: High-tensile steel core with embedded dual electrical conductors
  - Tape Sheath: Heavy-duty extruded clear nylon jacket resistant to abrasion, oils, and chemical leachates
  - Tape Graduations: Metric laser markings with 1 mm resolution (Imperial 0.01 ft options available)
  - System Resolution: 1 mm
  - Measurement Repeatability: ±3 mm
• CABLE REEL & SURFACE READOUT UNIT:
  - Alert Signals: Integrated audible buzzer and bright visual LED indicator
  - Power Supply: Standard 9V alkaline transistor battery (easily accessible and user-replaceable)
  - Reel Controls: Battery test push button, audible volume/sensitivity adjustment, mechanical friction lock
  - Stand Architecture: Rugged aluminum frame with high-impact composite storage drum
• SYSTEM COMPATIBILITY:
  - Guide Tubing Compatibility: Standard 1" Schedule 80 PVC pipe (33 mm OD) or 70 mm OD inclinometer casing
  - Anchor Compatibility: GEOKON Model 1900-5A/5B Datum Magnets, Model 1900-7A/7B Spider Magnets, Model 1900-4 Plate Magnets
  - Index Hardware: Supplied complete with casing collar index marker
• ENVIRONMENTAL SPECIFICATIONS:
  - Operating Ambient Temperature Range: -30°C to +80°C (-22°F to +176°F)
• Quality & Metrology Compliance: Conforms to American geotechnical engineering standards, ISO 9001 certified.