Mobrey Float Rod Assembly Inquiry

The Mobrey magnetic rod assembly is the mechanical heart of a Mobrey-style boiler water level control. A permanent magnet is fitted to a float or displacer on a vertical rod that slides inside a non-magnetic stainless-steel pressure tube/centre tube. As the water level changes the float (or displacer) and its attached magnet move, transmitting motion through the tube to one or more externally mounted Mobrey switch units. The switch units contain a unique three-magnet snap-action mechanism (primary/secondary/tertiary magnet arrangement) which gives positive, vibration-resistant latching and reliable switching without springs or mercury. This glandless, hermetically-sealed design makes the assembly particularly suited to the demanding environment of steam boilers — high pressure, high temperature and vibration — while minimising maintenance and eliminating leakage/contamination paths.

Technical highlights (concise)

  • Principle of operation: Float/displacer carrying a permanent magnet moves along a non-magnetic rod/tube; magnet action actuates external switch modules by magnetic repulsion/attraction (three-magnet snap-action).
  • Glandless / hermetically sealed options: Switch mechanisms are available fully hermetically sealed (gas-filled) for corrosive or low-temperature atmospheres — no moving contact exposure.
  • Multiple switch points: Switch units can be positioned along the rod to provide several independent alarm or control points (field-adjustable).
  • Switching variants & contact materials: Choice of general-purpose (silver cadmium oxide), gold-plated low-power contacts for intrinsically-safe circuits, and high-current variants — with rated switching capability up to about 10 A on selected mechanisms.
  • Ruggedness & reliability: No springs in the switching element means excellent resistance to vibration and contact bounce; long service life and low maintenance suitable for boiler service.
  • Pressure & temperature capability: Designed for boiler/process use — many models operate at high pressures and temperatures (example ratings shown in manufacturer data sheets: e.g., certain displacer models up to ~102 bar and temperature ranges up to +300 °C depending on model and options). Always select the model rated for your exact service conditions.
  • Approvals & standards: Available with international approvals (ATEX / IECEx flameproof options, FM/CSA explosion-proof, marine variants) and manufactured to recognised quality/pressure directives (PED/ASME where applicable).
  • Materials & wetted parts: Typical configurations use stainless steel (non-magnetic tube) and materials selectable for wetted parts (stainless steels, aluminium-bronze, etc.) to match boiler water chemistry and service. N

Typical specification summary (what to check when ordering / specifying)

  • Assembly type: float-rod or displacer-rod assembly (with or without float).
  • Mounting: direct vertical mounting or supplied in a chamber (side-and-bottom / side-and-side chambers available).
  • Switch options: SPST (single) or DPDT (double) and variants D4/D8 (general), X4/X8 (high current), P4/P8 (gold-plated low-power/IS), H4/H8 (hermetically sealed).
  • Electrical ratings: see manufacturer table — some high-current types approach 10 A; gold-plated types are for low-power/intrinsically-safe use.
  • Mechanical lengths: float/displacer and rod assemblies can be supplied to order (field-adjustable cable lengths also used on displacer types). Confirm exact travel / switch point spacing required.
  • Pressure/temperature: model-dependent — verify maximum working pressure and temperature for the selected rod/chamber/switch combination (examples in DS: up to ~102 bar and up to +300 °C for certain models).
  • Process connections: flanged or threaded process connections and chamber options; ensure compatibility with boiler penetration and pressure vessel rules

Benefits for boiler applications

  • Positive, repeatable level detection that tolerates vibration and steam environment.
  • Low maintenance due to sealed magnetic coupling (no dynamic seals/glands on the rod).
  • Safe for hazardous atmospheres with certified flameproof/intrinsically-safe options. D