Emerson Mobrey SMA1/RS Level Alarm Switch

Emerson Mobrey SMA1/RS Magnetic Float Level Alarm Switch

The Emerson Mobrey SMA1/RS (frequently designated under RS Stock No. 185-9847 or Delta-Mobrey M-Switch series) is a side-mounting, magnetically coupled float-operated liquid level switch engineered for high-integrity alarming and pump control in extreme marine and industrial process environments.

Constructed from corrosion-resistant AISI 316 (1.4401) austenitic stainless steel throughout both wetted parts and switch housing, the SMA1 eliminates mechanical gland packings, seals, or linkages between the process fluid and the electrical enclosure. Level detection is transmitted across a solid pressure-retaining stainless steel bulkhead via opposing permanent magnets. As liquid level traverses the horizontal float axis, the internal magnet snaps a single-pole change-over (SPCO / SPDT) microswitch with a positive, vibration-immune action that completely prevents contact hover, chatter, and bounce.

Rated up to 19 bar (275 psi) at 20°C with process operating temperatures up to 130°C (266°F), the SMA1 is the standard instrumentation choice for shipboard marine bilges, day-tank high/low alarms, lube oil sumps, expansion tanks, and boiler auxiliary condensate monitoring.

Model Details

• Manufacturer: Emerson Process Management / Mobrey Measurement (Rosemount Measurement Ltd. / Delta Mobrey)
• Full Model Code: SMA1 (Mobrey M-Switch Series)
• Commercial Part Number / RS Designation: SMA1/RS (RS Components Stock No. 185-9847)
• Internal Stock / Type Reference: CON964.006.0001 / Ref: 42286
• Mounting Pattern: Mobrey ‘A’ Flange (4-hole bolt circle, 92 mm PCD)
• Conduit Connection: Single M20 x 1.5 mm ISO metric entry
• Net Weight: 1.95 kg (4.30 lbs)

Technical Details

Features

1. Glandless Magnetic Repulsion Coupling: Liquid containment is absolute; the float mechanism inside the tank actuates the microswitch through a solid 316 SST barrier wall, eliminating leak paths, o-ring wear, and shaft seal degradation.
2. 316 Stainless Steel Wetted & Enclosure Construction: Fully resistant to marine salt-fog atmospheres, heavy fuel oils (HFO/MDO), lubricating oils, condensates, demineralized boiler feedwater, and aggressive chemicals.
3. High-Integrity Snap-Action SPDT Output: Features gold-flashed contacts rated up to 15 A at 250 VAC for direct relay operation, alongside low-power dry contact compatibility for PLC, DCS, and marine alarm monitoring system (AMS) digital inputs.
4. Dual Ingress Protection (IP66 & IP67): Heavy O-ring sealed housing withstands high-pressure washdown deck conditions, localized flooding, and prolonged weather exposure.
5. Compact In-Tank Footprint: Requiring only 153 mm (6.0 in) horizontal insertion depth and a float clearance swing of 112 mm (4.4 in), it mounts in restrictive standpipes, external float chambers, and narrow vessel nozzles.

Compatibility

• Direct Mounting Flanges: Mates with standard Mobrey ‘A’ weld-on companion flanges (P/N: J150 mild steel, J151 316 SST), Mobrey weld pads, and standard external float chambers.
• Marine Alarm Monitoring Systems (AMS): Direct dry-contact integration with Kongsberg K-Chief, Lyngsø Marine, Wärtsilä Valmarine, Praxis Mega-Guard, and Siemens S7-1500 / ET200SP digital input fail-safe loops.
• Pump Control & Starter Panels: Compatible with all single-phase and three-phase motor starters via interposing control relays or contactor coils. (Note: Must not be wired directly to start large electric motors exceeding 0.1 HP).
• Safety Relay Units: Interfacing capability with standard safety relays (PNOZ, Banner, Schneider XPS) configured in normally closed, fail-to-trip safety loop architecture.

Frequently Asked Questions

Can the Emerson Mobrey SMA1 be installed vertically through the top of a tank?

No. The SMA1 is specifically designed strictly for horizontal side mounting through the vessel shell or an external chamber. It relies on a balanced pivot arm that hinges vertically when liquid rises around the horizontal float. For top-entry vertical tank installations, vertical float level controls (such as the Mobrey S-series vertical air-break controls) must be selected instead.

How do you invert the switch operation from high-level alarm to low-level alarm?

The SMA1 provides a Form C SPDT contact block with Common (COM), Normally Open (NO), and Normally Closed (NC) connections. To change the functional alarm logic, you do not need to physically invert the switch body; you simply change your field wiring termination between the NO and NC terminals to suit either high-level trip or low-level cut-out logic.

What is the difference between Mobrey SMA1 and SMA2?

The SMA1 is a weatherproof industrial and marine model rated IP66/IP67 for non-classified or safe engine room spaces. The SMA2 is the explosion-proof/flameproof certified model built with an ATEX and IECEx approved Ex d IIC T6 enclosure for hazardous locations like cargo pump rooms, paint lockers, or flammable fuel storage zones.

What is the maximum fluid viscosity and minimum density the SMA1 can detect?

The SMA1 float will reliably operate in any clean process liquid with a minimum Specific Gravity (density) of 0.75 SG. It is suitable for lubricating oils, marine diesel, heavy fuels (at operating temperatures reducing viscosity below 150 cSt), and water-based solutions. Highly viscous, cold bitumen or unheated bunker oils that prevent free float movement must be avoided.

How can I test the SMA1 switch mechanism without filling the tank with liquid?

With the tank vented, depressurized, and isolated from power, you can unscrew the terminal end-cover to access the switch assembly. For non-pressurized vessels with manhole access, you can manually lift the float arm to check the magnetic snap-action and listen for the sharp microswitch click. Alternatively, when fitted in an external float chamber, close isolation valves and use the chamber drain/vent cock to cycle liquid locally.