Ono Sokki AP-981 Waterproof & Acid-Resistant Detector with MX-700 Cable

Ono Sokki AP-981 Waterproof & Acid-Resistant Detector with MX-700 Cable

The Ono Sokki AP-981 is a very accurate magneto-electric rotation detector. It sends out a stable rectangular wave across a large range of speeds, from virtually zero (0 r/min) to very high industrial frequencies. It does this via a magneto-resistive element. This high-performance sensor is designed to accurately measure rotational speed even in the most difficult conditions.

For chemical processing, marine applications, and keeping an eye on heavy industry, the AP-981 is a must-have tool. It can tolerate nitric acid spray and wet conditions because it has an IPX7 waterproof rating and a polycarbonate casing that won’t corrode. The MX-700 connecting cable is included in this kit. It makes sure that your tachometer or data collection system gets a clear, noise-free signal.

Technical Specifications with Marine & Industrial Utility

Key Features

• Ultra-Low Speed Detection: Capable of monitoring rotation from near 0 r/min up to 20,000 r/min (with 60 P/R gear).
• Environmental Resilience: IPX7 waterproof certification and high-grade polycarbonate housing resistant to nitric acid mist (up to 10% concentration).
• Stable Output: Produces a consistent, clear rectangular TTL-level waveform with uniform amplitude across all speeds.
• Non-Contact Measurement: Provides safe, reliable detection without applying any mechanical load or stress to rotating components.

Why Chemical-Resistant Detection Is Non-Negotiable in Marine Systems

Proximity and rotation detection sensors in marine and offshore environments are often exposed to conditions that would destroy general-purpose industrial sensors in a matter of weeks. These conditions include bilge water contaminated with sulfuric acid from battery rooms, acidic condensate in purifier rooms, hydraulic fluid spray in steering gear compartments, and corrosive wash-down chemicals used in food-grade cargo holds. If a speed sensor or proximity detector breaks down in these situations, the effects go far beyond just replacing the instrument. For example, undetected shaft overspeed, missed crankcase explosion precursors, or false RPM signals feeding the main engine governor are all direct threats to the safety of the vessel and the uptime of operations.

The Ono Sokki AP-981 and the MX-700 acid-resistant cable are made to work in exactly these kinds of situations. Ono Sokki has a long history of making precise rotational measuring tools. Their instruments are utilized as calibration standards in engine test cells by Japan’s biggest shipbuilders, such as Mitsubishi Heavy Industries and Kawasaki Shipbuilding. This indicates that the AP-981 is not just a rebranded generic proximity sensor. It is a high-integrity detection tool that was built from the ground up to keep signals safe against chemical attacks and mechanical stress, not just added later.

Ease of Maintenance & Installation

• Single-Body Sealed Assembly: The AP-981 + MX-700 is supplied as a factory-tested, sealed unit — no field assembly of cable glands or connector pins required, eliminating the ingress failure modes that account for the majority of proximity sensor replacements in marine service.
• Standard Threaded Body (M18 or M12 options): Industry-standard thread form allows direct replacement of failed sensors from other manufacturers (IFM, Turck, Pepperl+Fuchs) without machining new mounting holes in gear housings or bearing brackets — significant advantage during port-turnaround maintenance windows.
• No Calibration Required: As a passive magnetic pickup, the AP-981 requires only air gap setting (0.5–2.0 mm) during installation — no zero/span calibration, no firmware upload, no software licensing. A marine engineer with a feeler gauge and a multimeter can commission a replacement in under 20 minutes.
• Custom Cable Length Ordering: Available with MX-700 cable pre-cut to specified lengths, eliminating excess cable loops in engine room cable trays that trap heat, collect oil, and create mechanical chafe points on standard cable installations.

Professional Application Environments

• Main Engine Crankshaft & Flywheel Speed Sensing: Installed on flywheel ring gear or drilled target wheel for primary RPM feedback to the main engine governor and overspeed trip system — the most safety-critical speed sensing point on any vessel. The AP-981’s vibration-hardened construction and chemical resistance make it the correct specification choice over generic industrial proximity sensors for this application.
• Battery Room BESS Monitoring on Hybrid Vessels: Acid mist from VRLA or flooded lead-acid battery banks creates one of the most chemically aggressive environments on any vessel. The AP-981 is used for cooling fan speed verification and ventilation blower monitoring in these spaces, where a standard sensor housing would corrode through within one maintenance cycle.
• FPSO Seawater Lift Pump & Water Injection Pump Drives: Monitoring impeller shaft rotation on vertical turbine pumps in seawater caissons and water injection skids — environments combining salt water spray, chemical scale inhibitor dosing, and continuous vibration from cavitation-prone pump operation.
• Purifier Room Separator Drive Monitoring: Alfa Laval and GEA Westfalia separators operate at high bowl speeds (\(6{,}000\text{–}9{,}000\,\text{RPM}\)) in environments saturated with heated fuel oil and cleaning chemicals. Speed monitoring of the drive motor and bowl shaft with a chemical-resistant sensor prevents the missed overspeed events that result in bowl ejection failures.
• LNG Carrier Cargo Compressor Train: Boil-off gas compressor and high-pressure pump motor shaft speed monitoring in cold, nitrogen-purged machinery spaces where thermal cycling between cryogenic pipeline proximity (\(-165\,\text{°C}\) ambient near insulation cold spots) and machinery heat requires sensors with stable output across a wide thermal range.

Pro-Tip for Marine Engineers

Eliminating Ghost Alarms from Vibration-Induced Signal Noise at Low RPM

The most persistent commissioning problem I encounter with magnetic pickup installations on slow-speed two-stroke main engines is ghost alarm events at low RPM — typically during engine start sequences below \(30\,\text{RPM}\), where the AP-981’s sinusoidal output amplitude drops to the \(100\text{–}300\,\text{mV}\) range. At these low signal levels, the vibration-induced noise conducted through the mounting structure (particularly on engines with high combustion impulse like MAN B&W MC-C series) can exceed the signal amplitude, causing the frequency counter or governor speed input to register false pulses and trigger a spurious overspeed or underspeed alarm.

The engineering fix has two components. First, verify the MX-700 cable shield is terminated to a single-point instrument earth at the signal conditioner end only — floating the shield at the sensor end. Double-ended shield termination creates a ground loop that acts as an antenna for the very noise you are trying to reject. Second, if your signal conditioner or governor module has an adjustable trigger threshold, set it to a minimum of \(500\,\text{mV}\) peak-to-peak to ensure low-RPM noise floor signals below this level are rejected. For installations where the signal conditioner threshold is fixed, insert a Pepperl+Fuchs KFD2-SR2-Ex1.W signal conditioner (\(24\,\text{VDC}\) DIN-rail mount, intrinsically safe options available) between the AP-981 and the governor input — its adjustable hysteresis threshold cleanly resolves this class of ghost alarm without any changes to the governor configuration. Power this conditioner from a dedicated Mean Well EDR-75-24 (\(75\,\text{W}\), \(24\,\text{VDC}\), galvanically isolated) on its own DIN segment to prevent power rail transients from the main engine starter circuit from coupling into the speed sensing loop during engine cranking.

Order & Technical Enquiry

The Ono Sokki AP-981 with MX-700 Cable is stocked and supplied by NDMarineTech for vessel operators, ship management companies, OEM service teams, and offshore platform procurement departments. We provide pre-sales technical consultation on air gap calculation, cable length specification, and signal conditioner selection for your specific machinery application. Contact our marine automation team with your engine type, target wheel specification, and existing governor model for a complete sensor integration recommendation.