Level Alarm Probes

MCG 1090, MCG 1095

Self-testing overfill and high-level protection.

What It Is & How It Works

A level alarm probe is a dedicated point-level safety device that detects when liquid reaches a critical height in a storage tank. Unlike continuous gauges that track the full liquid level, an alarm probe delivers a single trip signal, high, high-high, or low, to alert operators or trigger automatic shutdown before an overfill or dry-run condition occurs.

The probe suspends a displacer on a spring-loaded magnetic shaft inside the tank. As liquid rises and contacts the displacer, buoyancy reduces its apparent weight, allowing the shaft to rise to its trip point and actuate built-in reed switches. Optional self-testing circuitry momentarily energizes an internal solenoid to physically move the shaft, verifying both mechanical travel and electronic switch response without removing the device from service.

The probe mounts on the tank roof through a flanged or threaded connection. Dual-displacer versions provide staged alarms or floating-roof monitoring from a single penetration. Explosion-proof housings and corrosion-resistant materials suit hazardous tank farm environments. Wireless models use a long-range 2.4 GHz frequency-hopping link to a base station, eliminating conduit runs and simplifying upgrades on older tanks.

Because the sensing principle is purely mechanical, the alarm point remains dependable regardless of changing product conditions, vapors, foam, or turbulence. Self-testing provides verifiable proof that the alarm will function on demand.

Continuous Gauge vs. Level Alarm

A continuous gauge (radar, servo, float & tape) tracks the full liquid level for inventory, custody transfer, and process control. A level alarm probe is an independent safety device: it trips at a fixed point to warn operators or shut down operations before an overfill occurs. Most tank farms deploy both — a gauge for measurement and an alarm for protection. Gauges measure; alarms protect.

When to Choose a Level Alarm Probe

Level alarm probes are the right choice when independent, verifiable overfill protection or point-level safety is required:

Petroleum & Petrochemical Tanks Overfill prevention and high-high shutdown for crude, refined products, and chemical storage under API 2350 and EPA SPCC requirements.
Refineries, Terminals & Depots Independent high-level and high-high alarm redundancy alongside continuous gauging systems for multi-tank operations.
API 2350 Overfill Prevention Dedicated safety-instrumented alarm layer providing independent protection separate from the primary level measurement system.
Remote or Unattended Sites Wireless models simplify installation and enable scheduled self-testing from the control room without tank climbing or site visits.
Pump or Valve Control Automatic start/stop based on high or low level detection in sumps, receivers, and process tanks.

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MCG 1090 MCG 1095

Why It Excels

Safety-Critical Installations Requiring Proof-of-Function

Built-in self-testing physically moves the displacer and verifies electronic switch response, providing documented proof that the alarm will function on demand.

Tanks Where Product Conditions Change Frequently

Mechanical displacer actuation is independent of dielectric constant, density range, or vapor conditions, maintaining a consistent trip point across product changes without recalibration.

Facilities Needing Independent Protection Separate from the Primary Gauge

The alarm probe operates on its own sensing principle and power circuit, functioning even if the continuous gauge fails.

Harsh Environments with Hazardous or Corrosive Atmospheres

Explosion-proof construction with sealed electronics and corrosion-resistant materials rated for Class I, Division 1 hazardous locations.

Older Tank Farms Requiring Retrofit Without New Conduit

Wireless models install on existing nozzles with battery power, adding certified overfill protection without trenching or cable runs.

What to Consider Alongside Level Alarm Probes

Consider an alternative when:

You need general-purpose liquid point-level switching with self-diagnostics and do not require self-testing proof-of-function. See Magnetostrictive Point Level Switches.
The material is a dry solid, powder, or granular product. See Ultrasonic Level Switches.
You need non-contact detection and switching in high temperature high pressure or abrasive products through a vessel wall or window. See Microwave Level Switches.
You need continuous level measurement rather than a high-level alarm. See Servo Level Gauges, Radar Level Gauges, or Float and Tape Gauges.

How This Technology Fits Into a Larger System

Pair with Servo, Radar, or Float & Tape Level Gauges as an independent alarm layer, satisfying API 2350 and IEC 61511 requirements for overfill protection that operates independently of the continuous gauge. See Servo Level Gauges, Radar Level Gauges, or Float and Tape Gauges.
Connect to FuelsManager® or SCADA for centralized alarm status, automated self-test scheduling, and compliance logging across the tank farm. See FuelsManager®.
Pair with Shand & Jurs conservation vents and flame arresters for a complete tank protection system covering both level safety and vapor control. See Conservation Vents.