Automatic Tank Gauge Product

Overview

Automatic Tank Gauges (ATGs) are the primary inventory management and leak detection systems at retail fuel stations. An ATG continuously measures fuel level in each Underground Storage Tank, calculates inventory volume (corrected for temperature), and transmits data to the Fuel Management System and back-office servers. By comparing inventory against dispensed volumes, the ATG detects slow leaks (under-pumpings) that would be invisible to daily visual inspections.

A typical ATG installation includes magnetostrictive Magnetostrictive Probe sensors (one per tank), a wall-mounted Console Display Unit with LCD display, temperature Temperature Sensor Assembly probes, and a modem for telemetry. The console powers all sensors from a regulated 24 VDC supply with battery backup, ensuring that level and temperature data persist during power outages.

Magnetostrictive Probes

The core sensor is the Magnetostrictive Probe, a non-contact guided-wave level transducer. The Magnetostrictive Waveguide Rod is a precision stainless steel rod suspended into the tank from the manway. A sealed Magnetic Float Cage rides on the rod, floating on the fuel surface. Embedded in the float is a neodymium magnet.

The probe head (mounted in the tank lid opening) houses a Magnetostrictive Excitation Coil that sends a low-frequency magnetic pulse (~1 MHz) down the rod. When the pulse reaches the magnet in the float, it produces a voltage transient that propagates back to the coil. The Signal Conditioning Module measures this return time; elapsed time is directly proportional to the distance (fuel level) traveled by the wave. A 30-foot stroke tank returns a measurement every 100 milliseconds with ±0.25% accuracy.

Console and Display

The Console Display Unit is a compact tabletop or wall-mounted unit displaying tank level in gallons, fuel temperature in degrees Celsius, and alarm status. The LCD Display Module shows 4 lines of information; the Input Keypad allows technicians to navigate menus, set tank capacity, configure alarms, and adjust temperature offset.

The Main Controller PCB microcontroller continuously acquires analog signals from probe and temperature sensors, applies calibration curves, and stores data in non-volatile memory (typically flash). Every 60 seconds or on demand, the console calculates current inventory as: Volume = (Measured Level − Calibration Offset) × Tank Capacity / Maximum Probe Stroke, then temperature-corrects using: Corrected Volume = Measured Volume × (1 − 0.0006 × (Fuel Temperature − 60 °F)).

Temperature Correction

Fuel volume expands and contracts with temperature. The Temperature Sensor Assembly assembly uses a Pt100 RTD Sensor Pt100 platinum thermometer installed in a stainless steel RTD Thermowell thermowell screwed into the tank side. The RTD element changes resistance ~0.385 Ω/°C; the console's analog-to-digital converter measures this and derives fuel temperature.

This temperature reading is critical for fiscal (sales) accuracy. A 1000-gallon tank of gasoline at 40 °F contains ~2% more mass than at 60 °F. Retail operators report volume and reconcile sales at a standard reference temperature (usually 60 °F), so the ATG's temperature correction is legally required in most jurisdictions.

Leak Detection Algorithms

The ATG continuously monitors inventory change rate. If inventory drops faster than fuel is being dispensed (accounting for pump meter error and drip loss), the console triggers a leak alarm. Configurable detection windows (1–8 hours) filter out false alarms from temperature swings and fill delays. Advanced ATGs perform mass-balance calculations: Leak Rate = (Previous Inventory − Current Inventory) − (Total Volume Dispensed + Delivery Volume) / Time Elapsed.

A leak rate of 0.1 GPH over an 8-hour window typically triggers alarm. The console outputs a discrete dry-contact relay signal to the Fuel Management System, which logs the alarm and may automatically shut down dispensers to prevent continued loss.

Temperature-Corrected Inventory Reporting

The console transmits inventory data to the back-office Fuel Management System at configurable intervals (1–4 hours typical). Transmitted data includes raw level, corrected volume, fuel temperature, leak alarm status, and transaction count. The Modem Communication Module uses either cellular LTE Cat-M or dial-up V.92 to reach the FMS server via public telephone network.

Back-office software reconciles all three inventory data streams: ATG gauge readings, dispenser meter totals, and delivery receipts. Discrepancies are flagged for investigation. A persistent inventory loss of 0.3% per month or higher indicates a significant leak requiring tank removal and testing.

Alarm Outputs

The Alarm Relay Output Module provides 4 independent dry-contact SPDT relay outputs:

• Tank low alarm (inventory below setpoint)
• Tank high alarm (overfill detected)
• Leak alarm (0.1 GPH+ loss rate)
• System fault (probe disconnected, RTD open circuit)

Each relay can drive external audible alarms, pump shutoff solenoids, or status lights. Relay contacts are rated 120 VAC 5 A, sufficient to cut power to the Pump Unit motor contactor.

Power and Backup

The Power Supply Module is fed from 120 VAC mains and provides regulated 24 VDC. A Backup Battery sealed lead-acid cell (12 Ah) is kept float-charged by a Battery Charger IC IC. If AC mains fails, the battery maintains console operation and modem communication for 24 hours, allowing remote FMS acknowledgment of the outage and delayed restart upon power restoration.

Installation and Maintenance

The probe Magnetostrictive Waveguide Rod is installed by lowering through the tank manway; the PTFE bushings ensure smooth float travel without friction. The Probe-to-Console Shielded Cable carries four shielded conductors (3 for RTD, 1 for probe signal return) from the probe to console, often 500–1000 feet in long station installations. Connector shields are bonded to both probe and console ground to prevent EMI from nearby electrical equipment.

Annual calibration checks verify probe accuracy against known fuel volumes or gravity measurements. Temperature sensors are tested against ice-water (0 °C) and boiling-water (100 °C) baths. Modern ATGs store historical data internally; download ports allow technicians to analyze inventory trends and detect slow leaks before they become liabilities.