UST Monitoring Systems - How They Work and Why Every Tank Owner Needs Them

Table of Contents

Photo of the Veeder-Root TLS 350 Tank Monitoring System.
Veeder-Root TLS-350 Tank Monitoring System

What is a UST Monitoring System?

A UST monitoring system is a release detection system that tracks fuel levels within an underground or aboveground storage tank over a period of time to see if the tank is leaking. It will also provide measurements of the fuel level, volume and temperature, water level and volume and high and low fuel level warnings.  Many systems are capable of monitoring double wall tanks and lines, pressurized piping and providing remote communication. UST monitoring systems are also referred to as fuel management systems, automatic tank gauge and leak detection systems.

Why do Tank Owners need UST Monitoring Systems?

Photo of UST technician manually measuring tank contents.
Technician measuring the product level in tank manually

There are the three main reasons tank owners require a tank monitoring system:

  • Regulatory Requirements
  • Product Management
  • Leak Detection

Automatic Tank Gauges (ATGs) are important to most tank owners because they help them meet state regulations which require tank and line tightness testing. ATGs also provide accurate product levels at a glance. With an ATG you won’t have to manually stick a tank to measure the tank volume. Knowing the tank volume will help provide accurate fuel inventories.  Accurate fuel inventories make for easy reconciliation of product use or sales. Most importantly, they eliminate manual measurements and calculation of monthly inventory by providing detailed reports of product delivered and product dispensed.

How do Tank Monitoring Systems work?

Photo of an automatic tank gauge probe being installed in an aboveground storage tank.
ATG probe being installed in aboveground storage tank

ATGs use probes to monitor both fuel and water height.  Most underground tanks will have some water at the bottom and regulations require that you maintain less than 1 inch of water in the tank at all times. Probes are accurate to 1/1000 of an inch for level measurements. These probes also measure temperature. Knowing the fuel temperature is important because fuel expands and contracts with temperature changes. The warmer the fuel, the larger the volume change. For example, a 1 degree Fahrenheit rise in a 10,000-gallon gasoline tank will increase the volume by 7 gallons.

The space between the inner and outer wall of a double wall tank is called the interstitial or annular space. A probe placed in a dry interstitial space checks for fluid that is either leaving the inner wall or entering from the outer wall. Some fiberglass underground tanks interstitial space is filled with brine or glycol and a probe is used to measure a drop in fluid level.  A level drop would indicate that the interstitial fluid has either left the outer wall of the tank or mixed with the product in the inner wall. Either of these cases indicates a leak to the ATG connected to this probe.

Alternate terms for the space between the inner and outer wall of an underground tank:

  • Annular space
  • Interstitial space
  • Secondary containment
Infographic of a UST monitoring system.
Click above graphic for detailed view.

UST Leak Detection

Most underground fiberglass and steel tanks are cylindrical. ATG probes are installed around a rib of a fiberglass tank and set on the bottom of the rib. The ribs are interconnected via small air pockets so a leak on one end will migrate to the other end.  Steel tanks typically utilize a float sensor located in a tube that connects to the bottom of the secondary containment. Any leaks in the secondary of a steel tank will trigger the sensor in the monitoring tube.

ATG’s are also used to monitor tank sumps, transition sumps and dispenser sumps.  Continuous monitoring of the sumps insures that no fuel leaves the tank without being accounted for.  Sensors are sometimes used in groundwater monitoring wells to check for fuel on water or for vapor monitoring.

These same sensors work for aboveground storage tanks to monitor for product level, water or loss of product.

Sensor Types - Discriminating and Non-discriminating

Photo of interstitial sensor that was set off by rising oil.
This interstitial sensor in submerged in oil which set off an alarm

A non-discriminating sensor is a float sensor that measures levels. No matter the type of liquid it is installed in, the sensor will indicate a level. These are useful for indicating a leak into an interstitial space, tank or dispenser sump. A discriminating sensor has poly carbon band on the outside of a set of floats. This poly carbon band expands when it comes in contact with fuel and increases resistance in the band. This increase in resistance in addition to the float sensor rising allows the ATG to tell the difference between fuel and water. Another type of discriminating sensor is the optical sensor. A beam of light is directed at a prism and if no liquid is present then the beam is returned to the sensor.  If liquid is present on the outside of the prism the beam is then reflected away. This would be a non-discriminating sensor if not for the addition of continuity pins. Continuity pins added to the exterior of the prism allows the ATG to measure for the presence of fuel or water.  If there is continuity between the pins then water is present and if there isn’t then fuel is present (water is conductive but fuel is not).


Photo of a tank monitoring system.
A look inside an intrinsically safe tank monitoring system

Gas Stations

Each sensor is connected to the ATG through an intrinsically safe set of wires. These wires must be incapable of producing heat or spark that would ignite fuel vapors. Intrinsically safe devices are low voltage circuits which protect the sensor wiring from producing a short and igniting fuel in the tank or sumps. ATGs are constantly monitoring the sensors installed at the site. These systems are also capable of shutting down dispensers, reducing flow of in-tank pumps and notifying the owner/operator that a problem has been detected.  Methods of notification include text, email, fax or external polling systems like a facility security alarm. ATGs are initially programmed by the tank service company to provide alerts at specific thresholds. Owners can specify a product level in inches or gallons that trigger a notification to a fuel delivery company.  When the delivery is made the ATG measures the new level and provides a delivery report of inches and volume gained.  In gasoline service stations, this volume is compared against sales at each pump, to reconcile the difference in the volume gained versus the fuel that was dispensed at the time of delivery.


ATGs are capable of monitoring levels and providing leak alarms but they also provide control of pumps. In the case of a generator where the fuel is supplied from a 110-gallon day tank or base tank, if the fuel level drops in the tank, the ATG can energize a transfer pump that will refill the day or base tank from a larger underground or aboveground tank.

Fuel dispensers have meters that measure the amount of fuel being pumped. ATG’s can indicate that a meter may be out of calibration if the inventory does not match the dispensed volume.

Oil Water Separator System

ATGs can be configured to monitor the oil level that accumulates in an oil water separator system (OWSS). The OWSS collects rainwater and separates out oil and other hydrocarbons before discharging clean water to storm drains. An ATG is used to notify the owner or waste management company to remove the accumulated oil when it reaches a certain thickness or depth in the tank.

Leak Detection Recertification

Photo of a technician documenting the ATG setting.

ATGs are capable of performing a static or statistical test that meets federal tank testing requirements. A static test is done when the tank is not accepting deliveries or dispensing product. The test monitors the tank for level changes over a 2-hour period. A statistical test is performed while the tank is still in service. It ignores the dispensing cycles and captures the levels in between those cycles to create enough data as captured in a static test.

  • Monthly test - 50% capacity at .2 gallon/hour detected leak rate
  • Yearly test - 90% capacity at .1 gallon/hour detected leak rate

In order to insure the ATG is capable of testing to the .1 gallon/hour rate the system needs to be calibrated and certified yearly to the manufacturer’s standard.  The New Hampshire Department of Environmental Services developed this ATG test form to document the required annual testing of ATG equipment. Massachusetts does not provide a test form so CommTank technicians use a form created in-house to document the recertification procedure. The technician performing the test must be certified by the manufacturer and be trained in the performance of the test.

Example of a tank monitor inspection report:

Graphic of an annual Veeder-Root tank monitor inspection report.

ATG System Manufacturers

The following is a list of electronic monitoring system manufacturers whose equipment can be found in USTs and ASTs at facilities throughout New England.