September 16, 2025
The difference between MESA 1-D and MESA 2-D: which test applies to your station
By Dagoberto Torres
Advanced Technician
The tightness test is an essential part of the maintenance of every fuel service station. In Panama, Master Services performs this test with the MESA equipment from ATS Environmental Services, a system that uses digital acoustic analysis to determine whether a tank or a line has a leak. The same equipment supports two test modes — MESA 1-D and MESA 2-D — and knowing when each applies helps to plan the work correctly.
What the MESA equipment does
MESA (an acronym for Mass Extraction Sonic Analyzer) is a tightness-test instrument manufactured by ATS Environmental Services in the United States. The basic procedure consists of introducing a microphone (called a sonde) into the tank, applying a controlled pressure or vacuum, and letting a computer with proprietary software digitally record and analyze the acoustic signals generated inside the vessel. At the end, the system issues a binary result: pass or fail. That result is documented in the digital record of the test, along with the environmental parameters and the tank conditions during the trial.
Both test modes — 1-D and 2-D — are run under Test Method #900 of the official ATS protocol.
MESA 1-D: positive pressure, vapor portion
The 1-D mode applies positive pressure to the tank and tests only the vapor portion (ullage), that is, the space above the level of the liquid product. This includes the upper part of the tank, the vents, the vapor lines, and the accessories associated with that zone.
1-D is appropriate when the interest is concentrated on the integrity of the upper part of the tank and its accessories, and when the installation's risk profile allows the test to be scoped to the vapor zone.
MESA 2-D: controlled vacuum, complete tank
The 2-D mode applies controlled vacuum and tests both the vapor portion and the liquid portion of the tank, that is, the complete vessel. This is the integral tightness test of the tank, the one operators normally seek when they want overall certainty about the condition of the vessel and the lines in contact with product.
When «the MESA test» is referred to generically for a service station, in most cases the requested scope is 2-D.
Technical scope common to both modes
The MESA equipment is certified for tanks of up to 30,000 U.S. gallons (approximately 113,550 liters), with a product level between 0% and 91% of fill. In practice, this covers the entire universe of tanks found in service stations, minor terminals, tanker truck compartments, and aboveground storage tanks (AST) in industrial and marine facilities.
Products compatible with the test include gasoline, diesel, kerosene, jet fuel, lubricating oils, hydraulic oils, and other liquids common in hydrocarbon operations. To apply vacuum (2-D) or pressure (1-D) safely, the equipment uses inert gases — typically nitrogen or CO₂ — together with a two-stage Gast vacuum generator, a 0–80 inches of water manometer, the MESA sonde, an ACTv1 water sensor, and the computer with the official software.
How to choose between 1-D and 2-D
In the initial conversation with the client, three elements are normally reviewed before deciding the scope of the test:
- The objective of the job: whether it is an integral verification of the vessel (2-D) or a validation focused on vents and the vapor zone (1-D).
- The age and history of the tank: old installations, with prior repairs or with findings in previous inspections, tend to benefit from the complete 2-D test.
- The documentation the client plans to submit to the National Secretariat of Energy (SNE), the Ministry of the Environment, or the Fire Department, and the specific requirements of their operation or license.
Master Services supports the client in this decision. On the prior technical visit we physically inspect the tank, its vents, the containment chamber, the product lines, and the installed leak detection system, and we recommend the appropriate test scope.
What we deliver when closing the test
At the end of the test — regardless of the 1-D or 2-D mode — we deliver the document package that backs the technical condition of the installation: the test record with the pass/fail result issued by the MESA software, technical report signed by the responsible engineer, photographic record of the work, and calibration certificates where applicable. If the test identifies findings, we document the repair or maintenance recommendations so the client can plan the next steps.
When a tank does not pass the test, we offer the complete cycle of diagnosis, leak location, repair or replacement of tanks, lines, and accessories, environmental remediation where applicable, reinstallation, and a new MESA test to document closure of the finding.
Why Master Services does this work
Master Services is a Panamanian company specialized exclusively in fuel station infrastructure, with 48 years of continuous operation since 1978. We are authorized users of the MESA equipment in Panama and our technicians are trained in the official ATS protocol. Since our beginnings we have served more than 350 stations across the country, with clients such as Delta, Texaco, Puma, Terpel, Ultra, MIT, CLH, and the Panama Canal Authority. We are members of the Petroleum Equipment Institute (PEI), the leading international association in the sector.
To schedule a MESA 1-D or 2-D test, or to ask which scope applies to your station: +507-274-0836 · info@masterservices.org.