An automatic tank gauging (ATG) system does more than measure level — it measures product level to ±0.5 mm,multi-point temperature to ±0.1°C for volume correction,water bottom interface to ±1 mm,and in some configurations,product density for mass calculation. For a refinery with 40 tanks holding $200 million in crude and finished product inventory,that sub-millimeter accuracy translates directly to financial reconciliation. The decision between wired and wireless ATG architecture turns on three factors that have nothing to do with the sensor itself: hazardous area wiring regulations,the cost of Ex-rated conduit and IS barriers,and the availability of a reliable power source at each tank.
What a Wired ATG System Actually Connects
A traditional wired ATG uses a servo-driven displacer or a radar gauge head mounted on the tank roof,connected by a multi-conductor armored cable to a field junction box at grade,then via trunk cable to a central ATG console in the control room. Each tank requires a minimum of three cable pairs: one for the gauge head (power + Modbus or proprietary serial),one for the multi-point temperature probe (RTD array,typically Pt100 elements spaced at 1–3 meter intervals),and one for the water bottom sensor (capacitance or conductivity probe). In an ATEX Zone 0 environment — the vapor space inside a gasoline or crude oil tank is Zone 0 — every conductor entering the tank must pass through a galvanic isolator or intrinsic safety barrier. A single tank's worth of IS barriers and Ex-rated cable glands can cost $800–1,500 before the first meter of cable is pulled.
For a refinery tank farm with 40 tanks in a 500-meter radius,the installed cost of the fieldbus trunk cable,junction boxes,IS barriers,and terminations for a wired ATG typically runs $200,000–400,000 — comparable to or exceeding the cost of the gauge heads themselves. This is where wireless ATG becomes economically interesting.
Wireless ATG: Removing the Cable,Keeping the Accuracy
A wireless ATG system replaces the trunk cable from tank to control room with a WirelessHART or ISA100.11a radio link. The gauge head,temperature probe,and water bottom sensor remain at the tank — unchanged. A wireless field adapter mounted at the tank collects the Modbus or HART data from all three instruments and transmits it to a wireless gateway located at the control room. The gateway reconstitutes the fieldbus data stream and presents it to the ATG console exactly as if it came from a wired fieldbus card.
The critical question is whether the wireless link can support the data rate and latency requirements of custody-transfer tank gauging. For inventory management — reading level and temperature once every 15 minutes — WirelessHART's typical 10–30 second update rate is more than sufficient. For custody transfer — where you need to record level before,during,and after a 30-minute pipeline transfer — a 10-second polling interval on a WirelessHART network with 20–40 devices is achievable if the gateway is within 200 meters of the farthest tank. Beyond 200 meters,or with more than 50 devices per gateway,the polling cycle stretches to 30–60 seconds,which is marginal for dynamic transfers but still acceptable for daily inventory reconciliation.
The Endress+Hauser Proservo NMS81 servo tank gauge achieves ±0.4 mm level accuracy with a modular design that integrates a multi-point RTD temperature probe and a water bottom sensor in a single tank opening — reducing the number of tank nozzles and the associated flange and gasket maintenance. It supports both wired Modbus RS-485 and WirelessHART via the NMS8x platform,so the same gauge head can start wired and convert to wireless later when an ISA100 or WirelessHART gateway is installed.
When Wireless ATG is the Right Choice
Three conditions favor wireless ATG. First,existing tank farms with no spare cable trays or conduit — running new Ex-rated trunk cable through a live refinery requires hot work permits,scaffolding,and confined-space entry procedures that can double the installed cost per meter relative to greenfield installation. Second,remote tanks more than 300 meters from the control room — a single wireless gateway can cover a 500-meter radius,eliminating a dedicated fiber or copper trunk cable run to the distant tank cluster. Third,tanks that currently have mechanical float gauges with no electrical connection at all — adding wireless ATG provides real-time inventory visibility without any trenching.
When Wired ATG Still Wins
Wired ATG remains the right choice for greenfield tank farms with a planned cable infrastructure,for custody-transfer applications that require sub-second polling during high-flow-rate pipeline transfers,and for tanks located in dense metallic environments — inside a floating-roof tank with a steel dome,a 2.4 GHz WirelessHART signal attenuates by 40–60 dB,which effectively blocks the wireless link. Wired ATG also avoids the battery replacement cycle: wireless field adapters in Ex zones typically use intrinsically safe battery packs rated for 3–5 years,and replacing 40 tank-mounted battery packs on a scheduled maintenance rotation adds roughly $200–300 per tank per replacement cycle in labor and battery cost.
Does wireless ATG work through a floating roof?
No — a steel floating roof is essentially a Faraday cage. The wireless antenna must be mounted outside the tank shell,typically on a short mast above the gauge head housing. The signal passes through a polycarbonate or glass radome that maintains the Ex rating. For tanks with an internal floating roof inside a fixed-roof tank,the antenna can be mounted on the fixed roof with the gauge head still connected through the roof penetration — the antenna and the gauge electronics are at different physical locations,connected by a short coaxial cable. This arrangement adds approximately $500–800 per tank to the antenna and cable cost.
For more on the level measurement technologies behind ATG systems,browse our radar level transmitter and guided wave radar ranges. For ATEX-certified signal conditioning,see our intrinsic safety barrier catalog.
