By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed August 17, 2026
Sulfuric acid tank level is normally measured with a non-contact radar transmitter carrying a PTFE-faced antenna. Concentrated acid (93 to 98 percent, specific gravity about 1.84) destroys most wetted sensor materials, yet reflects radar strongly. Hydrostatic and differential pressure instruments still work in the right concentration range, but the span must be built on the acid density, not on water.
The acid adds three complications that a water tank never shows. Corrosivity changes direction with concentration. The headspace carries acid mist and sometimes hydrogen gas. Density runs near twice that of water.
This guide walks through the technology choice, the concentration and material logic, the radar details, the hydrostatic math, and the overfill layer.
Contents
- Why acid is different
- Choosing the technology
- Concentration and materials
- Radar on acid tanks
- Hydrostatic and density
- Overfill protection
- Installation rules
- FAQ
Why acid is different
Three properties of stored sulfuric acid drive the instrument choice. First, density. Commercial 93 percent acid has a specific gravity of 1.834, and 98 percent about 1.84. Every pressure-based reading, float depth and load calculation therefore runs near twice the water value.
Second, corrosivity: which materials survive depends on concentration, and the relationship is not intuitive. Third, the headspace: concentrated acid gives off fine acid mist, and in carbon steel tanks the corrosion reaction releases hydrogen gas.
Concentrated acid is also hygroscopic. It pulls moisture out of any air that enters through the vent. The absorbed water forms a slightly diluted layer at the surface and around the vent nozzle.
Diluted acid attacks steel far faster than concentrated acid. That is why corrosion often shows up first at the top of the tank rather than at the wetted wall.
None of this makes the measurement itself hard. An acid surface is calm, conductive and an excellent radar reflector. The engineering problem is keeping the instrument alive, which is why the selection below starts from what touches the acid.
Choosing the technology
The table compares the level technologies that appear on acid storage tanks. The short version: non-contact radar is the default for concentrated acid. Pressure-based instruments suit dilute acid when the wetted face is fluoropolymer. Anything with moving wetted parts needs a strong reason.
| Technology | Behavior on H2SO4 | Where it belongs |
|---|---|---|
| Non-contact radar | Nothing touches the liquid; PTFE or PVDF antenna face resists the mist; strong echo from the high-dielectric surface | Default for all concentrations; see the radar for corrosive liquids |
| Submersible hydrostatic | PTFE-jacketed probe survives dilute acid; span must use the acid density | Dilute acid below roughly 70 percent; see the anti-corrosive submersible transmitter |
| DP with remote seals | Tantalum or Hastelloy diaphragm faces the acid; transmitter sits away from the tank | Closed or blanketed tanks; see diaphragm seal transmitters |
| Capacitance probe | Fully PTFE-insulated rod works in stable acid; coating and condensate shift the reading | Smaller dosing tanks; see the capacitance level transmitter |
| Float and magnetostrictive | Moving wetted parts; the float also rides high in a 1.84 SG liquid, so buoyancy must be re-checked | Point-level use with PTFE floats only; see the multipoint float switch |
| Ultrasonic | Acid mist and blanketing gas attenuate the acoustic pulse; readings drop out during filling | Not recommended on concentrated acid headspace |
Behavior notes assume storage at ambient temperature. Hot or fuming acid narrows every option; confirm materials against the datasheet.
Point-level protection is a separate decision. An external clamp-on ultrasonic switch reads through the tank wall with zero wetted parts. That makes it one of the cleanest high-level options on acid; the ultrasonic level switch covers that approach. The overfill section below explains where it sits in the architecture.
Concentration and materials
Sulfuric acid corrosivity runs backwards from intuition. Concentrated acid at 93 to 98 percent is routinely stored in plain carbon steel tanks. A thin ferrous sulfate film that forms on the wall protects the steel.
Dilute and mid-range acid destroys that film, attacks carbon steel and most stainless grades aggressively, and forces fluoropolymers or exotic alloys. The industry standard for carbon steel acid tanks, NACE SP0294, covers concentrations above 70 percent for exactly this reason.
| Concentration | SG at 20 °C | Instrument wetted parts that survive |
|---|---|---|
| Below 30% | 1.00 to 1.22 | PTFE, PVDF, Alloy 20; carbon steel and 304/316 corrode |
| 30 to 70% | 1.22 to 1.61 | PTFE, Alloy 20, Hastelloy; the harshest band for every common steel |
| 70 to 93% | 1.61 to 1.83 | PTFE, tantalum, Hastelloy; carbon steel enters service near the top of this band |
| 93 to 98% | 1.83 to 1.84 | PTFE, tantalum; carbon steel tanks standard at ambient with corrosion allowance |
| Oleum | Above 1.84 | PTFE and selected alloys only; PVDF is not recommended at any temperature |
SG anchors from published density tables at 20 °C: 70% = 1.61, 90% = 1.81, 93% = 1.834, 98% = 1.836 to 1.84. PVDF handles concentrated acid at ambient only; hot concentrated acid calls for PTFE or tantalum.
Two material traps are worth calling out because they appear in vendor copy. PEEK, an excellent engineering plastic almost everywhere else, is dissolved by concentrated sulfuric acid and has no place in the wetted path.
And 316 stainless has only two narrow windows: cold acid below roughly 15 percent, or above roughly 85 percent at low velocity. Across the mid range it corrodes severely. The general method for reading a datasheet wetted-parts list is covered in our wetted parts guide.
Carbon steel tanks add one more actor: hydrogen. The corrosion reaction that forms the protective film also releases hydrogen gas.
Rising bubbles scrub the film off the wall and cut curved grooves, a failure mode called hydrogen grooving. The gas then collects in the headspace, where it is flammable. That argues for non-contact instruments with no spark path into the vapor space. It is also one reason hot work on an acid tank in service is forbidden.
Radar on acid tanks
Sulfuric acid is close to an ideal radar target. Instrument dielectric tables list the static value for concentrated acid at 84 to 100, on par with water at about 80. Even the lower values quoted at radar frequencies sit far above the 1.5 to 3 minimum a non-contact radar needs.
The echo is strong, the surface is calm, and there is no foam. Radar trouble on acid tanks never comes from the physics. It comes from the antenna material and the mist.
Frequency matters mostly through beam width. An 80 GHz transmitter with a full-size antenna focuses to roughly a 3 degree beam. A 26 GHz unit with the same antenna size spreads to about 10 degrees.
At 6 m range that is a 0.31 m footprint versus 1.05 m. The difference decides whether the beam clears a narrow nozzle, an internal ladder or the fill stream. Small-antenna 80 GHz units spread wider, so check the datasheet beam angle rather than the frequency alone.
Acid mist condenses on any cold surface, including the antenna face. A PTFE lens antenna sheds condensate better than a metallic horn. Mounting the transmitter away from the fill nozzle keeps splash off the face during deliveries.
If a purge is fitted, it must run on dry instrument air. Purging with moist air delivers water vapor straight to the hygroscopic acid, right at the antenna.
Application example
Chemical plant, Argentina. The site stored sulfuric acid and caustic soda in separate tanks, and both media destroy common wetted sensor materials. Rather than pick a different technology per tank, we proposed corrosion-resistant non-contact radar for both services. One instrument approach across an acid tank and a caustic tank means one spares position and one calibration routine instead of two.
Hydrostatic and density
Every pressure-based reading on an acid tank runs through the density. The base relation is the same as for any liquid:
P = ρ × g × h
3 m of 93% acid (1,834 kg/m³) = 1,834 × 9.807 × 3 = 54.0 kPa, about 540 mbar, where 3 m of water would give 294 mbar
The same density converts level to inventory. A vertical tank 8 m in diameter holds 50.3 m³ per meter of level. That meter weighs 92.2 t as 93 percent acid, against 50.2 t if the tank held water.
Getting the density wrong therefore corrupts both the level and the mass figure. The mechanics of head pressure are covered in our hydrostatic pressure guide.
The failure that goes unnoticed is a concentration change. A transmitter spanned for 98 percent acid (SG 1.84) that receives a delivery of 70 percent acid (SG 1.61) reads 12.5 percent low. The lighter column simply produces less pressure at the same true level. Even a drift to 90 percent (SG 1.81) reads 1.6 percent low.
Density is also how the plant checks concentration itself: 66 degrees Baume, SG 1.835, is 93 percent acid by definition. An inline instrument such as a tuning fork density meter turns that into a continuous concentration readout.
Overfill protection
An acid overfill is a small-volume, high-consequence event. Under the US CERCLA rules the reportable quantity for sulfuric acid is 1,000 lb. At SG 1.84 that is only about 247 L, roughly 65 gallons of 98 percent acid.
Note that SPCC spill plans under 40 CFR 112 cover oil, not acid. An acid tank answers to CERCLA reporting, state chemical storage rules and plain engineering practice instead.
The architecture that practice expects has two independent layers. The continuous transmitter, radar in most cases, feeds control and inventory. A separate high-level switch, on its own power and wiring, trips the inlet valve or an alarm. Because it shares no failure mode with the transmitter, a fault in one layer never blinds both.
On acid, the cleanest switch choices are the external clamp-on ultrasonic type that reads through the wall, or a PTFE-bodied float switch inside the tank.
Test the switch layer on a schedule and log it. A high-level switch that has never been exercised since commissioning protects nothing. Lifting the level to the switch point during a controlled fill, or using the vendor test function, takes a few minutes per tank per year.
Secondary containment sized for the tank plus rain sits under all of this. The same layered thinking runs through the rest of our chemical industry overview.
Installation rules
Keep every penetration above the liquid line if the layout allows it. A top-mounted radar, a top-entry switch and a top bracket for wiring leave the shell unbroken below the surface. That removes the leak paths that matter most on an acid tank.
Where a bottom connection is unavoidable, as with a submersible sensor cable gland or a DP seal nozzle, use PTFE or PTFE-faced gaskets. Standard graphite and elastomer gaskets are attacked.
Mount the radar nozzle away from the fill line so the beam never crosses the falling stream. Give the antenna a full-bore nozzle without weld protrusions.
Keep the vent path in mind. A dryer or scrubber on the vent slows the moisture uptake that feeds the diluted surface layer. It also spares the antenna a permanently wet headspace.
Finally, respect the dilution exotherm during any flushing or decommissioning around instruments. Mixing water into concentrated acid releases about 95 kJ per mole of acid, enough to boil the water locally and spatter acid. Lines and cavities are flushed acid-into-water, never the reverse. An instrument pulled from an acid tank gets rinsed by immersion, not by a water jet into a cavity full of concentrate.
FAQ
How to measure tank level?
Pick the technology by what the liquid allows. On sulfuric acid that means non-contact radar for concentrated acid and PTFE-wetted hydrostatic sensors for dilute acid. Closed tanks take DP transmitters with tantalum or Hastelloy remote seals. A separate high-level switch backs up whichever continuous instrument you choose.
How to measure sulfuric acid concentration?
Through density at a known temperature. Published tables tie specific gravity to concentration; 66 degrees Baume, SG 1.835 at 15.6 °C, corresponds to 93 percent acid. In a plant, an inline density or concentration meter reads this continuously, and a lab hydrometer does the same job on a sample.
How to measure level in a closed tank?
Use an instrument that does not need an open surface to the atmosphere. Non-contact radar works through a top nozzle at tank pressure. A DP transmitter with remote seals reads the level as the difference between bottom and top taps, so the blanket gas pressure cancels out.
What is a tank level indicator?
Any device that converts liquid height into a reading you can act on. The range runs from a sight gauge to a 4-20 mA transmitter feeding a control system. On sulfuric acid the term usually points to a radar transmitter plus a local display. Sight glasses and dip methods put people next to the acid.
Request a quote
Tell us the acid concentration, temperature, tank height and material, and whether you need continuous level, a high-level switch or both. We will match antenna and wetted materials to the concentration and propose an instrument set with the datasheets. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.