Technological Advances and Practical Implementation of Hydrostatic Level Measurement with Smar Pressure Transmitters

Contemporary industrial instrumentation demands levels of precision, stability, and reliability that transcend the simple capture of analog signals. In the vast field of process automation, level measurement in tanks and reservoirs stands out as one of the most critical applications, directly impacting operational safety, inventory control, and the productive efficiency of chemical, petrochemical, food, and sanitation plants. Among the various technologies available, hydrostatic pressure measurement has established itself as the most versatile and widely used method, based on robust physical principles and the electronic evolution of intelligent devices. In this context, Smar plays a pioneering role, especially through the development of capacitive sensors that eliminate the need for internal analog-to-digital (A/D) conversion, providing a direct digital reading free from conventional thermal drift.

Theoretical Foundations and Physical Principles of Hydrostatic Measurement

Level measurement based on hydrostatic pressure is founded on Pascal's Law and Stevin's principle, which states that the pressure exerted by a column of liquid at rest is directly proportional to the height of the column, the density of the fluid, and the local acceleration due to gravity. The fundamental mathematical relationship governing this phenomenon is expressed by the classic hydrostatic equation:

In this equation, P represents the hydrostatic pressure detected at the measurement point, ρ (rho) is the density or specific mass of the process fluid, g is the acceleration due to gravity, and h is the vertical height of the liquid column above the sensor. For practical instrumentation and calibration of devices in the field, the industry often adopts units of millimeters of water column (mmH2O), simplifying the direct conversion between physical height and measured pressure:

This simplification assumes that the density of water at 4°C is exactly 1.000 g/cm³. However, in real-world applications, the accuracy of level measurement intrinsically depends on the constancy of the fluid density. Significant variations in process temperature can alter the density, introducing errors that must be compensated for by the control system or through internal transmitter algorithms, such as those found in the Smar LD300 and LD400 series.

The Architecture of the Smar Capacitive Sensor

The technological differentiator that positions Smar as a leader in pressure and level measurement lies in the architecture of its capacitive sensor. Developed and improved since the 1980s, this sensor is based on a differential capacitance cell where a central sensor diaphragm moves between two fixed metallized surfaces, isolated by a ceramic or glass material. Unlike traditional piezoresistive sensors, which depend on the variation of the electrical resistance of a material under deformation, the Smar sensor uses the minimal deflection of the diaphragm to alter the capacitance of an oscillator circuit.

The capacitance variation is directly converted into a frequency variation. Since frequency is a digital quantity by nature, it can be processed directly by the transmitter's CPU without the need for A/D converters. This feature is vital for eliminating quantization errors and thermal drifts associated with analog conversion components. The signal remains digital from the sensor capture stage to the communication output (HART, Foundation Fieldbus, or Profibus PA), ensuring exceptional long-term stability, with warranties of up to 12 years on high-performance models.

The table below details the main comparative performance characteristics between the LD300 and LD400 series, highlighting the evolution of Smar technology towards high-demand applications and functional safety.

Tank Geometries and Linearization Challenges

The application of pressure transmitters for level measurement requires a detailed analysis of the tank geometry. Tanks are primarily classified as linear and non-linear, a classification that determines the need for complex transfer functions to calculate volume or mass.

Linear Tanks - Simplicity and Proportionality

Linear tanks are those whose cross-section remains constant throughout the entire usable height. Typical examples include vertical cylinders, rectangular and square silos. In these configurations, the fluid level is directly proportional to the stored volume. The Smar transmitter configuration in these cases is simplified, where the Lower Range Value (LRV) is associated with the 0% level and the Upper Range Value (URV) with the 100% hydrostatic pressure level corresponding to the maximum height of the tank.

Non-Linear Tanks - The Need for Tonnage Tables

In tanks where the cross-section varies with height—such as horizontal cylindrical, spherical, conical, or domed-bottom tanks—the relationship between height (linear level) and volume (volumetric level) is not constant. Although the pressure transmitter measures the height correctly, the resulting volume requires a mathematical conversion.

Smar addresses this challenge by implementing a 16-point "Linearization Table" or "Point Table," freely configurable by the user. This table allows the entry of XY value pairs, where X represents the linear level (pressure or percentage of height) and Y represents the corresponding volume.

Practical Example - Horizontal Cylindrical Tank

One of the most complex cases in the industry is that of the horizontal cylindrical tank. Consider a reservoir with a diameter of 2m and a length of 10m. The total volume of the tank is approximately 31.42m³. However, when the tank is 10% full (0.2m), the occupied volume is only 5.2% of the total, due to the narrow base of the cylinder. As the level approaches the center (50% height), the rate of volumetric change reaches its maximum.

For this scenario, the Smar LD301 or LD400 transmitter is configured with the "TABLE" transfer function. The microprocessor performs linear interpolation between the entered points, ensuring that the 4-20 mA output or digital signal reflects the actual volume and not just the height of the liquid column.

Installation Architectures and Internal Pressure Compensation

Correct transmitter installation is essential to avoid systematic measurement errors. The main distinction lies in the tank pressure condition: atmospheric or pressurized.

Measurement in Atmospheric Tanks

In tanks open to the atmosphere, the pressure above the liquid is the local barometric pressure. To measure the level, the high-pressure side (H) of the differential transmitter is connected to the base of the tank, while the low-pressure side (L) remains open to the atmosphere. In this way, the effect of atmospheric pressure acts on both sides of the sensor and cancels each other out, resulting in a net reading of the hydrostatic pressure.

A critical aspect is the transmitter's position relative to the desired zero level. If the instrument is installed below the tank bottom (due to accessibility for maintenance), the static liquid column in the impulse line will exert additional pressure that must be electronically suppressed in the transmitter (zero suppression).

Measurement in Pressurized Tanks

In pressure vessels, boilers, or tanks with an inert gas cushion, the internal pressure acts on the surface of the liquid and would add a massive error if measured against the atmosphere. To compensate for this pressure, the low (L) side of the transmitter must be connected to the top of the vessel. There are two main techniques for this connection:

Dry Leg Leg

The dry leg is applied when the gas or vapor at the top of the tank does not condense at operating or ambient temperatures. The piping remains filled only with process gas, transmitting the upper static pressure directly to the L side of the sensor.

Range Calculation (Dry Leg):

• LRV (0% Level): P_H - P_L = (H_distance.ρ) – 0;
• URV (100% Level): P_H - P_L = (H _height + H_distance.ρ) – 0.

Wet Leg Leg

A wet leg is indispensable when vapors from the top of the tank tend to condense, which would erroneously fill the impulse line and cause measurement errors. In this method, the reference line is deliberately filled with a stable liquid (usually water, glycol, or the condensed process fluid itself). A condensate pot at the top ensures that the level of this reference column remains constant, draining the excess back into the tank.

Since the wet leg exerts constant pressure on the low side (L), the differential pressure measured when the tank is empty will be negative, requiring a "zero lift" setting on the Smar transmitter.

Where SG _ref x is the specific gravity of the fluid in the wet leg, and x is the height of the leg.

The Smar Pressure and Level Transmitter Ecosystem

Smar offers a complete line of instruments that cater to applications ranging from basic systems to critical safety systems.

LD300L Level Transmitter

The LD300L is a model specifically designed for tank mounting via direct flanged connection. This design eliminates the need for impulse lines, reducing leak points and simplifying cleaning. It is available with diaphragm extensions, allowing the sensor face to be positioned flush with the inner wall of the tank, preventing the accumulation of solids in viscous processes.

LD400 Series and High Technology HART 7

LD400 series represents the state of the art in Smar's HART instrumentation. With the accuracy of a dedicated mathematical processor, the LD400 offers advanced control functions directly in the field.

• Integrated PID Control: The transmitter can act as a local controller, comparing the process variable (level) with a setpoint and generating a control signal for a valve, which reduces latency and increases system safety;
• Loop Diagnostics: The instrument continuously monitors the integrity of the 4-20 mA loop and the power supply voltage, alerting to possible wiring faults or terminal corrosion;
• Flow Totalization: When used in level measurements that infer flow in open channels (such as Parshall flumes), the LD300/400 has non-volatile totalizers that store the accumulated volume even in case of power loss.

LD300S and LD400S Sanitary Transmitters

For the pharmaceutical and food industries, hygiene is paramount. Smar's sanitary models utilize Tri- Clamp connections and electropolished surface finishes that meet international 3A standards. These instruments support CIP (Clean-in- Place) and SIP (Sterilization - in-Place) sterilization processes, resisting high temperatures and aggressive chemical agents without compromising sensor integrity.

LD400S – SMS Connection 

LD300S – Tri- Clamp Connection

SR301 Remote Seals - Isolation and Protection

Often, the process fluid has characteristics that prevent direct contact with the metallic isolation diaphragm of the transmitter. For these scenarios, the SR301 remote seal system is used .

Application Scenarios for Remote Seals

• Corrosive Fluids: Fluids such as hydrochloric acid or caustic soda can corrode standard 316L stainless steel. Remote seals allow the use of noble materials such as tantalum, Hastelloy C-276, or Monel only on the face in contact with the process;
• High Temperatures: If the process operates above these temperatures, the heat can damage the electronic components. The capillary tube of the remote seal acts as a heat sink, allowing the transmitter body to operate at a safe ambient temperature.
• Fluids that solidify: Materials such as asphalt, polymers, or saturated solutions can crystallize within conventional thrust lines. The remote seal provides a flat surface that prevents this buildup.

Total Probable Error (TPE) and the Influence of Capillaries

The accuracy of a remote seal system is influenced by the capillary length and the type of filling fluid (Silicone, Halocarbon , Neobee , etc.). Variations in ambient temperature cause the fluid to expand or contract, generating an additional pressure error. Smar provides the "ETP" software so that application engineers can predict and minimize these deviations during the design phase, selecting the most suitable capillary diameter and filling fluid for the temperature range of the installation site.

Configuration and Digital Management Tools

The transition from analog to digital instrumentation has allowed instrument configuration to become a task based on software and interactive menus.

Local Adjustment with Magnetic Key

A classic Smar innovation is the local adjustment via magnetic key. Without the need to open the electronic compartment covers—which is crucial in explosion-proof classified areas—the user can insert a small magnetic tool into the holes (Z) for Zero and (S) for Span . This interface allows:

1. Adjust the values of 0% and 100% (LRV and URV).
2. Change the pressure unit (bar, psi , mmH2O, Pa ).
3. Configure damping to smooth readings in tanks with agitation .
4. Perform pressure trimming to align the transmitter reading with a reference standard.

Integration with FDT/DTM and AssetView

For modern asset management, Smar provides DTM ( Device Type Manager ) drivers based on the FDT standard. These drivers allow the transmitter to be viewed graphically and completely on any control system or maintenance PC. Through the DTM, it is possible to access the error log, check operating hours, and back up all instrument settings.

AssetView system takes this concept to the cloud or local network, enabling proactive monitoring of thousands of instruments. It analyzes calibration deviation trends and identifies impending failures before unscheduled process downtime occurs, optimizing preventive and predictive maintenance routines.

Considerations on Density and Concentration Measurement

In addition to level measurement, Smar differential pressure transmitters are frequently used to measure the density of liquids in real time. The intelligent density transmitter (such as the DT300) uses two diaphragms separated by a fixed distance.

Since the distance between the pressure taps is constant, the measured differential pressure is directly proportional to the density of the liquid, regardless of the total tank level (provided both sensors are submerged).

This application is vital in fermentation processes (Brix or Plato degrees ), sugar refining, and chemical industries where the concentration of a solution must be continuously monitored to ensure the quality of the final product.

Conclusion and Future Perspectives

Tank level measurement has evolved from simple mechanical rulers and floats to highly complex and intelligent electronic systems. Smar, with its capacitive sensor technology and direct digital readout, has solved historical challenges of instability and drift that plagued older analog transmitters.

The flexibility of the LD300 and LD400 series, combined with the robustness of the SR301 remote seals, allows virtually any industrial fluid to be monitored with millimeter precision. The ability to perform volumetric calculations in non-linear tanks and act as local PID controllers demonstrates that the modern transmitter is not just a sensor, but an active processing element in the control loop.

With the advent of Industry 4.0 and WirelessHART , the trend is for the connectivity of these devices to increase, allowing level and diagnostic data to feed artificial intelligence algorithms for logistical optimization and overall energy efficiency of industrial plants. Smar continues to be at the forefront of this evolution, ensuring that the physical basis of the measurement — hydrostatic pressure — is captured with the highest possible digital fidelity.

Bibliography

SMAR Technology Company . "Technical Article: Pressure Transmitter with Capacitive Sensor: High Accuracy with Direct and Fully Digital Reading".

SMAR Technology Company . "Continuous Measurement of Density and Concentration in Industrial Processes - Technical Article".

SMAR Technology Company . "LD300 and LD400 Series Pressure Transmitters - Instruction and Operation Manuals".

Technical Documentation. "Differential Pressure Level Measurements - R1".

Automation Forum . "Guide to Calculations for Dry Leg and Wet Leg in Differential Pressure Transmitters".

SMAR Technology Company . "LD300 and LD400 Series - Technical Catalogs for Pressure and Level Transmitters".

SMAR Technology Company . "SR301 Remote Seal - Instruction, Operation and Maintenance Manual".

ADRIANO MARCELO CORTEZE  
NOVA SMAR S/A  
09/MAR/2026

Equipment links

LD300Series - Pressure, Level and Flow Transmitter - SMAR Technology Company

LD400 - HART Pressure Transmitters - SMAR Technology Company

LD400WH - WirelessHART™ Pressure, Level and Flow Transmitters - SMAR Technology Company

DT300Series - Density Transmitters - Technology - SMAR Technology Company