Differential Pressure Level Transmitter with Double Flange and Capillaries for Sealed Tank Measurement

Sealed and pressurized tanks cannot be measured with a single pressure point: the vapor space pressure must be subtracted to find the true level. This differential pressure level transmitter uses a double flange configuration with capillary tubes to measure sealed tanks accurately, with 4-20mA HART output and reliable performance in high-temperature and vacuum service.

Why Do You Need a Differential Pressure Level Transmitter?

In sealed tanks, reactors and pressurized vessels, the pressure above the liquid is not atmospheric: it may be positive, from process pressure or vapor, or negative, from vacuum. A single pressure sensor measures the total pressure at its mounting point, which includes both the liquid column and the vessel pressure, so the level reading would be wrong whenever the vessel pressure changes. A differential pressure transmitter solves this by measuring the pressure difference between two points: the high-pressure side at or near the bottom of the tank, and the low-pressure side connected to the vapor space at the top. The difference between these two pressures is exactly the hydrostatic pressure of the liquid column, independent of the vessel's internal pressure. For sealed tanks, distillation columns, reactors and vessels operating under pressure or vacuum, the differential pressure method is the standard, accurate solution, and the double flange design with capillaries keeps both process connections isolated from the electronics.

Working Principle: Differential Pressure with Capillary Transmission

The transmitter compares the pressure at two points in the vessel. The high-pressure flange is mounted at the lower part of the tank, where it senses the combined pressure of the liquid column plus the vessel pressure; the low-pressure flange is mounted at the top, in the vapor space, where it senses only the vessel pressure. Capillary tubes filled with a transmission fluid connect both flanges to the transmitter's sensing element, allowing the flanges to be located remotely from the electronics, which is essential when the process is hot or the mounting position is inaccessible. The transmitter subtracts the two pressures, and the difference is the hydrostatic pressure of the liquid column, which is converted into an accurate level value using the configured liquid density and mounting geometry. The 4-20mA output with HART communication delivers the level to the control system while allowing remote configuration and diagnostics.

Structural Advantages of the Double Flange Transmitter

• Differential measurement: accurate level in sealed and pressurized tanks.
• Double flange with capillaries: remote mounting for hot processes.
• Vapor space compensation: immune to vessel pressure changes.
• 4-20mA + HART output: standard integration and configuration.
• Wide temperature capability: suitable for high-temperature service.
• Vacuum capable: measures vessels under negative pressure.
• No moving parts: low maintenance and long service life.
• Rugged flange construction: leak-free process connections.

Selection and Installation Guide

Select the flange ratings and materials to match the process pressure, temperature and media, and specify the capillary length according to the distance between the flanges and the transmitter mounting position. The high-pressure flange should be mounted below the minimum liquid level, and the low-pressure flange in the vapor space above the maximum level. The transmitter electronics can be mounted at a convenient, accessible location, connected by the capillaries. During commissioning, the zero point is set with a known empty condition, and the level span is configured using the liquid density and the vertical distance between the flanges. For vacuum service, the capillary fill fluid and sealing must be specified for vacuum operation. Our engineers support the complete configuration.

Pain Point Analysis: 4 Challenging Working Conditions

Application Cases

Product Advantages

• Differential pressure principle for sealed tanks
• Double flange with capillary transmission
• Accurate under pressure and vacuum
• 4-20mA with HART communication
• Remote electronics for hot processes
• Wide temperature and pressure capability
• No moving parts, low maintenance
• Leak-free rugged flange connections

Technical Parameters

SEO FAQ

Q1: Why is differential pressure needed for sealed tanks?

In a sealed tank, the pressure above the liquid is not atmospheric; it can be positive, from process pressure or vapor, or negative, under vacuum. A single pressure sensor measures the total pressure at its point, which is the sum of the liquid column pressure and the vessel pressure. When the vessel pressure changes, the reading changes even though the level is constant, making the measurement unreliable. A differential pressure transmitter measures the difference between the bottom pressure and the vapor space pressure; the vessel pressure appears in both measurements and cancels out, leaving exactly the hydrostatic pressure of the liquid column. This is why differential pressure is the standard method for sealed, pressurized and vacuum vessel level measurement.

Q2: What is the purpose of the capillaries?

Capillaries are the tubes that connect the flanges at the vessel to the transmitter's sensing element, filled with a transmission fluid that transfers the process pressure hydraulically. They serve two main purposes. First, they allow the transmitter electronics to be mounted remotely from the process, which protects the electronics from high temperatures, vibration and inaccessible locations while the flanges remain at the measurement points. Second, they allow the two pressure signals to be brought to a single sensing element at a convenient location. The capillary length and fill fluid are specified for the application, considering temperature and vacuum, so the pressure transmission is accurate and stable over the life of the instrument.

Q3: How is the level calculated from the differential pressure?

The differential pressure measured by the transmitter equals the hydrostatic pressure of the liquid column between the two flange mounting points. Using the relationship where pressure equals density times gravity times height, the electronics calculate the liquid height above the high-pressure flange by dividing the differential pressure by the product of liquid density and gravity. The total level is then obtained by adding the known vertical distance from the high-pressure flange to the tank bottom. The liquid density is configured during commissioning, and the mounting geometry is entered as part of the setup. The result is delivered as a level value in meters or other units through the 4-20mA/HART output.

Q4: Can this transmitter measure vacuum vessels?

Yes. The differential pressure method measures the difference between the bottom pressure and the vapor space pressure, so it works correctly whether the vessel pressure is positive, atmospheric or negative. In a vacuum vessel, the low-pressure side senses the vacuum, and the difference still equals the hydrostatic pressure of the liquid column. However, vacuum service places special demands on the capillary fill fluid and sealing: the fill fluid must not vaporize or outgas under vacuum, and the seals must hold the reference reliably. When ordering for vacuum duty, specify the vacuum level and temperature so the correct fill fluid and configuration are supplied.

Q5: What temperature and pressure ranges are supported?

The transmitter supports a wide range of process conditions, with the double flange and capillary design allowing operation at process temperatures up to 200 °C because the electronics are isolated from the process by the capillaries. The flange ratings determine the maximum process pressure, and flanges can be selected to match your vessel's pressure rating, from low-pressure atmospheric tanks to high-pressure vessels. The capillary fill fluid is chosen for the temperature range of the application. When ordering, provide your process temperature, pressure, vacuum level and media so our engineers can confirm the correct configuration for your service.

Q6: What support and customization do you offer?

As the manufacturer, we provide direct technical support including flange selection, capillary length specification, fill fluid selection and installation guidance. Customization options include flange size and rating, capillary length, diaphragm material, measuring range, output protocol, housing options, labeling and packaging, plus OEM and ODM programs. Every transmitter is factory-calibrated and tested before shipment, including the complete capillary assembly. Contact us with your vessel details, process conditions and control system requirements for a tailored recommendation and quotation.

Why Choose Our Differential Pressure Level Transmitter?

Sealed tanks keep their secrets from single-pressure sensors. This double flange differential pressure transmitter reveals the true level by subtracting the vapor space pressure, delivering accurate measurement in pressurized, vacuum and high-temperature vessels where other methods fail. Capillaries keep the electronics safe and accessible, and 4-20mA HART output integrates seamlessly with your control system. As the manufacturer, you receive a factory-calibrated capillary assembly, application-matched configuration and direct engineering support. When your tank is sealed and your level must be exact, choose the differential pressure transmitter that measures through the pressure difference.