Why use a differential pressure gauge to measure flow?

A differential pressure (DP) gauge is used to measure flow by converting the pressure drop created by a flow restriction into a readable flow rate based on established fluid dynamics principles. In many industrial applications, engineers need a reliable way to monitor liquid or gas flow but may not require the complexity or cost of an electronic flow meter. In these cases, a DP gauge provides a simple, mechanical solution for flow indication.

With more than 170 years of experience in pressure and flow measurement, Ashcroft supports engineers working in harsh and demanding process environments where measurement reliability matters.

In this article, you’ll learn why flow monitoring is important, how differential pressure relates to flow and the most common ways DP gauges like the ones offered by Ashcroft, are used to measure flow in industrial applications.

Why is monitoring flow important in industrial processes?

Monitoring flow is important because changes in flow often indicate conditions that can affect safety, product quality, or system performance. A decrease in flow may signal a clogged line, fouling, or blockage, while excessive flow can point to valve issues or improper system setup.

Across industries such as oil and gas, water and wastewater, chemical processing and HVAC flow monitoring helps operators:

• Maintain safe operating conditions
• Protect equipment and downstream processes
• Identify abnormal conditions early
• Support consistent and efficient operation

While electronic flow meters are commonly used, they are not always necessary when the requirement is basic monitoring rather than automated control.

How can differential pressure be used to measure flow?

Differential pressure can be used to measure flow because flowing fluids create predictable pressure changes when they pass through a restriction. When fluid velocity increases, static pressure decreases, and that pressure difference can be measured.

By measuring the pressure upstream and downstream of a restriction, a DP gauge provides a differential pressure value that can be related to flow rate. Once the system is properly designed and calibrated, this relationship allows flow to be calculated or displayed directly on the gauge dial.

What role does Bernoulli’s principle play in flow measurement?

Bernoulli’s principle explains the relationship between fluid velocity and pressure that makes DP-based flow measurement possible. In a horizontal pipe, higher fluid velocity corresponds to lower pressure, while lower velocity corresponds to higher pressure.

When a restriction is introduced into the flow path:

• Fluid velocity increases at the restriction
• Pressure decreases at that point
• A measurable pressure difference is created

The flow rate is proportional to the square root of the measured differential pressure. This principle is widely used in applications such as water distribution, air handling systems, chemical processing, and oil and gas operations.

What are the most common ways to measure flow using a DP gauge?

The most common ways to measure flow using a DP gauge are with orifice plates, venturi tubes and pitot tubes. Each method applies Bernoulli’s principle but differs in design, pressure loss, and typical application.

It’s important to note that DP gauges are visual indicators. They provide local flow indication but do not transmit signals to control systems in the same way electronic transmitters do.

How does an orifice plate work with a DP gauge?

The orifice plate is the most commonly used method for flow measurement. It consists of a thin plate with a precisely sized hole or orifice.  When placed in the pipeline, the flow through the pipe causes a large drop in differential pressure.  The DP gauge is placed across the orifice plate and connected to the pressure taps. As fluid passes through the orifice, velocity increases and pressure decreases, creating a differential pressure proportional to flow.

Orifice plates are cost-effective options that are suitable for many liquid and gas applications

Figure 1. The Orifice Plate

When is a venturi tube used instead of an orifice plate?

A venturi tube consists of a constricted pipe section that causes the fluid velocity to increase, resulting in a decrease in pressure. It is used when pressure loss must be minimized. Unlike an orifice plate, a venturi gradually accelerates and decelerates the fluid, resulting in a smaller permanent pressure drop.

Venturi tubes are often selected when:

• Higher flow rates are required
• Energy efficiency is important
• Excessive restriction could disrupt the process

The DP gauge measures pressure at designated taps along the venturi to determine flow.

Figure 2. Venturi Tube

Why are pitot tubes commonly used for airflow measurement?

Pitot tubes are commonly used for airflow measurement because they measure velocity pressure at a specific point in the flow stream. Unlike orifice plates and venturi tubes, pitot tubes do not measure the full pipe cross-section.

They are typically used in:

• HVAC ducts
• Air handling systems
• Ventilation monitoring

Flow is calculated by measuring the difference between static pressure and velocity pressure.

Figure 3. Pitot Tube

How do you measure flow using a differential pressure gauge?

Flow is measured using a DP gauge by matching the expected pressure differential to the desired flow range and configuring the gauge accordingly. Once the restriction device is selected, the pressure drop associated with the target flow rate is typically provided by the manufacturer of the orifice plate, venturi tube, or pitot tube.

The basic steps include:

1. Installing the orifice plate, venturi tube, or pitot tube in the system
2. Connecting the DP gauge to the appropriate pressure taps
3. Measuring the differential pressure created by flow
4. Calculating the flow rate or applying a custom flow scale
5. Designing a dial that displays flow units, differential pressure, or both

Flow measurement accuracy depends on factors such as flow element design, fluid properties, installation conditions and calibration practices.

For accurate and repeatable results, engineers should reference applicable industry standards and guidelines. Standards from organizations such as ISO, ASME, and ISA address the design, installation and use of differential pressure flow elements, including orifice plates, venturi tubes and pitot tubes. Following recognized standards helps reduce installation-related errors and supports consistent flow measurement performance.

When is a DP gauge a practical alternative to a flow meter?

A DP gauge is a practical alternative when local flow indication is sufficient and electronic output is not required. In harsh environments where simplicity, durability and ease of maintenance are priorities, DP gauges can provide dependable flow monitoring without adding system complexity.

By understanding Bernoulli’s principle and selecting the appropriate flow element, engineers can confidently use differential pressure gauges as an effective solution for many flow measurement applications.

Ready to learn more?

Now you know how differential pressure gauges can be used to measure flow, how Bernoulli’s principle applies, and which DP-based methods are commonly used in industrial applications. Understanding these fundamentals can help you determine when a DP gauge is a practical solution for flow monitoring in your process.

If you’d like to go deeper, explore related articles on pressure measurement fundamentals and gauge selection, or contact us with additional questions about your specific application. 

In the meantime, download our guide to learn about the pressure and temperature instruments available for harsh and demanding applications.