The idea behind differential pressure meters is to restrict a pipe’s flow partially. As a result, the static pressure on the device’s upstream and downstream sides differs. The differential pressure, which is measured and used to calculate the flow rate, is the difference in the static pressure.

At least 40% of industrial flow meters currently in use are considered differential-pressure devices, with the orifice plate being the most common type. Differential pressure meters are ubiquitous. From gases to highly viscous liquids, differential-pressure meters have been used to measure a wide range of different fluids.

Differential-pressure flow meters are ubiquitous, in part because of their straightforward construction and affordable price. You will have a much clearer understanding of the advantages, practical metering options, and uses for employing differential-pressure meters after reading this article.

**Differential Pressure Flow Meter**

The idea of measuring flow rate by the pressure drop fluid experiences as it passes through a restriction in a pipe was first proposed by Bernoulli in the 18th century.

The idea of differential pressure. The static pressure difference upstream and downstream of the restriction is measured by manometer tubes.

To maintain the mass flow past the limitation, a fluid must accelerate to a more incredible velocity (V2 > V1), which lowers its static pressure. The flow rate via the device is then determined by this differential pressure (p).

Simply put, the higher the p, the higher the flow rate for a given restriction size.

Bernoulli’s equation derives the link between the differential pressure and flow rate.

It can be demonstrated that the differential pressure produced is proportional to the square of the mass flow rate, Qm (kg/s), using Bernoulli’s equation and the principle of conservation of mass.

Many available Δp meters operate on this tenet of monitoring the pressure differential between upstream and downstream. However, some meters, such as variable area meters, utilize the differential pressure in other ways.

**Types of Differential Pressure Flow Meter**

The following differential pressure meter types are the most popular:

- Orifice plates
- Venturi tubes
- Cone meters (e.g., V-cones)
- Low-loss meters for nozzles (e.g., Dall tubes)
- Area meters with a range
- Flow meters for inlets
- Vaporizer cones
- Nozzles for venturi
- Pull plates

**Advantages and disadvantages of DP meters**

**The majority of DP meters share several general benefits:**

- They are easy to manufacture and don’t have any moving parts.
- Their effectiveness is well known.
- When compared to other meters, they are affordable, particularly for larger pipes.
- They are adaptable to any orientation.
- They work with the majority of gases and liquids.
- For some applications, some types don’t need calibration.
- The main disadvantages of DP meters are:
- Less rangeability (turndown) compared to the majority of other flow meter types
- There could be significant pressure losses.
- The output signal is flow-dependently non-linear.
- The pipe configuration or flow characteristics may impact the discharge coefficient and accuracy.
- They may experience the effects of aging, such as the accumulation of deposits or the erosion of sharp edges.

**Common terminology**

**Beta (β)**

The ratio of the orifice’s or device’s throat’s diameter to that of the pipe is known as the diameter ratio or beta (also known as the beta ratio).

To match a specific pipeline size, p meters are sometimes specified in terms of their beta value and diameter, for example, a 4-inch = 0.6 Venturi tube.

A low beta ratio, such as = 0.2, indicates that the limitation size or hole size of the plate is small.

As a result, the pressure loss across the p meter is increased. To counter this increased pressure loss and maintain the flow rate that can be achieved with a larger beta p meter, a pump with a higher discharge pressure (therefore more expensive) or compressor may be required.

A higher differential pressure, however, can typically be detected more precisely than a lower one.

**Effect of using different values of beta**

- The discharge coefficient’s uncertainty growing
- Lower differential pressure across the orifice plate is being recorded (and this can be more difficult to measure)
- To guarantee the velocity profile of the flow through the orifice plate is stable and symmetrical, longer lengths of upstream straight pipe are necessary.
- The roughness of the pipe walls has a more significant impact on the flow profile of the flow through the orifice.
- There are several sizing packages for orifice plates that can determine the needed plate dimensions. Empirical formulas based on actual testing are used by the software. The majority of results are accessible for beta values between 0.3 and 0.7.

**Advantages:**

- Low price
- Installation simplicity
- A full standard is available (ISO 5167-2)
- There is no need for calibration because C is a common value.
- Different designs are available, such as those for viscous fluids, bi-directional flows, and suspended solids.

**Disadvantages:**

- Low turndown (which can be enhanced by dual range p cells)
- High-pressure loss (ranging from 35% to almost 100% of measured p)
- Errors brought on by erosion or harm to the upstream edges
- High sensitivity to upstream installation errors

**Discharge coefficient (C)**

- When using p meters, the discharge coefficient, or C, is a parameter that accounts for non-ideal effects like energy losses from friction.
- The ratio of the measured mass flow rate to the actual mass flow rate is known as the discharge coefficient. The discharge coefficient can either be:

**Determined from a standard**

- Offers accurate flow measurements at a fair price.
- Is particularly suited when repeatability overrides accuracy.

**Determined by calibration**

- Lowers measurement uncertainties for the flow.
- The flow closely follows the tube border in nozzles and Venturi tubes, and the value of C is typically close to one.
- However, C has a value of about 0.6 for orifice plates. For nozzles, Venturi tubes, and orifice plates that are produced to the standard’s specified tolerances, values of C can be found in the standard (ISO 5167).

**Turndown of a DP meter**

The ratio of the maximum to the least flow rate that can be precisely measured is known as a meter’s turndown. To measure a wide variety of flow rates, a high turndown ratio is ideal.

**Relationship between flowrate and p that is square:**

- The p is at 25% of the complete p scale if the flow rate is 50% of the full scale.
- The p is at 6.25% of the complete p scale if the flow rate is 25% of the full scale.

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