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Pressure measurement principles

Why is pressure measurement important?

Pressure is one of the most measured process variables in industry. By measuring pressure it is possible to ensure safety and quality across different industrial processes. But what is pressure? In short, pressure can be defined as a force that is applied and equally distributed within a surface area. Pressure measurement is often used to indirectly determine other process variables such as flow, level and density.

Mathematically, pressure is defined as:

Pressure (Pa) = Force (N) ÷ Area (m2)

The SI unit for pressure is Pa (Pascal), which represents 1 Newton per square meter (N/m2); however, different engineering units can be found. The most common are: bar, psi, kgf/cm2, kPa, mmH2O and mmHg.

With pressurized liquids and gases, the pressure contained in a vessel is equally distributed across all its internal area – this has been defined by the physicist Blaise Pascal in Pascal’s Law.

Figure 1  – Forces distributed within an surface area

Figure 1 – Forces distributed within an surface area

What is the difference between absolute pressure and gauge pressure?

Figure 2 - Pressure scales

Figure 2 - Pressure scales

Pressure transmitters can measure process pressure using two different scales: the absolute and the gauge pressure scale. The main difference between absolute and gauge pressure is the reference they use, the absolute scale starts at the absolute vacuum, while the gauge pressure scale starts at the atmospheric pressure. Absolute and gauge pressure transmitters have different designs. The main reason for this is that, for gauge pressure, the atmospheric pressure changes from one place to another and depending on the weather, and thus requires continuous compensation.

Absolute pressure transmitters

Absolute pressure transmitters measure the process pressure using the absolute vacuum as a reference. The absolute vacuum is an immutable value and, for this reason, the measured pressure does not require further compensation. As the absolute pressure scale starts at 0 bar abs, this scale has no negative values.

At sea level, an out-of-the-box sensor, with no extra pressure applied to it, will indicate approximately 1.013 bar abs, which is the atmospheric pressure.

Absolute pressure can be represented mathematically as:

Pabs = Pgauge + Patm

Where:

Pabs = absolute pressure

Pgauge = gauge pressure

Patm = atmospheric pressure

Absolute pressure transmitters are typically used in industry for vacuum applications, such as vacuum packaging, vacuum dryers and also for volume compensation of gases.

Figure 3 - Absolute pressure sensor construction

Figure 3 - Absolute pressure sensor construction

Gauge pressure transmitters

Figure 4 – Gauge pressure sensor construction

Figure 4 – Gauge pressure sensor construction

Gauge pressure transmitters, also known as relative pressure transmitters, measure the process pressure using the atmospheric pressure as a reference, but, because the atmospheric pressure, varies from one location to another and depending on the weather conditions, it needs to be compensated.

The transmitter is designed in such a way that the sensor measures the process pressure and a small opening to the atmosphere allows the compensation of the atmospheric pressure; consequently, the gauge pressure is measured. The relative pressure scale starts at 0 bar g and can have negative values up to -1.013 bar g, which is the absolute vacuum.

An out-of-the-box relative pressure sensor, with no extra pressure applied to it, will indicate approximately 0 bar g, regardless of whether it is located at sea-level or at different altitudes.

Gauge pressure values are represented by a pressure engineering unit followed by “g” or simply the pressure engineering unit. E.g.: 10 bar g or 10 bar.

The gauge pressure can be represented mathematically as:

Pgauge = Pabs - Patm

Where:

Pabs = absolute pressure

Pgauge = gauge pressure

Patm = atmospheric pressure

Gauge pressure transmitters are used for a wide range of applications, such as pressure monitoring and control of hydraulic and pneumatic systems, tanks, pipes, air ducts and level measurement on open tanks.

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Differential pressure transmitters

Differential pressure transmitters are used for measuring the pressure difference between two points. They are designed in such a way that the sensor has two process connections, called high-pressure and low-pressure connections, or simply represented as HP or LP respectively. The pressure measured on the high-pressure side is subtracted by the pressure measured on the low-pressure side, and as result, the differential pressure is measured.

Differential pressure transmitters are versatile devices and can be used in different industry applications, such as level measurement on pressurised tanks, flow measurement of liquids, gases and steam and density measurement of liquids.

Figure 5 - Differential pressure sensor construction

Figure 5 - Differential pressure sensor construction

Hydrostatic pressure transmitters

Figure 6 – Hydrostatic pressure transmitter

Figure 6 – Hydrostatic pressure transmitter

Hydrostatic pressure transmitters are used for level measurement. By measuring the pressure exerted by a liquid column above the sensor, it is possible to determine the level. The pressure measured by the sensor is proportional to the liquid column height that it is above the sensor, regardless of the container shape, as stated by the physicist Simon Stevin in Stevin’s Theorem.

Hydrostatic pressure transmitters follow the same measuring principle as gauge pressure transmitters; however, they are designed as a submersible probe. Since the sensor is submerged, a vent tube for the atmospheric pressure compensation is mounted enclosed with the electrical cable. The vent tube should never be blocked, otherwise, the sensor accuracy can be compromised.

Conventional gauge pressure transmitters are also often used to measure level on open tanks; however, on some applications like boreholes or underground tanks it is not possible to install a transmitter externally mounted in the tank wall – that is where submersible hydrostatic pressure transmitters can be used.

Figure 7 – Gauge pressure transmitter (left); Hydrostatic pressure transmitter (right)

Figure 7 – Gauge pressure transmitter (left); Hydrostatic pressure transmitter (right)

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How to select a pressure transmitter?

Selecting a suitable pressure transmitter can be challenging due to the variety of models available to cover the wide range of different applications that exist in industry, with different requirements and process conditions. Answering a few questions will provide better insight into which device to select: .

1) What is the application?

The application requirements will determine the type of device needed. For some applications a gauge pressure transmitter needs to be used, for others, an absolute pressure transmitter would be better. If triggering an alarm or relay when a certain pressure is achieved, pressure switches are recommended. If just a local indication is needed, pressure gauges can be a cost-effective solution.

2) Which measuring range is demanded?

A transmitter with a compatible measuring range for the application should be selected to ensure its maximum efficiency. Overly large transmitters will have accuracy problems when working with low pressures. Transmitters that are too small simply will not measure any pressure above its maximum range and the sensor could also be damaged by excessive pressure. Excessive vacuum can also damage some sensors; consequently, if the application involves vacuum, it is important to check the vacuum resistance of the selected sensor.

3) What are the medium properties?

Certain types of liquids and gases can react chemically to some materials. Fluids containing particles can be abrasive to certain materials, thus leading to premature wear and the compatibility between the measured fluid and the sensor materials therefore needs to be checked. The most common sensor materials are: stainless steel, ceramic or stainless steel coated with special alloys like gold-rhodium. Metallic sensors can work with higher pressures compared to a ceramic sensor; however, ceramic sensors can perform and resist better under vacuum applications. In terms of resistance, ceramic sensors are more resistant to abrasion, chemical corrosion and pressure shocks compared to metallic sensors.

4) Which accuracy is required ?

Depending on the application, the accuracy can be a key factor to maintain quality standards in the process or in the final product. Different models of sensor and transmitters can have different levels of accuracy. There are sensors suitable for every type of application: for applications where there a high level of accuracy is critical or for applications where the accuracy is not that crucial.

5) What is the process temperature?

Every transmitter will have a temperature range that it was designed to work within. It is thus worth checking if the selected transmitter is suitable for the required process temperature. Some sensors are specially designed to work under cryogenic or high temperatures.

6) Which process connection is needed?

The process connection is the mechanical part that attaches the sensor to the process. Adapters should always be avoided; therefore, it is important to select a sensor with a compatible process connection. For hygienic applications, it is recommended to use process connections with hygienic approvals to avoid contamination in the process.

7) What is the output signal?

If the measured value needs to be sent to a control system or any other equipment, it is worth checking if the transmitter output signal is supported by this device. The most common types of outputs are 4-20 mA and 0-10 V for pressure transmitters, PNP/NPN and relay for pressure switches. Devices with industrial communication protocols like HART communications and IO-Link are also common.

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