Pressure Sensing 101 – Absolute, Gauge, Differential & Sealed pressure

Pressure can be defined as the continuous physical force exerted on or against an object by something in contact with it or “the force per unit area” and can be calculated by the following formula:

P = F/ A

As an example, the force produced by 1lb of weight due to gravity acting on 1 inch x 1 inch (1sq inch) surface would produce a pressure of 1lb/sq inch, which is often written as 1 PSI (1 pound per square inch). If the same 1lb was applying a force to only half the area, the pressure would equal to 2 PSI. Using a more practical example, in our day to day experience we may pump our car tires from 32 to 40 PSI.

In real-life systems and electronics, pressure is always measured with respect to either a constant or a varying reference in all industries and applications. Changing the reference, in fact, changes the type of pressure sensor used. There are four main types of pressure sensors:

• Absolute
• Sealed Gauge
• Gauge
• Differential

It is critical to choose the correct pressure sensor type for your application. Overlooking this might result in measurement errors and malfunctions in system performance. In the following article, we are going to go through each pressure sensor type briefly and associate some common application examples with each respective pressure sensor type.

What is absolute pressure?

An absolute pressure sensor measures pressure (P1) with reference to the vacuum of free space (zero pressure) as illustrated in the figure below.

Typical applications of absolute pressure sensors

Absolute pressure sensors are used in applications where a constant reference is required. These type of sensors would be typically found in high-performance industrial applications such as monitoring vacuum pumps or industrial packaging. Absolute pressure sensors are also widely used in industrial process control, liquid pressure measurement & aviation and navigation inspection.

Absolute pressure sensors

ES Systems offers three families of high-end absolute pressure sensors designed and optimized to handle targeted media and operating conditions, as derived from customer requirements and feedback, each serving very demanding applications in the field. These families are:

• ESCP-BMS1, ideal for industrial, HVAC and medical applications that require very low total error band.
• ESCP-MIS1, specifically designed to sustain high measurement quality in harsh environments and media.
• ESCP-SAPT, designed and developed according to the very demanding standards of the European Space Agency (ESA) in order to monitor the pressure of the propellant fluid in propulsion systems of satellites. The developments under this activity were also funded by ESA.

What is sealed gauge pressure?

A sealed gauge pressure sensor measures pressure (P1) with reference to the atmospheric pressure (usually at sea level, 1013.25 mbar) as illustrated in the figure below.

This type of pressure sensors is less common compared to the other three. Sealed pressure is referred to as atmospheric pressure trapped within the back of the diaphragm, similarly to absolute. However, it does not use a vacuum, but a predetermined reference point, thus venting on the backside is not needed for pressure to flow in or out.

Sealed gauge pressure can assume both positive and negative values. For positive values, it is called overpressure. The measured pressure is then higher than the standard atmospheric pressure and is equal to the absolute pressure minus atmospheric pressure.

Po = Pabs – Patm

If the measured gauge pressure is negative, it is called underpressure or partial vacuum. The measured pressure is then lower than the standard atmospheric pressure and is found by subtracting the absolute pressure from the atmospheric pressure.

Pu = Patm – Pabs

By mentioning that it is a partial vacuum we do not need to use the minus sign. If a vacuum cleaner operates at an absolute pressure of 0,8 bar, we could also say that it operates at 0,2 bar underpressure.

In general, absolute pressure sensors are preferred compared to sealed gauge, as the vacuum in the cavity offers measurements less prone to inaccuracies due to temperature variations.

What is gauge pressure?

A gauge pressure sensor measures the pressure (P1) with reference to the variable atmospheric pressure as illustrated in the figure below.

Typical applications of gauge pressure sensors

Gauge pressure, also called relative pressure, is the most common pressure sensor type used, found in almost every application and industry. Some of its medical or industrial applications include air compressors, automated pneumatic assembly equipment, factory automation, industrial controls, industrial gas supply, liquid level metering, oil pressure sensing, oxygen concentrators, pressure valves, process control pumps and medical instrumentation.

A gauge sensor should be used when measuring or monitoring pressure where the process is influenced by a change in atmospheric pressure. For example, if you are looking to measure liquid level in a vented tank you would select a gauge sensor. Most gauge sensors contain a single pressure port on the process side and ambient pressure is applied through to the back of the sensing element via a vent.

Gauge pressure sensors

ES Systems offers two families of gauge pressure sensors:

• ESCP-BMS1, ideal for industrial, medical and HVAC applications that require very low total error band.
• ESPP-MIT1, ideal for engineering machinery, hydraulic & pneumatic systems, pumps, compressors and refrigeration systems that require excellent performance at low cost.

What is differential pressure?

A differential pressure sensor measures pressure (P1) with reference to the variable pressure (P2) as illustrated in the figure below.

There can be a little confusion about differential pressure, as it is more complicated than gauge and absolute. Differential pressure is regarded as the diversity between two points (Δp = p1 – p2). For that reason, differential pressure sensors must include two separate pressure ports in order to identify the difference between the two separate physical areas, either with a tube or threaded connections. Therefore, only if the measured values differ from each other, a differential pressure will be indicated.

Differential pressure, in general, is a measure of pressure where the reading and reference values are variable. Differential pressure is calculated by subtracting one of these values from the other. If Pipe A flows at 100 psi and Pipe B flows at 30 psi, the differential pressure would be 70 psi.

Typical applications of differential pressure sensors

Differential pressure sensors usually measure very low-pressure ranges (down to ± 1mbar) and are typically found in HVAC and medical applications such as anesthesia equipment, CPAP equipment, respiratory equipment, nebulizers, sleep apnea equipment, spirometers, ventilators, airflow monitoring, clogged filter detection and filter monitoring.

Differential pressure sensors

ES Systems offers one family of differential pressure sensors:

• ESCP-BMS1, ideal for industrial, medical and HVAC applications that require very high accuracy and low total error band.

Choosing the appropriate pressure sensor

Keeping the facts in mind, and understanding the application details, selecting the best applicable pressure sensor can be an easy job. ES Systems, with a long experience in innovative MEMS, has designed high quality and high-performing pressure sensors that allow for more precise processes with outstanding reliability and efficiency in competitive prices.

You can use the following quick reference table for the appropriate sensor type selection.