Relative and Absolute Pressure: The Differences and How to Measure It

Pressure Transmitters in Liquid Level Monitoring

Pressure measurement is essential in many industrial sectors, from food production to chemical processes, from hydraulic systems to HVAC management. Knowing the difference between relative pressure and absolute pressure is essential to choose the right instrument and ensure precision, safety and efficiency. In this article we will see clearly and practically what distinguishes them, how they are measured and which instruments to use.

Difference between relative and absolute pressure

The difference between relative pressure and absolute pressure is in the starting point of the measurement:

An absolute pressure transmitter measures the pressure starting from a vacuum (i.e. absolute zero), so it also takes into account the atmospheric pressure.

A relative pressure transmitter, on the other hand, measures the pressure with respect to the environment, so it only shows the difference with respect to the atmospheric pressure.

In short:

Absolute = total pressure
Relative = pressure more (or less) than the air around us.

How is relative and absolute pressure measured?

Relative pressure is measured by comparing the process pressure to atmospheric pressure.

Absolute pressure is measured against absolute vacuum. The sensor is sealed and contains a vacuum chamber, so it detects the actual total pressure.

In short:

Relative = difference from atmospheric pressure
Absolute = total pressure against vacuum

Instruments for measuring relative pressure

Instruments for measuring relative pressure can be:

Analog pressure gauges: mechanical instruments with a pointer.
Digital pressure gauges: more precise, with a display.
Relative pressure transmitters: used in systems to send electrical signals.
Pressure switches: activate an electrical contact when a certain pressure is reached.

All these instruments measure the difference compared to atmospheric pressure.

Instruments for measuring absolute pressure

To measure absolute pressure, more sophisticated instruments are used, designed to ensure maximum precision in critical contexts.

Absolute pressure transducers or transmitters are electronic sensors equipped with a vacuum reference chamber. They measure the real total pressure and convert it into a proportional electrical signal (typically 4÷20 mA), useful for monitoring and controlling sensitive industrial processes.

Alternatively, digital pressure gauges for absolute pressure are less common than those for relative pressure. They are always in the digital version, since they require an internal sealed vacuum reference to function. They are used mainly in scientific contexts, vacuum chambers and meteorological applications.

Unlike relative pressure instruments, they are not influenced by atmospheric pressure, because they do not have direct contact with the environment.

The measurement starts from absolute zero, ensuring a precise value of the actual pressure.

Importance of correct measurement in industrial processes

There are many reasons why correct pressure measurement is essential in industrial processes, but the most relevant are:

Operational efficiency: optimizes the operation of the plants, reducing losses and improving yield.
Cost reduction: prevents failures and interruptions, optimizing the consumption of resources.
Product quality: ensures optimal conditions to guarantee high-quality production.

How to choose the right instrument to measure pressure?

Choosing the right instrument to measure pressure depends on various factors related to the specific needs of the industrial process.

Here are the main aspects to consider:

Type of pressure to be measured: relative pressure: if the measurement must be made with respect to atmospheric pressure, for example for hydraulic systems, pneumatics or tire pressure gauges, a relative pressure gauge or a relative transmitter are ideal.
Absolute pressure: if you need to know the total pressure, including atmospheric pressure, for applications such as vacuum systems or barometric monitoring, you must choose an absolute pressure gauge or an absolute pressure transducer.
Accuracy and measurement range: the accuracy required by the industrial process is essential. Instruments such as digital transmitters or high-precision pressure gauges are suitable for applications that require precise measurements. The pressure range must be compatible with the range required by the application. For example, for very high pressures, a Bourdon tube pressure gauge may not be sufficient, while for very low pressures a sensitive transducer may be necessary.
Operating environment: consider factors such as temperature, humidity, vibration and the presence of corrosive substances. Instruments such as digital pressure gauges or rugged transmitters are suitable for harsh working conditions, while simpler instruments may be suitable for more controlled environments.
Required output: if the system requires an analog or digital signal for integration with PLCs or monitoring systems, opt for transmitters or transducers that provide signals in 4÷20 mA or 0÷10 V format.

Se è necessaria una lettura visiva diretta, un manometro analogico o digitale con display può essere la scelta migliore.

Why is it important to choose the right tool?

Choosing the right pressure measurement instrument is essential to ensure safety, efficiency and quality in industrial processes.

Accurate pressure measurement not only prevents failures and risks, but also helps reduce operating costs and improve process control.

Regular maintenance and calibration of instruments are essential to maintain high performance and precision over time.

For specific doubts, it is always advisable to consult a technical expert or the supplier to choose the instrument best suited to your needs.

Riels Instruments recommends the following products for your pressure measurements:

Pressure Transmitters and Transducers
Digital Manometers With Data Logger Function
Differential Pressure Transmitters
Analog and Differential Pressure Gauges

Frequently Asked Questions about Relative and Absolute Pressure

How can I choose between a pressure gauge and a pressure transducer?

If you need a direct visual reading, a pressure gauge (analog or digital) is the best choice.If you need an electrical signal to integrate the system with PLC or remote monitoring, a pressure transducer is more suitable.

In what applications is an absolute pressure transmitter used?

Absolute pressure transmitters are ideal in applications where total pressure must be monitored, such as vacuum systems, atmospheric monitoring or critical industrial processes.

What are the common applications for gauge pressure transmitters?

Gauge pressure transmitters are widely used in hydraulic systems, industrial automation, HVAC and process control.

Quali fattori influenzano la scelta dello strumento per la misurazione della pressione?

I fattori principali sono: il tipo di pressione da misurare (relativa o assoluta), il range di misura richiesto, l'ambiente operativo (temperatura, umidità, vibrazioni), la precisione necessaria e l’output richiesto (analogico o digitale).

What factors influence the choice of pressure measurement instrument?

The main factors are: the type of pressure to be measured (relative or absolute), the required measurement range, the operating environment (temperature, humidity, vibration), the required accuracy and the required output (analog or digital).

What are the main advantages of a digital pressure gauge compared to an analog one?

Digital pressure gauges offer greater accuracy, ease of reading and additional functions (data storage, alarms, etc.), while analog pressure gauges are cheaper, simpler and more durable in harsh environments.