Pressure Calibration

November 3, 2021

Pressure is the scalar quantity that relates the force to the surface on which it acts, that is, it is equivalent to the force that acts on the surface. That is to say:

The pressure unit of the International System of Units is Pa [kg / (m ∗ s2)], although in practice other units are also used such as Bar, psi, kg / cm2, mmHg, etc.

There are 3 types of pressure, absolute pressure, relative pressure and differential pressure:

  • Absolute pressure : it is the total real pressure that is exerted on a surface, the absolute vacuum is used as a reference of zero.
  • Relative pressure : relative to the atmosphere that is exerted on a surface, atmospheric pressure is used as a reference to zero.
  • Differential pressure : relative to another pressure that is exerted on a surface. Differential pressure meters are widely used for flow measurement and in clean rooms.

Pressure calibration is the comparison of the output of one device used to measure pressure with that of another

The outputs of the devices are compared at one or more pressures, typically from the lowest to the highest readings of the full scale range of the device under test, or the range for which it is normally used. pressure measuring device, or with a pressure measuring standard.

This normally involves connecting the device under test (DUT) to the standard device and generating a common pressure in the measurement circuit.

The comparison process is carried out on a chain from the highest level of production of fundamental pressure, to the pressure measuring devices of each day, to ensure that the pressure measurements are accurate and that they comply with accepted or mandatory standards.

Pressure calibration

Pressure measuring equipment loses its accuracy for various reasons, especially due to overloads or sudden variations in applied pressure.

The calibration of the manometers is carried out by comparison with a standard of better accuracy (at least 3 times better) which can be a digital manometer or a dead weight scale.

The medium of use of the pressure gauge must be taken into account when requesting a calibration. If a pressure gauge is used with a pneumatic system (gas or air) it must not be calibrated with a hydraulic system. A hydraulic pressure gauge can be calibrated with a pneumatic system, but there is the possibility of contaminating the standards and damaging them even with contamination traps.

Equipment for pressure measurement

The  liquid column manometer  is widely used to measure very small differential pressures, as is the case in clean rooms where a different pressure is maintained than that of the outside to avoid the entry of particles.

The  Bourdon manometer  consists of a flattened tube that tends to return to its previous (circular) shape when pressure is applied. As the pressure increases and the tube returns to its original state it tends to unwind, thereby moving the needle. In general, this type of pressure gauge presents very large errors when its full scale is 10 bar or less. Bourdon pressure gauges tend to lose their accuracy due to a zero shift, especially if they are subjected to pressure fluctuations.

A Bourdon gauge with zero adjustment and good accuracy can cost more than a digital gauge. Bourdon gauges should not be used as a standard for calibrations.

The  digital gauges are increasingly based on pizoelectricos. The pizoelectric is a quartz that when a pressure is applied generates a DC potential. This potential is linear with pressure. This type of sensors can be used in low or high accuracy equipment depending on the purity of the quartz, the cut and how it is characterized.

These gauges can reach accuracies as good as 0.01% of reading and are generally used as pressure standards. Care must be taken not to use a digital pressure gauge that is specified for use with gas in a hydraulic system, as this can ruin the instrument.

The  deadweight balance  consists of masses stacked on a piston that apply a force due to gravity, as the piston has a known area, then it generates a pressure P according to the formula:

P = mass * gravity / area

This type of equipment is the standard par excellence in calibration laboratories since the drift of this type of equipment can be as small as 3ppm (0.0003% of reading). These equipments are the most complicated to use, they require highly qualified personnel for their correct operation and a hermetically stable environment free of vibrations, with which they can only be used in a laboratory.

The working principle of deadweight balances is to balance a known force (defined by the masses placed on the piston and local gravity) with the force produced by pressure of the fluid on the piston. When these forces are balanced, the pressure in the system equals the pressure generated by the deadweight scale.

Differential pressure sensors

Differential pressure sensors are commonly used in the process industry and cover a variety of applications. To understand what a differential pressure pressure sensor is, it is important to contrast it with other types of pressure measurement. The most common types of pressure measurement are absolute, gauge, and differential.

Gauge pressure: Gauge pressure is the pressure difference in reference to barometric (or atmospheric) pressure. This is the most widely used type of pressure measurement in the industry today.

Absolute pressure: Absolute pressure is when zero pressure refers to absolute vacuum. This is accomplished by pushing to a very high vacuum, getting as close to absolute zero as possible, and then referencing the sensor zero to that vacuum point. Absolute sensors often use a gauge sensor and a barometric sensor and calculate absolute pressure by subtracting the barometric pressure from the gauge pressure.

Differential pressure: Differential pressure (DP) can be independent of atmospheric and absolute pressures. It is the pressure difference between two applied pressures. These sensors are very useful for determining the pressure difference between two sites or systems and are frequently used in calculations of flow, filtration, fluid levels, density and viscosity.

The periodic calibration of instrumentation is essential to ensure the quality and proper functioning of industrial facilities. In many cases calibration can be carried out easily and with sufficient precision directly on site in the process and it is not necessary to move the instrument to a specialized laboratory.

Dr. Loony Davis5
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Born and raised in Brussels in an English family, I have always lived in a multicultural environment. After several work experiences in marketing and communication, I came to Smart Water Magazine, which I describe as the most exciting challenge of my career.
I am a person with great restlessness and curiosity to learn, discover what I do not know, as well as reinvent myself daily, someone who is curious about life and wants to know. I enjoy sharing knowledge.
This is my personal project but I also collaborate in other blogs, it is the case, the most important web on water currently exists in the US, if you are interested you can read my articles here.

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