Pressure Gauge

We are constantly exposed to different pressures in our immediate environment, even if we are not consciously aware of them. Different pressure gauges have been developed to measure these pressures. Whether it is blood pressure, which is one of the standard measurements when it goes about health problems, air pressure, which has a great influence on the weather and, for some people, on their well-being, the pressure in car tyres or bicycle inner tubes, in water, heating or ventilation pipes, or the pressure that a force exerts on its point of application – without a suitable pressure gauge, we cannot objectively assess the pressure values. In this purchase advice, an attention is paid to the pressure gauges for liquids and gases that are suitable for technical measurements and environmental measurements. First, however, we will give an overview of different units for pressure measurement.

Which pressure indicator is suitable for which application? This question often arises before the purchase of a pressure gauge. This text below attempts to cover and explain this and other questions.

First, it must be clarified whether the pressure gauge should measure the pressure of air, liquids, gases etc. and visualize it directly on site. In this case, a so-called manometer as a pressure gauge for the measurement task is recommended. If, for example, the pressure indicator is to be located in a control cabinet or control center, it is recommended that a pressure transmitter with standard signal output should be used and, as a pressure indicator – a digital display with a corresponding input. This could be, for example, a 0 ... 10 V or 4 ... 20 mA signal. In this case, the external pressure gauge must then be scaled to the measuring range of the pressure transmitter. In concrete terms, this means that if a pressure sensor with a measuring range of 0 ... 16 bar is used, it must be set in the pressure gauge that, for example, an analog measured value of 4 mA corresponds to the pressure of 0 bar. Finally, the upper scale value is defined as follows: 16 bar correspond to a current of 20 mA. The setting process is the same for 0 ... 10 V pressure gauges. Whether a pressure indicator with 0 ... 10 V input or 4 ... 20 mA is used depends on the distance between the pressure sensor and the display. Typically, we recommend a sensor with 4 ... 20 mA as the best solution.

When selecting pressure gauges or manometers, it is important to pay attention to the device having the correct design for the application. As a rule, for industrial measurements, the pressure gauges with a standard threaded connection are used. The separation membrane between the sensor and the medium can vary, depending on the application. Here, the pressure displays with stainless steel or ceramic membrane can be selected. For foodstuff applications or hygienic pressure measurements, a pressure indicator with an easy-to-maintain separation membrane is offered. Furthermore, the pressure gauges differ in accuracy. The accuracy of the pressure gauge is always specified by the full end value of the measuring range. Typical accuracy classes of the PCE pressure gauge are 1%, 0.5%, 0.2%. Another option for this type of pressure indicators is the connecting thread itself. For many different applications, the pressure gauges from PCE instruments have ¼ ", ½" ... connecting threads. This always depends on the installation situation.

In the case you cannot decide what type of the pressure gauge may be the best fitting one for your application or whether the selected device meets all the requirements, as the customer of PCE Instruments, you are welcome to call the technical service department of PCE Instruments. The professional consultation is intended to facilitate your choice and clarify many issues connected with your measuring case.

The text below is intended to provide you with the useful information concerning the selection of the suitable pressure gauge, in other words, to give you, as the user, some tips as for what aspects to pay attention before making the final decision about the purchase.

The modern progress leads the presence of a wide range of different versions of principally one device, which besides the main measuring task (the measurement of absolute, relative and differential pressure), may have auxiliary functions or differ in design, measuring methods, size, measuring accuracy, measuring possibilities etc.

A pressure gauge as a modern device has not become an exception and is present on the market as a digital device. For those who prefer mechanical measuring method, there are analogue models. These devices are very easy-to-use and even though there is no up-to-date electronics in them, they deliver reliable measuring results and are quite popular in industrial environment and for stationary application.

As for the digital versions, pressure gauges are absolutely convenient, since there is no need to search for the mains and the battery operation allows for mobile applications, wherever it is required. Due to the installed electronics, the device enables its users to enjoy the advantage of additional measuring functions as well as easy switching between the measuring units (up to 11), making the application flexible and convenient.

Absolute pressure
When it goes about pressure, the reference value is so-called zero pressure, existing in the vacuum space. Any pressure measured related to the value of the absolute vacuum, is called absolute pressure.

Relative pressure
Relative pressure is measured related the atmospheric pressure (type of pressure of a crucial importance for life on Earth). Usually the value of this pressure is the absolute pressure minus the atmospheric pressure (may fluctuate depending on the time, temperature and altitude above the sea level). Depending on the relation between the measured pressure and the barometric or atmosphere pressure, it may be divided into positive (over-pressure) and negative. The former definition is used when the measured pressure is higher than the atmospheric, otherwise – it goes about negative relative pressure, when the measured with the help of the pressure gauges value is lower than the atmospheric pressure. Positive overpressure is measured in positive units, while the negative goes from – 103 to 0 kPa.

Differential pressure
Differential pressure, as the name suggests, comes when it goes about the comparison of two different pressures, at two points, and none of them is equal to the atmospheric pressure. Application of the pressure gauges for determining the differential pressure is highly recommended when it goes about maintenance and inspections of the equipment and various facilities, since the measurement result may be a very accurate and precise sign of the malfunction, incorrect work of the system or some particular part of it, like, for example, filter in the HVAC system. If the measurement is carried out on both sides, and the differential pressure value may indicate the necessity to replace the filter, or clean it, or on the contrary, may indicate that the system is working flawlessly. Handheld pressure measuring gauges from the range of our products offer a very convenient way of locating the problems and due to that, help to keep the systems and equipment (sluices, ducts, pipelines, reservoirs, HVAC etc.) at the enterprise in the best condition which is the pledge of safety and successful work.

What is the pressure gauge supposed to measure?
Before making the final solution bout the purchase, it is desirable to clearly understand what types of measurements will the device be applied for. When it goes about pressure it not necessarily means air, but also gases and liquids. Not all of the devices available are multi-applicable in terms of the medium, and in order to avoid the measuring error, it is a must to check the technical description and specifications of the model that has been selected. Also, the sphere of application plays not the least important role. If the measurements will be carried out in the food production area or medical/pharmaceutical field, the best choice make the devices with the components made of stainless steel, since the devices must survive intensive cleaning and sterilization. Whether the medium is acidic or abrasive plays also an extremely important role, since the device (or the parts of it, like ceramic diaphraghm) may be seriously damaged after the contact with the harmful particles. If various measurements have to be carried out, it may be possible to acquire the device that can be adjusted and configured in accordance with the particular measuring task. Taking all of these points into account helps to seriously narrow the range of the devices and point out the most suitable one. Safety note: never use our pressure gauges for the measurements of explosive and corrosive gases or liquids!

Portable or permanently installed devices?
PCE Instruments has got pressure gauges for different users and types of application, from continuous, long-lasting measurements and monitoring, to quick on-site measurements on demand. The pressure gauge in all possible meanings should fit for the measuring task. It means that if there is only one location where the device should be installed and only one type of measurements that is required, the handheld device won’t be the best choice, and vice versa, for a big number of measurements, carried out at different locations, directly on site, the stationary device that has been designed for being built-in in the system, will make little sense.
Built-in devices are perfect for application in industrial environment, since they possess a special robust housing and type of protection, are reliable, often with a big display, with an option of switching between the units (in the case of pressure: bar, psi, Pa and others). In addition, there is a choice between digital and analogue devices. No doubt, this type of devices is the correct choice for the technological field.

From the other side, handheld pressure gauges are a wonderful alternative to the stationary devices, when their user has to be mobile and move from one measuring spot to another. They are battery-operated, light, and absolutely convenient for quick, sometimes even spontaneous measurement, for quality controls or troubleshooting. They are perfectly good for the series of measurements such as a general inspection or when searching for the problem-spot in the system. Laboratory is also one of the perfect application fields, where multiple measurements are often required. Handheld power gauges possess a function of storing the MAX. and MIN. values as well as to store the measurement data over a certain time period (SD card memory is available). After the application of the portable device, it is possible to evaluate and study the data on the computer, since it can be easily transferred from the gauge and the gauge includes the software allowing for convenient data processing.

Helpful selection criteria: accuracy / measuring range
As mentioned above, the pressure gauges vary a lot and clear idea about the future measurements may also play a very helpful role. The approximate values to be expected should be seen on the display of the device, that is why the measuring range of the particular gauge is to be carefully studied. Also, depending on the application field, different accuracy requirements may be needed. PCE Instruments offers the gauges for permanent installation of the accuracy classes 0.05 / 0.1 / 0.2 and 0.5, and the measuring range goes from 0.1 to 600 bar. The accuracy of the handheld devices varies between 0.3% and 2%, with the measurable pressure of +/- 1000 to +/- 7000 mbar.

Selection of the connection thread and the membrane
In the paragraph about the medium where the pressure gauge is supposed to measure, we have touched upon an important issue of selecting the device with the components made of the right for the measurement material, like, for example, ceramic or stainless-steel membrane. Besides, up to 6 threaded connection are possible, for example, G ½ "DIN 3852 or ¼" NPT, which allows the user to configure the purchased gauge for individual application. It is necessary to select the required thread. A professional consultation from one of our technicians may significantly facilitate the process of choosing and may help to avoid the mistake of buying the wrong device. No question is unimportant when it goes about choosing a reliable measuring device.

Pressure is defined as a technical quantity as the quotient of force and stressed area. Over time, special measurements and their own units have become established for different applications, but these can be converted into the internationally used SI unit Pascal. Also due to regional conditions, many different units for pressure are still in use. Some of them refer directly to the structures used in the past to measure pressure, e.g. millimetres of mercury or centimetres of water. The modern pressure gauge with an electronic evaluation unit can often convert the measurement result directly and display it in the selected unit.

Internationally used units for pressure gauges

Units used mainly in English-speaking regions

Force gauges that are used as pressure gauges for pressure measurements on solids often display the pressure force in the unit Newton or in kilonewton. If necessary, however, these measured values can be converted into Newton per square metre, Newton per square centimetre or Newton per square millimetre with the help of the area dimensions. Some pressure gauges can also give the values directly as pressure or as surface pressure.

Important terms for a pressure gauge for gases or liquids
In order to be able to classify pressure data correctly, it is important to know whether an absolute, a relative or a differential pressure has been determined. The atmospheric air pressure is always measured as absolute pressure. These pressure gauges are often called barometers. The lowest value for the absolute air pressure is zero and is only reached in airless spaces – iь.e. in a vacuum.

The usual absolute air pressure at sea level is about one bar. The higher you go up, the more the atmospheric air pressure decreases. This reduction is not only due to the fact that the height of the oppressive air column is lower, the higher air layers also have a lower density. However, atmospheric pressure is not only dependent on the altitude, but also on temperature, wind and other environmental variables.

The pressure gauge that can measure overpressure or underpressure does not determine an absolute value, but a relative value. These pressure gauges determine the pressure difference in ventilation systems or compressed air systems compared to the prevailing ambient air pressure. The measurement of the relative air pressure is therefore always a differential pressure measurement. However, differential pressure measurements are also carried out independently of the ambient air pressure to determine the pressure drop in the pipe systems for gases or liquids. The pressure gauge can measure a differential pressure of one bar both between a vacuum and the ambient air pressure and between different areas of the compressed air systems, gas or water pipes.

Pressure gauge for absolute pressure
The absolute air pressure is indicated, among other things, in the ambient air pressure. In the pipe systems, the absolute pressure describes the pressure independent of the prevailing ambient air pressure. The lowest absolute pressure zero prevails in the vacuum. Therefore, the pressure gauge for absolute pressure cannot measure negative values. The pressure gauges with a closed vacuum chamber are often used for measurement. A device for measuring the absolute pressure is therefore also called a closed system. The pressure gauge only needs one connection through which the medium to be measured is routed to the measuring chamber.

Pressure gauge for relative pressure
If only the pressure difference to the ambient air pressure is to be measured, the pressure gauge is used whose measuring device also records the ambient pressure. These pressure gauges are therefore called open systems. The usual ambient air pressure at sea level is 1,013 millibars or hectopascals. Extreme values are 925 mbar in strong winter lows and 1070 mbar in exceptional cold highs. As altitude above the sea level increases, the atmospheric pressure falls steadily as fewer and lighter air masses press on the surfaces that are higher.

The pressure gauge for relative pressure has two measuring openings, but the opening for the ambient air is not always recognisable as such. However, in the device description and the operating instructions for the device models that are only connected to one measuring point it must always be indicated whether the absolute or the relative pressure is measured there. The relative pressure can be higher or lower than the ambient air pressure. If it is higher, it is called overpressure. Lower values are called negative pressure or negative overpressure.

Examples of technically used overpressure are pressure sprayers, the air pressure in vehicle tyres, the compressed air for pneumatic tools, the pressure of liquids in containers and pipes such as in heating systems and in the drinking water network, or the pressure in gas containers. Negative pressure is used, for example, in ventilation systems or to suck in vapours, dusts or materials.

Pressure gauge for differential pressure
The pressure gauge for determining differential pressures is always equipped with two similar connections for recording the pressures to be compared. These pressure gauges can measure both relative pressure – in which case one of the ports records the ambient air pressure – or are used to quantitatively record the pressure differences within closed systems. For example, the difference in pressure before and after pumps, valves or other components can be determined in this way. Permanently installed models can also be used to control technical systems.