Differential Pressure Meter

Purchase Advice on Differential Pressure Gauges Pressure gauges are measuring devices for determining the pressure in gases or liquids. With the differential pressure gauge, the difference between two pressures can be determined in one measurement. In this case, the pressure difference refers either to two different areas of the system operated with pressure or to the pressure difference between the pressure of the ambient air and the positive or negative pressure in a ventilation system. Differential pressure gauges enable the correct setting, monitoring and control of the pressure conditions in the pressurised systems or pressure vessels. In ventilation systems for clean rooms, operating rooms or intensive care units, a differential pressure manometer can also be used to determine whether the filters are damaged or clogged to the extent that they need to be replaced. Differential pressure gauges can also be used for leak tests on technical components and buildings. Differential pressure gauges should be calibrated regularly to ensure that the assured measuring accuracy is maintained.

Types of differential pressure gauges
The differential pressure manometer can, for example, via a liquid-filled U-tube or two measuring chambers separated by a membrane make the difference in pressure recognizable. The higher the pressure difference between the two tube ends or chambers is, the more the liquid or the membrane is deflected in the direction of the lower pressure. This deflection is registered and evaluated and then displayed as a value for the differential pressure. Differential pressure gauges built in this way cannot measure absolute pressures, only pressure differences. If the value of the absolute pressure is also required, care must be taken that the differential pressure manometer has the equipment for this additional function.

The majority of differential pressure manometers have connections for two flexible hoses with, for example, a diameter of 4, 5 or 6 millimetres. The pressurised medium is conducted through these hoses to the measuring chamber. If the pressure difference to the ambient air is measured, only one of the connections is needed. When measuring, make sure that overpressure and underpressure are connected to the respective input provided.

In some types of devices, one of the connections is suitable for a specially shaped rigid pipe or such a pitot tube is already fitted. With these devices, the flow velocity can be determined from the pressure difference between the flowing and still medium. For the measurement, the upper opening of the pitot tube is aligned against the flow in such a way that the upper tube section runs parallel to the flow and the flowing medium flows without deflection into the open tube and further to the measuring chamber. This pressure is compared with the resting pressure introduced via additional openings.

Units for pressure measurement in gases and liquids
Pressure is generally defined as force per unit area. Due to regional conditions and subject-specific peculiarities, many different units are still in use for the indication of the pressure values. Many pressure units result already from the different combination of force and area units. In addition, there are the units that have resulted from earlier measuring methods for pressure in gases and liquids.

For comparison:

> Air pressure at sea level is 101,325 pascals, which is 101.3 kilopascal or 1,013 millibar or one physical atmosphere.
> The pressure in drinking water pipes should be 2 to a maximum of 8 bar in the house, and at least 1 bar at the top tap, so that water flow was fast enough.

Measuring range and load limit of a differential pressure manometer
Differential pressure gauges are among the devices that can be destroyed by overloading. It is therefore essential to choose a model with the appropriate for the task measuring range. In some devices, the burst pressure is specified in addition to the maximum measurement value. The pressure values between the maximum measured value and the burst pressure can no longer be measured, but do not immediately lead to the destruction of the measuring device.

Additional output values, data storage and interfaces
Differential pressure gauges are used to determine pressure differences in gases or liquids in industry, medicine and air conditioning. With some types of devices, the temperature can also be measured immediately in addition to the pressure. Other devices determine flow velocities and flow rates in the closed pipelines with the help of the entered additional values or have special functions for leak testing. Many of the modern digital models can not only display but also store the measured data. The range of possibilities extends from the internal memory for the smallest and largest value of a measurement series MIN/MAX to the programmable data logger for continuous recording of the measurement data over longer periods of time.

For the transmission of the measurement data from the memory or for current measurements, many of the differential pressure manometers have a built-in RS232 or USB interface. Using an adapter cable, the measurement data can also be transferred from the RS232 interface to the USB port of a computer. Special software solutions simplify the evaluation of the data. Differential pressure gauges with analogue or switching outputs can transmit measured-value-dependent signals directly to the valves, switches or control units and thus be used for automatic control of the pressure systems.

Pressure is one of the crucial parameters, which plays a huge role in many industrial and engineering processes. Differential pressure may be an indicator that some processes are running wrong and immediate adjustment of the system is required; it may point to the problems in pipes, HVAC systems, volume / mass flow processes, pumping etc. A modern differential pressure meter is produced in various versions, and is suitable for application under hard conditions, is resistant to chemicals or influence of other external factors. Application of a differential pressure meter / systems helps to regulate the air volume in the ventilation systems and detect contaminated filters, or, if applied in the heaters, to achieve the optimum combustion mode and better performance of the whole system. Detection of pressure fluctuation / drop points to the failure in a certain system or fault in the process and helps to prevent shutdown of the system.

Control and continuous measurement of the liquid level, mass and volume flow provided by the electronic differential pressure meter is safe and reliable. Not always the pressure differences between two parts of the systems are bad. But without application of auxiliary devices an operator cannot detect and correctly evaluate all the processes going on inside of the complex mechanisms and hydraulic, pumping, ventilation, heating etc. system. Differential pressure meter suitable for installation in the most difficult to reach places indicates when this difference is getting negative and even dangerous, and timely troubleshooting may be carried out.

The choice of differential pressure meter depends on many parameters, measuring technology and specialization of the enterprise. The truth is that no matter what it goes about: gas, liquid, pressed air etc., static or flowing medium, the pressure parameter must be monitored regularly. For some technological processes just absolute pressure measurement, with no relation to atmospheric pressure. For many, on the contrary, just differential pressure matters and the latter is measured not depending on the absolute one. Another possibility is to measure differential pressure in connection to the atmosphere (for these measurement additional sensors will be required).

The fact to remember about the differential pressure is that it never shows the absolute and relative value, its main purpose is to show the difference between the pressures in two different parts of the reservoir, vessel, container. It is easy to underestimate the importance of monitoring particularly this type of pressure by a differential pressure meter, but it is a faulty way. To know what is going on in the system inside is of crucial importance for understanding whether the process flows as required and the dynamics is positive or immediate changes should be made. A quick change of pressure may influence the auxiliary devices applied and bring their settings down. For each system there is a permissible difference between the pressures and to prevent shutdown of the whole system in case of unexpected deviation from the norm the differential pressure regulators / switches are installed to provide stability, since in case of emergency they react immediately.

It is especially highly recommended for the HVAC systems. Here, the maximum and minimum permissible pressure and temperature limits as well as pH of the medium under monitoring should not be exceeded. In each particular situation the switches should be selected in accordance with individual parameters, since it happens, for example, that the differential pressure required for the correct technological process is from the beginning beyond the “safe” range of some elements in the regulator. Before applying differential pressure meter and auxiliary regulators, it is necessary to select the elements with the appropriate ranges for every individual task.