Pressure Measurement & Control


Pressure measurement reveals more than just pressure

January 2001 Pressure Measurement & Control

Pressure sensors do not have an easy job. They have to work under the most extreme conditions, often in the presence of corrosive substances, dangled down mineshafts, into cesspits, or in other harsh environments. They carry out these tasks tirelessly, detecting the slightest blips on the graph, and supply precise data over many years. So what is new in pressure measurement?

Pressure-detection plays an important part in many industrial areas, such as process technology. The system virtually always involves a mechanical technique for detecting the degree of pressure, which then converts the reading into an electrical signal for subsequent control or data-display purposes. The current development trend in this respect - at least where industrial applications are concerned - is to make the costly measuring transducer perform such important additional functions as automatic monitoring, fieldbus connection and reading error compensation.

The measuring principle is virtually always the same: pressure acts on a determined surface, causing it to deform. This (minimal) deformation is then used in different ways, depending on each manufacturer's sensor system. In the case of pressure-measuring transducers with VEGA ceramic detection cells, this surface functions as one plate of a condenser, whose capacitance alters with the degree of deformity. The capacitance level of the condenser is then transformed into a discreet value of practical use, as either a 4-20 mA electrical signal or a digital signal suitable for transfer to a bus system.

Absolute or relative?

An important factor where pressure measurement is concerned is that the reading is defined relative to a reference pressure value. If the reference pressure is a vacuum, we refer to the procedure as 'absolute' pressure measurement. This measuring technique is used when the pressure in question is contained in sealed containers or pipe systems.

In many cases however, ambient air pressure - rather than a vacuum - performs this reference function, where the technique is known as 'excess' or 'relative' pressure detection. This type of detection is used when it is not the pressure itself, but a factor depending upon it, that needs to be measured - ie the contents of an open container or the level of a liquid. In this case, the sensor must detect the actual hydrostatic pressure of the container contents, along with the atmospheric pressure above its surface. If the atmospheric value were to change due to air-pressure fluctuations, an 'absolute' sensor would produce a different (ie false) reading. A change in the weather that resulted in an air-pressure fluctuation of - for example - 30 mbar would result in a 3% rate of error in the case of a sensor with a measuring range of 1000 mbar (absolute), although there are sensors (with an error tolerance of 0,1%) capable of producing considerably more accurate readings. The 'relative' pressure sensor works - as opposed to its 'absolute' counterpart - with air pressure as its reference value. The reference pressure and actual air pressure are thus subject to the same fluctuations. In this way, neither the height of the measuring point nor possible fluctuations in air pressure have any bearing on the level reading. One more important factor needs to be taken into account where level measurement by pressure sensor is concerned. If the density of the container contents changes (eg due to an increase in temperature), the level of the contents rises; but this is not detected by the pressure sensor because the mass - and therefore the weight of the container contents - remain the same.

If a reference value other than atmospheric pressure is used, the technique is referred to as 'differential' pressure detection. A practical example of this method is the system used to monitor the filters installed inside pipelines. The pressures upstream and downstream of the filter are compared and, whenever a preset differential value is exceeded, the system warns that the material being handled is no longer passing smoothly through the filter, because it has become clogged.

Another type of differential pressure detection is involved when the level of the contents in a sealed container with overlying gas pressure needs to be determined. If the container in question were - for example - a 20 m high tank, the gas (or a pipe containing a pressure-sensitive liquid) would have to be fed from the top of the tank, to the differential pressure sensor at the bottom, via a 20 m long pipe. However, it is far simpler just to use two individual sensors: one at the bottom to measure total pressure, and one further up the tank to detect gas pressure only. An evaluation device then calculates the actual level (total pressure minus excess pressure), thus supplying the user with three different readings. This technique is used in breweries, for example, to take level readings in fermentation tanks that contain beer with overlying carbon dioxide.

Inclusive evaluation

The use of these evaluation devices permits a further application, namely the calculation of the volume of a container's contents. In the case of containers with straight-line geometry, volume is proportional to filling level, which does not suppose any complicated mathematical operation (doubling the level means doubling the contents). But what happens if the tank is spherical, for example? In this case, the evaluation device has to be provided with information on the tank proportions in the form of a linearisation curve. This information can be transferred to the evaluation device in several different ways. The curve can be entered by means of 32 keyboard coordinates or (using the more sophisticated method) by means of Vega Visual Operating software, which uses the container dimensions and diagram data to calculate the curve and transfer the values to the evaluation device. This allows any simple 4-20 mA sensor to be used to determine the contents of a container. Vega's HART and Profibus PA sensors are supplied with this function as a built-in feature, thus allowing them to be used as standalone volume detection units.

These measuring transducers are suitable for many different applications.

Example from the paper industry

The pressure at the headbox nozzle has to be monitored and kept constant. The sensors used for this purpose must be absolutely flush at the front, in order to avoid any turbulence occurring in the water/paper mixture. This is the only way to keep the thickness of the paper consistent as it is being made. Extra abrasion resistance is also important where the paper industry is concerned. Dissolved paper paste from the wastepaper recycling plant is transported, via a system of pipes, to the next stage in the process. The purpose of pressure monitoring here is to help control the pumps. The wastepaper paste is extremely abrasive, as it contains many hard and/or metallic foreign objects, such as sand and paper clips. Only absolutely front-flush sensors with detection cells made of Alpha ceramic are capable of withstanding this strain in the long term.

Brewery example

In breweries, the contents of high fermentation and storage tanks are determined using pressure sensors. A vital factor in this respect is the need to use equipment approved for handling products for human consumption. This is why the sensor membrane must be as smooth and crack-free as the interior wall of the tank, in order to prevent any build-up of germs.

Example from a refuse-incinerating plant

In order to be able to feed fresh air into the combustion chamber of a refuse-incinerating plant, it is necessary to measure the ambient atmospheric pressure around the outside of the plant. The output signal from the measuring transducer, which has been set to a measuring range of 0 to 1200 mbar (absolute), goes directly to the input connection of a process control system - where it is used for adjusting the air-regulating flaps.

The places where these pressure-measuring transducers are used are so varied that the units must be supplied in several different designs. Whenever the sensor is directly submerged, along with its connection lead, into the substance being handled - rather than being linked to the substance via the processing system - we refer to the device as a 'suspended' pressure-measuring transducer. The connection cable now contains of course (if relative measurement is to be carried out) a capillary tube to provide air pressure compensation in the detection cell. This configuration is used mainly in the area of water/sewage disposal, reservoirs and deep wells.

Add-on attachments with different processing connectors, materials, approval ratings and functions such as temperature-measurement, detection of tendencies, scalable display and remote operation facilities are all available for use in processing technology.



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