Most uses for pressure gauges are as a simple indicator, but if you want to carry out additional operations an intelligent gauge opens up a wealth of functionality that will help you expand it's use in a variety of monitoring setups.
Changing Pressure Units
The most requested feature for digital pressure gauges is the ability to change pressure units and most gauges provide this function. Not all gauges will convert every pressure unit though, it can vary from a choice of just bar/psi, to many more, so it is worth checking the spec first to make sure all the units that you need to convert are included.
Max/Min Values
You can't always be present to check the pressure of a process, so how do you know whether the pressure has risen or fallen while you were away? A maximum/minimum or peak/valley hold feature would give you some idea of what has happened between visual checks. This feature will continually monitor the pressure and update the memory with the highest and lowest reading reached.
Is Measurement Resolution the same as Accuracy
You will find mentions of resolution and accuracy on many product information sheets for pressure measuring equipment, however when discussing the performance of equipment the two terms often get confused as meaning the same.
Resolution defines the ability to distinguish one reading from another. For a digital pressure gauge the resolution is normally referred to as the number of readable digits, e.g. 2 bar range with a 5 digit display would have a 0.1 mbar resolution.
Resolution defines the ability to distinguish one reading from another. For a digital pressure gauge the resolution is normally referred to as the number of readable digits, e.g. 2 bar range with a 5 digit display would have a 0.1 mbar resolution.
Using an Absolute Sensor to Measure Gauge Reference Pressure
When it is necessary to present gauge reference pressure data, an absolute pressure sensor can be used in some situations if a gauge pressure sensor is not available.
An absolute pressure sensor will measure total pressure independent of atmospheric pressure changes, unlike a gauge pressure sensor which measures pressure relative to atmospheric pressure. To convert the absolute reading to a gauge reference type the atmospheric pressure should be subtracted from the total measured pressure.
An absolute pressure sensor will measure total pressure independent of atmospheric pressure changes, unlike a gauge pressure sensor which measures pressure relative to atmospheric pressure. To convert the absolute reading to a gauge reference type the atmospheric pressure should be subtracted from the total measured pressure.
What to investigate before selecting an IP68 Pressure Transducer for measuring Liquid Level
To avoid fitting an unsuitable device or one that will not last a long time, it pays to consider all the factors that will influence the final specification you choose.
What type of liquid is to be measured?
This will help select a device with the most compatible construction materials. Submersible pressure sensors are comprised of sensing module, body, cable and seals, all of which must be compatible with the substance in which the sensor will be immersed.
What is the specific gravity or density of the liquid compared to freshwater?
The hydrostatic pressure generated by a fixed height of liquid is proportional to the specific gravity. 10 metres of water is a very close approximation to a 1 bar hydrostatic pressure, but if the specific gravity is say 0.9 of water (fresh water has a specific gravity of 1.0) a 10 metre head of this liquid would generate a 0.9 bar hydrostatic pressure instead. The output of some submersible pressure sensors can be scaled to correct for specific gravity without the need for the installer to compensate for it within the signal conditioning instrumentation.
What type of liquid is to be measured?
This will help select a device with the most compatible construction materials. Submersible pressure sensors are comprised of sensing module, body, cable and seals, all of which must be compatible with the substance in which the sensor will be immersed.
What is the specific gravity or density of the liquid compared to freshwater?
The hydrostatic pressure generated by a fixed height of liquid is proportional to the specific gravity. 10 metres of water is a very close approximation to a 1 bar hydrostatic pressure, but if the specific gravity is say 0.9 of water (fresh water has a specific gravity of 1.0) a 10 metre head of this liquid would generate a 0.9 bar hydrostatic pressure instead. The output of some submersible pressure sensors can be scaled to correct for specific gravity without the need for the installer to compensate for it within the signal conditioning instrumentation.
Determining the Signal Cable Length of a Suspended Liquid Level Transducer
Deciding the correct length of cable for a suspended IP68 transducer is an important part of specifying, if the cable is too short it is difficult and expensive to join two cables together and make the seal liquid tight.
There are two separate parts of the installation that need to be considered to determine the overal length which is the suspended length and any extra length that is needed to reach the termination point from the point of suspension.
There are two separate parts of the installation that need to be considered to determine the overal length which is the suspended length and any extra length that is needed to reach the termination point from the point of suspension.
Why are Submersible applications so demanding of Pressure Transducers?
Placing a sensing device in a liquid is one of the most hostile environments to ensure a long reliable service life.
The extra considerations for design engineers include:
The extra considerations for design engineers include:
- Keeping pressurised liquid out.
- Preventing condensation damage to the internal electronics.
- Corrosion protection for all the materials used in the transducer construction.
- Adequate protection from lightning surges in exposed installations.
There are many types of place where liquid level is measured such as Tanks, Reservoirs, Boreholes, wells, Rivers or Lakes, but the basic rules for the design of a submersible pressure transducer are the same for all of these situations.
Transforming a 2 wire Current Loop into a Voltage Output Signal
If you are running a signal cable over a long distance or have a spare current output pressure transmitter, it is possible to convert the output signal to a voltage type to make it compatible with voltage only input devices.
A typical current output is 2 wire 4 to 20 mA which is designed to be connected in a series with the power supply and other components included in the circuit. The pressure transmiitter acts as a current source regulating the current at a particular pressure for a range of load impedences as long as there is sufficient supply voltage to power it.
A typical current output is 2 wire 4 to 20 mA which is designed to be connected in a series with the power supply and other components included in the circuit. The pressure transmiitter acts as a current source regulating the current at a particular pressure for a range of load impedences as long as there is sufficient supply voltage to power it.
Does the type of Engineering Units matter for Pressure Transmitters
When selecting a part number for a pressure transmitter you will need to choose a pressure range. Usually a manufacturer will designate pressure ranges in the most common unit used by it's customers.
For the USA the most common unit is psi and for Europe it is bar, but what if you are working with other units such as kPa or inHg, is it possible to use a pressure transmitter scaled in bar or psi for other types of unit?
For the USA the most common unit is psi and for Europe it is bar, but what if you are working with other units such as kPa or inHg, is it possible to use a pressure transmitter scaled in bar or psi for other types of unit?
Considerations for monitoring Landfill Water Level
In recent years regulations for environmental control, health and safety issues have become much more stringent for existing and new landfill sites. The regulations encompass many considerations and some involve the measurement and control of water level in many positions around a waste disposal site.
One application is ensuring contaminated water is contained within a tanked zone and does not escape into the surrounding environment. Water level is also monitored at syphoning points where the contaminated water (Leachate) is removed for processing.
In locations where the leachate is pooled there will also be relatively high concentrations of volatile hydrocarbon gases expelled due to the decomposition process within the landfill. Due to the presence of flammable gases, areas within landfill sites are classed as hazardous zones. It is requirement that any measurement instrumentation that is to be located inside a hazardous zone, should be approved and installed to the appropriate ATEX classification for that area.
One application is ensuring contaminated water is contained within a tanked zone and does not escape into the surrounding environment. Water level is also monitored at syphoning points where the contaminated water (Leachate) is removed for processing.
In locations where the leachate is pooled there will also be relatively high concentrations of volatile hydrocarbon gases expelled due to the decomposition process within the landfill. Due to the presence of flammable gases, areas within landfill sites are classed as hazardous zones. It is requirement that any measurement instrumentation that is to be located inside a hazardous zone, should be approved and installed to the appropriate ATEX classification for that area.
Converting 4 to 20mA Linear Signal to Square Root Extraction Output
Some pressure measurements are used to indirectly derive another type of measurand. One of these is the Rate of Flow of a gas or liquid. The flow rate along a closed pipe is directly proportional to the square root of the pressure drop or differential pressure between two points.
Since the relationship is non-linear there is a greater change in flow at low pressures compared to higher ones. In order to optimise the resolution of flow measurement the output on some differential pressure transmitters can be altered so that it is directly proportional to the flow rate rather than the differential pressure.
Since the relationship is non-linear there is a greater change in flow at low pressures compared to higher ones. In order to optimise the resolution of flow measurement the output on some differential pressure transmitters can be altered so that it is directly proportional to the flow rate rather than the differential pressure.
How does changes in Local Gravity affect Hydrostatic Pressure
The value of hydrostatic pressure for a fixed height column of fluid is directly proportional to the gravitional strength at a particular geographical location.
Generally if the height above sea level is increased the hydrostatic pressure will decrease. Also since the world is wider at the equator than the poles the gravitional pull will increase as you move closer to either the north or south pole.
Generally if the height above sea level is increased the hydrostatic pressure will decrease. Also since the world is wider at the equator than the poles the gravitional pull will increase as you move closer to either the north or south pole.
Why is Temperature included in Height of Fluid pressure units
Pressure units which are related to the height of a liquid often include a reference temperature e.g. 50 mH2O @ 4degC.
The hydrostatic pressure of a fluid at a certain height is determined by the total height of fluid above that point. If the temperature of the liquid is increased it will expand in volume increasing the fluid level, however the hydrostatic pressure will remain the same. Therefore when quoting a pressure in terms of fluid level it is useful to know the temperature.
The hydrostatic pressure of a fluid at a certain height is determined by the total height of fluid above that point. If the temperature of the liquid is increased it will expand in volume increasing the fluid level, however the hydrostatic pressure will remain the same. Therefore when quoting a pressure in terms of fluid level it is useful to know the temperature.
The different ways of pressure sealing a BSP parallel thread
There are two distinct styles of seal typically used for pressure instrumentation with a BSP thread fitting.
Pressure connections which include the suffix EN 837 or DIN 16288 are sealed on the end of the thread with a small diameter O ring which is placed around the post protruding from the end of the thread. In the female mating part of the pressure connection there is a hole with sufficient diameter to locate the male end post without pinching the O ring.
Pressure connections which include the suffix EN 837 or DIN 16288 are sealed on the end of the thread with a small diameter O ring which is placed around the post protruding from the end of the thread. In the female mating part of the pressure connection there is a hole with sufficient diameter to locate the male end post without pinching the O ring.
How Gravity affects the performance of Pressure Transducers
Any pressure transducer which relies on a force collecting diaphragm for sensing pressure, will be sensitive to a change in orientation.
A pressure transducer diaphragm is designed to flex when a force is applied to it. If the diaphragm is orientated horizontally it will sag slightly under the force of its own weight. If the diaphragm is then flipped upside down it will sag in the opposite direction. The movement in the diaphragm will translate as a slight change in pressure when monitoring the output signal of the device.
A pressure transducer diaphragm is designed to flex when a force is applied to it. If the diaphragm is orientated horizontally it will sag slightly under the force of its own weight. If the diaphragm is then flipped upside down it will sag in the opposite direction. The movement in the diaphragm will translate as a slight change in pressure when monitoring the output signal of the device.
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