Pressure Measurement Glossary-Pressure Unit

Bar - is a widely used metric unit of measure for pressure and 1 bar converts precisely to 100,000 Pascals.

Kilopascal (kPa) - is a 1000x multiple of the Pascal unit which is the SI unit for pressure. 1 Kilopascal equals 1,000 Pascals.

Megapascal (MPa) - is a 1000000x multiple of the Pascal unit which is the SI unit for pressure. 1 Megapascal equals 1,000,000 Pascals.

Millibar (mbar, mb or mbr) - is a metric unit of pressure mainly used in European countries and is derived directly from the Bar pressure unit which equals 1,000 mbar. In SI units 1 mbar equals 100 Pascals.

Pascal (Pa) - is the SI unit for pressure and is derived from other SI units using the following relationships: Pa = (kg.m/s²)/m² = kg/m.s² = N/m². Since 1 pascal is a very low pressure being 1/100 of a millibar it's use is limited to ultra low pressure applications such as measuring the pressure differences in ventilation systems.  

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Pressure Measurement Glossary-Pressure Type

Absolute Pressure - is one which is measured relative to a perfect vacuum. 

Barometric Pressure or Atmospheric Pressure - is the total outside air pressure measured with reference to absolute vacuum. The pressure varies depending on geographical location, altitude and local weather conditions. For weather reporting purposes the barometric pressure is normally adjusted to a sea level value so that all locations can be compared independent of the altitude at each location

Differential Pressure (DP) - is the difference in pressure between two separate points. 

Gauge Reference Pressure or G - is a pressure measured relative to atmospheric or barometric pressure. 

Hydrostatic Pressure - is what is exerted by a liquid when it is at rest. The height of a liquid column of uniform density is directly proportional to the hydrostatic pressure.

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Pressure Measurement Glossary-Accuracy and Precision

Does Precision mean the same as Accuracy for Pressure Measurement Devices?

In the context of a pressure measurement device, precision refers to how well it provides repeatable and linear results with increasing or decreasing pressure over many cycles. It is therefore an indication of the quality of measurement technology utilised and is a major factor effecting the cost of an instrument.

Accuracy is very closely connected with precision, but unlike precision it is constrained by a series of fixed reference points or absolute values that all readings are compared with to define the exactness of the instrument. Typically these comparison absolute values are read from a more accurate instrument ideally with a 10 to 1 ratio difference in accuracy to ensure it does not contribute significantly to the overall error calculation.  However in practise it is not always possible to achieve such a high ratio difference in accuracy. When this is the case the overall accuracy should also include the reference instrument uncertainty which better reflects the true accuracy of the device.

A sensors measurement error is normally defined as precision because tight electrical tolerances are difficult to achieve with an analog output signal. Once the zero and full scale points have been ascertained, device scaled via a digital panel meter or other analog to digital converter instrument, the error can then be expressed as accuracy rather than precision. 

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Why Pressure Sensors are designed with stainless stell

There are many types of materials used for constructing pressure sensors in a variety of applications, but those made from stainless steel are considered the most reliable and durable.

High Tensile strength
In hydraulic applications where pressure overloads are commonplace a stainless steel pressure sensor can withstand a high number of pressure cycles without fatigue damage.

Durability and Hardness
In demanding environments a high reliability and durability is required to withstand the extremes of shock, vibration and temperature. An all stainless steel design is ideal for these applications due to its high toughness.

Corrosion Resistance
For the majority of fluids and gases, stainless steel provides excellent protection against short and long term corrosion due to a naturally occurring resistant surface layer formed by a self passivisation process.

Welded Seals
Over many pressure and temperature cycles an all welded mechanical assembly provides a high integrity pressure seal for very high pressure and vacuum measurement applications.

Cleanliness and High Purity
For food, beverage and pharmaceutical industries, stainless steel is the preferred material because of its high polished surface which makes it easy to clean and discourages bacterial growth during use. The other benefit is that all the mechanical piece parts exposed to the media can be made from one material and welded together so that there are no crevices to attract contaminants or process media residues.

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Advantages and Disadvantages of using a 4 to 20 mA Signal

1. The 4-20mA current loop is a common method of transmitting sensor information in many industrial process-monitoring applications – typically in systems monitoring pressure, temperature, pH, or other physical factors. These systems employ the familiar 2-wire, 4-20mA current loop, in which a single twisted-pair cable supplies power to a module and also carries the output signal.
   
2. Transmitting sensor information via a current loop is particularly useful when the signal has to travel long distances – 1,000 feet is not uncommon.
   
3. The use of basic two-wire technology makes the installation both inexpensive and simple to wire, maintain, and troubleshoot.
   
4. The loop’s operation is straightforward: A sensor’s output signal is first converted to a proportional current, with 4mA normally representing the sensor’s zero-level output and 20mA representing the sensor’s full-scale output. A reading of 20 mA means that a valve, for example, is 100 % open, and a reading of 4mA means that it’s closed. Readings between the maximum and minimum values mean the circuit is controlling the valve.
   
5. Why is 4mA chosen as the lower threshold representing the ‘off’ or ‘closed’ position? The design takes into account that there must be a means of representing a fault caused by an open circuit or a lost feed. Therefore, a reading of 0mA is essentially “reserved” to signal a fault in the system, while a reading of approximately 4mA or 20mA would indicate a properly working circuit.
   
6. Key among the advantages of 4-20mA current loop technology is that the accuracy of the signal is not affected by voltage drop in the interconnecting wiring, and that the loop can supply operating power to the device. Even if there is significant electrical resistance in the line, the current loop transmitter will maintain the proper current, up to its maximum voltage capability.
   
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Sendo Sensor: Types of Pressure Measurements

Sendo Sensor: Types of Pressure Measurements: Pressure S ensors can be classified in terms of pressure ranges they measure, temperature ranges of operation, and most importantly the...

Types of Pressure Measurements

Pressure Sensors can be classified in terms of pressure ranges they measure, temperature ranges of operation, and most importantly the type of pressure they measure. In terms of pressure type, pressure sensors can be divided into five categories:

 Absolute pressure sensor

This sensor measures the pressure relative to perfect vacuum pressure (0 PSI or no pressure). Atmospheric pressure, is 101.325 kPa (14.7 PSI) at sea level with reference to vacuum.

 Gauge pressure sensor

This sensor is used in different applications because it can be calibrated to measure the pressure relative to a given atmospheric pressure at a given location. A tire pressure gauge is an example of gauge pressure indication. When the tire pressure gauge reads 0 PSI, there is really 14.7 PSI (atmospheric pressure) in the tire.

 

Vacuum pressure sensor

This sensor is used to measure pressure less than the atmospheric pressure at a given location. This has the potential to cause some confusion as industry may refer to a vacuum sensor as one which is referenced to either atmospheric pressure (i.e. measure Negative gauge pressure) or relative to absolute vacuum.

  

Differential pressure sensor

This sensor measures the difference between two or more pressures introduced as inputs to the sensing unit, for example, measuring the pressure drop across an oil filter. Differential pressure is also used to measure flow or level in pressurized vessels.

  Sealed pressure sensor

This sensor is the same as the gauge pressure sensor except that it is previously calibrated by manufacturers to measure pressure relative to sea level pressure

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