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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How Piezoresistive Pressure Sensors Work

A Piezoresistive Pressure Sensor contains several thin wafers of silicon embedded between protective surfaces. The surface is usually connected to a Wheatstone bridge, a device for detecting small differences in resistance. The Wheatstone bridge runs a small amount of current through the sensor. When the resistance changes, less current passes through the pressure sensor. The Wheatstone bridge detects this change and reports a change in pressure.

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How pressure transmitters work

Pressure transmitters are commonly used for industrial processes and they are used to gauge the pressure and control the operation of industrial equipment. Pressure transmitters can sense physical variables such as pressure and flow-rate. This can be brought into existence due to the differential measurements.

A pressure transmitter contains a pressure sensor that can process fluids and calculate an output. This output will be the answer to pressure applied on it by the fluid.

What is an isolation system? – A pressure sensor can combine with a process fluid due to an isolation system. The isolating system comprises of an isolation diaphragm. During this process, there is no physical contact between the isolation fluid and the process fluid. This extends from the pressure sensor to the isolation diaphragm.

Isolation diaphragm- An isolation diaphragm is important as it separates the pressure sensor from any corrosive process fluids. During this process, pressure passes from the isolation diaphragm to the sensor. All these pressures pass through an incompressible isolation fluid which is then carried in the passageway.
Differential pressure – The pressure difference between two process pressures is measured by a differential pressure transmitter. A differential pressure transmitter requires two connections to the process fluid. This type of pressure transmitter measures the gage or absolute pressure of a processing fluid. It needs a single union with the process fluid chamber. This differential pressure transmitter comprises of a main body that has an internal enclosure. This internal enclosure contains a measurement chamber which can hold a pressure transducer. Differential pressure sensors are mainly used to measure small pressure differences over an absolute pressure range.

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What is Absolute Pressure ?

Pressure is represented by defining absolute vacuum as zero. Gauge pressure is generally used. Absolute pressure is used for scientific calculations, etc., and is distinguished from gauge pressure by putting “abs” after the unit (for example, mmHg abs).

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Absolute Pressure Sensor

 

What is Gauge Gressure ?

Pressure is represented by defining atmospheric pressure as zero. Pressure higher than atmospheric pressure is positive pressure, while pressure lower than atmospheric pressure is negative pressure.

Note: Atmospheric pressure varies depending on the weather, the altitude, etc. Therefore, a gauge pressure measuring instrument in which atmospheric pressure is defined as zero may not display the pressure value under absolute vacuum as “-760 mmHg”.

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gauge type