Hi! Friends! When you are working in the industry, then the four most common types of sensors you find there are Temperature, Pressure, Flow, and Level measurement sensors.
So today in this article we will study the basics of pressure measurement sensors.
Basics of Pressure Measurement
Pressure measurement is a very common requirement in the industrial process. We can say that most of the operation takes place in the industry at some pressure like fluid flowing through the pipe, gas flowing through the pipe, flow measurement by measuring differential pressure, etc.
For that, we have to study, first what is pressure in terms of Industrial Automation?
What is Pressure?
The pressure is the measurement of the force exerted per unit area.
Types of Pressure
Atmospheric Pressure(Patm) is the surface pressure of the earth and is available due to the weight of the gases in the earth’s atmosphere.
Absolute Pressure(Pabs) is the pressure measured relative to the full vacuum.
Differential Pressure(DP) is the pressure measured at two different pressure points with the same reference point.
Gauge Pressure(Pgauge) is the pressure that is measured against the atmospheric pressure.
Pressure Measurement Methods
There are many methods used to measure the pressure in the industry.
1) The first method involves balancing the unknown pressure against the pressure produced by a column of liquid of known density.
U Tube Manometer
The manometer is the simplest example of this type of pressure measurement device. The manometer is a U-shaped glass tube in which liquid is filled. The unknown pressure is applied to the one arm of the tube, this results in the deflection of the liquid height to the other arm. This height is measured and pressure is converted using this height.
Advantages of U-tube Manometer:
- Simple in construction
- Low cost
- Very accurate and sensitive
- It can be used to measure other process variables.
Disadvantages of U-tube Manometer:
- Fragile in construction.
- Very sensitive to temperature changes.
- The error can happen while measuring the h.
2) The second method involves the object in which unknown pressure is applied directly, this pressure results from stress or strain on this object.
Any change in pressure causes the object to deflect. This deflection is mechanically amplified by using a suitable gear and linkage mechanism. Then it is indicated on the calibrated dial. They are also called mechanical pressure measurement devices like bourden gauge, diaphragm gauge, or bellow gauge.
The mechanism of this gauge consists of a coiled tube with one end is sealed and the other end is open to measure the pressure.
When the pressure is applied to the measurement end of the coil it straightens up and causes deflection, this deflection results from the movement of the needle which moves on a calibrated dial.
The diaphragm consists of a circular membrane, which is in a flat or corrugated shape made up of sheet metal with a precise dimension.
This gauge works similar to the bourden gauge but consists of a diaphragm that deflects when the pressure is applied. This deflection is indicated with the needle on a calibrated scale.
The deflection is caused by bellows which are driven indicating a needle over a calibrated scale. This is suitable for a very low-pressure measurement.
Bellows are usually made up of thin-roll form by pressing mechanically or hydraulically. The material used to create a bellow are mainly Brass, Stainless Steel, Phosphor Bronze, and Beryllium Copper.
3) The third method is the electrical pressure measurement method in which mechanical deflection is converted into electrical signals. Pressure transducers are used for that.
Pressure Transducers are useful to measure pressures continuously such that the electrical signal supplied to some control system can be used to monitor the pressure variations.
Some typical types of pressure transducers can be capacitive, magnetic, resistive, or piezoelectric.
Resistive Pressure Transducer
This type of pressure transducer consists of a stable main body with a thin diaphragm. The main element is a diaphragm which is equipped with a strain gauge, a strain-sensitive element.
When the pressure is applied to the diaphragm, it deflects. Thus the strain gauge attached with a diaphragm will compress and its electrical resistance changes. This change of resistance is directly proportional to the pressure.
That’s the way the pressure sensor converts the mechanical pressure into a proportional electrical signal.
Capacitive Pressure Transducer
Capacitive pressure sensors use a thin diaphragm as one plate of a capacitor. When the pressure is applied to the diaphragm, its deflection causes changes in capacitance value.
Secondary electronics measure this capacitance and convert it into a proportional pressure signal. Capacitive pressure measurements have a high sensitivity, allowing very low pressures to be measured.
Piezoelectric Pressure Transducer
image courtesy PCB.com
Piezoelectric pressure sensors use quartz crystals as an element to measure the pressure. When pressure is applied to the crystals, it generates a positive or negative charge. This charge generates an electrical voltage proportional to the measured pressure.
This voltage generates an electrical current that can be amplified and converted into an output signal of the transducer.
Piezoelectric pressure sensors are mainly used to measure dynamic pressure. They are generally not suited for static pressure measurement because the polarization of the piezoelectric crystal can fade under static conditions.
Inductive Pressure Measurement
The principle of inductive pressure measurement is based on the change in impedance of a coil due to the displacement of a ferromagnetic core that is firmly connected to the pressure sensing element.
The elements used in this type of pressure measurement can be a bourden tube, bellow, diaphragm, or a capsule that is directly connected with the ferromagnetic core.
The movement of the pressure sensing element caused by the pressure applied to it will be measured using a Linear Variable Differential Transformer(LVDT).
I hope you like this blog about industrial pressure measurement. If you want more blogs like this in the field of instrumentation then comment down below.
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