As with any other measuring or monitoring device,Pressure switchThere are several selection criteria to consider. Selecting the right pressure switch for a specific application can reduce costs and extend the service life of your equipment.

Process fluids:

The chemical properties of the process fluid determine the type of material required for the liquid receiving parts. The liquid connection component is the pressure side of the port, seal and pressure sensing assembly. These parts must be able to withstand any chemical or physical erosion from the process fluid. The mechanism of part degradation may be through corrosion, oxidation or erosion. The most commonly used materials for rigid parts are steel, brass, stainless steel, PTFE or PP; The elastic pressure sensing parts and seals are made of nitrile butadiene rubber, EPDM rubber and fluorine rubber.

Working temperature:

The operating temperature will affect the material used. Certain materials degrade at high temperatures. Materials suitable for high temperature service are FKM and stainless steel 316. The temperature of the measured medium must be within the temperature range specified by the manufacturer for the switch.

The effect of temperature on accuracy must also be considered. When the pressure switch is configured at room temperature, it may be necessary to readjust the set point if the process is at a higher temperature. Fitting connection sizes range from 1/8 to 1/2 NPT.

Pressure range:

The pressure range defines the limits for adjusting the cut and cut off pressures. This is often referred to as the operating range of the pressure switch. It is recommended to set the set point at 40% to 60% of the pressure range to anticipate any adjustments or field changes.

Type of pressure:

Pressure switches are often used in positive pressure systems. But there are also cases where they are used for vacuum applications. For negative pressure systems, pressure switches specified for vacuum and compound pressures must be used.

Switching function:

Switches can be characterized by the number of poles and the number of throws. The number of poles refers to the number of circuits the switch can control, while the number of throws is the number of connections the switch can make. Both pole and toss can be single or double. Switch functions are classified as:

Single Pole Single Throw (SPST) :

This is the basic switch. It can only be NO or NC.

Single Pole Double Throw (SPDT) :

Due to its versatility, this is the most common. It can be used as a NO, NC or CO switch. It can also have three positions, with the off position of the CO switch in the middle. This is called a single-pole three-throw and is rarely used for pressure switches that typically have only two positions.

Double Pole Single Throw (DPST) :

This is the same as two SPST switches connected in a common actuator.

Double Pole Double Throw (DPDT) :

This is the same as two SPDT switches controlled by a common actuator.

Differentiation, dead zone, or lag:

This is the difference between cut in and cut out pressure. The dead zone of the pressure switch can be adjustable or fixed. Adjustable dead zones are widely used for pumping services. Fixed dead zones, on the other hand, occur in sets of equipment and alarm systems where modifications are not required or avoided to prevent any unintentional modifications to the system. Diaphragm and Bourdon tube pressure sensing elements typically have narrower dead zones than pistons.

Voltage resistance:

Voltage resistance refers to the maximum pressure that a switch can withstand without changing its characteristics or performance. This is also known as overrange capacity or maximum system pressure. The identification proof pressure takes into account any pressure spikes or surges that occur in the system.

Accuracy:

This is the maximum positive or negative deviation from a set point or a specified characteristic curve under certain conditions and operations. Accuracy is an even more important factor in choosing analog pressure sensors and electronic pressure switches. For these devices, having higher accuracy can significantly increase the cost of the pressure switch. Accuracy is expressed as a percentage of full scale (FS) values. The typical pressure switch accuracy for diaphragm and Bourdon tubes is ±0.5%, while the piston pressure switch accuracy is ±2%. On the other hand, electronic pressure switches have better accuracy of ±0.2 to 0.5% depending on the manufacturer.

Repeatability:

Repeatability is the deviation between measurements or activations at the same pressure. This is different from accuracy because the device can have high repeatability but low accuracy. The pressure switch can be activated repeatedly under a certain pressure, but the activation is far from the set point. As with accuracy, repeatability is specified as a percentage of full scale.

By bike:

This is the expected time between activations. This factor must be taken into account, as the continuous deformation of the pressure sensing element causes continuous fatigue, thereby reducing its service life. Piston tubes and bourdon tubes operate according to deformation principles and are suitable for low cycle applications. For high cycles, use pistons and electronic pressure switches. Because the drive depends only on the movement of the piston or plunger, the fatigue level of the piston pressure switch is low. Electronic pressure switches have the same experience because strain gauges deform much less than mechanical sensing elements.

Service life:

This is directly affected by the speed of the ride. Service life refers to the expected number of times a switch can be activated and deactivated before it fails. Since electronic pressure switches are solid-state devices with no moving parts, they have a better service life, which is expected to be in the millions or more. In mechanical pressure switches, the service life of piston switches is better than Bourdon tube and diaphragm switches.

Control system voltage:

This specifies the electrical characteristics of the control circuit. The rated current, voltage and frequency of the power switch must be the same. Otherwise, the switch, especially the electronic switch, may not activate or the accuracy may be poor. The control circuit using the pressure switch is usually DC. However, in some cases, AC voltage is also used. Common DC voltages are 8, 12, 24, and 30 volts, while 60 Hz AC voltages are 24, 120, 240, and 480 volts.

Accessory:

The fitting connection type on the pressure switch must match the process stub connection or pressure port. External and internal connections are widely used to install pressure switches. Joint connection sizes range from 1/8 to 1/2 inch. In addition to size and type, the material for connecting the specified accessories is also based on the type of environment and match. The main reason is to prevent corrosion, whether from the atmosphere or from the plating process.

Housing protection class:

This determines the environment the switch housing can withstand. Because pressure switches are widely used in almost every industry, a variety of housing designs exist to balance robustness and cost. The protection level of the enclosure is specified by NEMA and IP number. In general, a higher NEMA number indicates a better level of protection. IP numbers, on the other hand, have two digits. The first indicates a solid or particle protection level, while the second indicates a liquid. For general purpose indoor use, NEMA 1 to 2 or IP 10 to 11 are used where only human contact needs to be prevented. NEMA 3S to NEMA 4X or IP 54 to IP 64 are sufficient to protect against dust, rain and snow outside. In the case of occasional rinsing and soaking, NEMA 6 and IP 68 are usually used.

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