PRESSURE SWITCHES

A pressure switch is a form of switch that makes electrical contact when a certain set pressure has been reached on its input. This is used to provide on/off switching from a pneumatic or hydraulic source. The switch may be designed to make contact either on pressure rise or on pressure fall.

CALIBRATION
Before we get down to the nitty-gritty of how to calibrate and adjust a pressure switch, let us get to understand some basic concepts with pressure switch calibration:

Setpoint:
This is the pressure at which the pressure switch is required to operate. A pressure switch may be set to operate on either a rising pressure (high level alarm) or a falling pressure (low level alarm). Most switches are designed to operate at a 'gauge' pressure setpoint i.e. relative to atmospheric pressure. Some applications require an 'absolute' pressure setpoint i.e. relative to absolute zero pressure, and an absolute pressure switch is required for these. Ideally the range of the switch should be chosen such that the setpoint is between 25% to 75% of this range.

Dead-band or Reset:
This is a setting that determines the amount of pressure change required to re-set the switch to its normal state after it has tripped. The dead-band or reset or switching differential is the difference in the rising and falling pressures at which the pressure switch operates. For a fixed differential output switch this is typically about 1% to 3% of the switch range. For an adjustable differential output switch it may be adjusted from about 5% to 12% of the switch range.

The pressure switch is a ubiquitous device. It is practically everywhere in your plant. But how do you calibrate this simple device? The answer is here. Just follow the simple steps that I have outlined below.

Before you calibrate your pressure switch, confirm the following:

• The setpoint of the pressure switch
• The dead-band of the switch

Also depressurize and isolate the pressure switch from the process. If opening the switch exposes voltages or energy that is not intrinsically safe, please follow the specified procedure for your plant. For example, if in an explosive environment, use a continuously monitoring gas detector to monitor for the presence of explosive gasses.

Calibration Procedure of the Pressure Switch

Step 1:
Connect the pressure switch to a pressure source e.g air supply via a hand pressure regulator and test gauge, as shown in the diagram above.

Step 2:
Use an Ohmmeter or a Digital Multimeter (DMM) set to the continuity range to check and verify that the switch contacts are as indicated: NO (Normally open) and NC (Normally close).

 Step 3:
Connect the Ohmmeter or DMM between the normally open contacts(NO) and the common terminal (C) of the switch. The meter should read "open circuit". Adjust the hand pressure regulator to increase the pressure to the setpoint of the pressure switch until the contacts change over. The meter should now read "short circuit". Note the pressure reading and write it down. This pressure is the switch setpoint for a "rising" pressure.

Step 4:
Increase the pressure to the switch to its maximum rating. Slowly reduce the pressure to the switch until the switch changes over from closed to normally open again. Note and write down this pressure reading. This pressure is the switch setting for a "falling" pressure.

Step 5:
From the readings you have taken work out the pressure difference between the rising and falling pressure settings. This is called the "dead-band" of the switch. The dead-band calculated should be equal to or less than the manufacturers’ dead-band.

The maximum dead-band is usually stated by the manufacturer. The switch is unserviceable if the maximum dead-band is more than the manufacturer's recommendation (dead-band on the nameplate of the switch)

GAS PRESSURE SWITCH


Gas-pressure switches are available in two basic types:
• Falling-pressure switches
• Rising-pressure switches

In the falling-pressure switch, decreased pressure on the diaphragm actuates the device. The switch is designed to lock out when the pressure falls to the setpoint (minus differential). As the pressure increases, the diaphragm rises and the switch is deactuated (except on manual reset models). An adjustable spring-loaded diaphragm determines the amount of pressure required to actuate the switch.

In a rising-pressure switch, the switch is actuated by increased pressure on the diaphragm. As the pressure falls, the diaphragm lowers and the switch is deactuated (again, except on manual reset models). An adjustable spring-loaded diaphragm determines the amount of pressure required to actuate the switch.

The pressure required to move the diaphragm in these switches is adjustable within the pressure range stamped on the switch nameplate. The pressure switch shown in Figure can be adjusted by removing the cover and turning the adjustment screws clockwise. This action raises the actuation point of the switch. Turning the screws counterclockwise lowers the actuation point. The range scale plate in the switch is marked for four relative pressure settings. Setting A corresponds to a minimum range, D to a maximum range, and both B and C to intermediate ranges.

A typical wiring diagram for a single-pole double-throw SPDT switch is shown in Figure. An SPDT switch may be wired to open or close the circuit on pressure rise.

On switches equipped with manual reset, the switch contacts open on pressure rise or pressure drop (depending on which model is used) and remain open regardless of pressure change. The reset button is pushed to close the switch after the pressure has returned to an acceptable level.

REFERENCE

http://instrumenttoolbox.blogspot.in
http://www.heaterheating.com

Posted in: Calibration, Instruments, Pressure, Pressure switches