Pneumatic Transmitter Positioner Description
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Pneumatic Transmitters, Pneumatic Positioners
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Pneumatic transmitters and pneumatic positioners represent two of the four distinct elements in pneumatic process control systems. Pneumatic transmitters sense process variables such as temperature, flow, pressure, or liquid level and transmit proportional three to 15 pounds per square inch (psi) process signals corresponding to 0 to 100 percent of full range to control instrumentation. Pneumatic positioners receive output signals from pneumatic controllers to position the control valve's opening percentage to keep a process variable at a desired set point.
Pneumatic Transmitters
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Pneumatic transmitters use a force balance principle that balances an input force change to an output force change to nullify internal motion and increase accuracy. Pressure transmitters usually apply process pressures to diaphragms or bellows to generate linear force that is sensed by a pneumatic pilot that facilitates the output change to the control instrument while internally rebalancing the forces between input and output. Temperature transmitters employ sealed helium bulb elements and bellows to convert temperatures to pressures and linear forces according to the Ideal Gas Law that states that a fixed amount of gas in a fixed volume will increase its pressure proportionately to an increase in temperature. Pneumatic flow transmitters most often use a differential pressure principle which resolves a balance between two opposing pressure diaphragms which is then fed as a single force to the output section of the device, similarly to the simple pressure transmitter. Level sensors may use the differential pressure principle that measures liquid head, or a proportional float mechanism that measures the force change of a partially submerged float as liquid level goes up and down.
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Transmitter Versatility
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In most cases, the output pilot and force balance feedback mechanisms are identical for all types of pneumatic transmitters since they all convert process inputs into measurable, scalable forces. Scaling or calibration adjustments allow pneumatic transmitters to be adjusted to different ranges according to the specific application. For example, a food manufacturer may require a temperature range of 0 to 250 degrees Fahrenheit (degrees F) in a steam cooker, and 0 to 600 degrees F in an oven. Their freezer may require minus 30 degrees F to 100 degrees F. Calibration of zero and span allow the user to set the low point that will transmit three psi output, or zero percent, and 15 psi output corresponding to 100 percent of full range. The same calibration principle would apply to pressures, flows, levels or any other variable transmitted pneumatically.
Pneumatic Positioners
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Pneumatic positioners are usually supplied as integral parts of pneumatic diaphragm or cylinder actuated control valves. They translate three to 15 psi output signals from remotely located pneumatic controllers to exact linear or rotational valve stem positions. They continually monitor the valve's opening position and correct it as required. This is extremely important to the overall precision and effectiveness of the control system, particularly where the medium flowing through the valve may be under extremely high pressure, or may impart momentum forces to the valve element that would offset the valve from its correct position.
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