How a Current to Pressure Transducer (I/P) Works

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A “current to pressure” transducer (I/P) converts an analog signal (4 to 20 mA) to a proportional linear pneumatic output (3 to 15 psig). Its purpose is to translate the analog output from a control system into a precise, repeatable pressure value to control pneumatic actuators/operators, pneumatic valves, dampers, vanes, etc.

The  I/P converter provides a reliable, repeatable, accurate means of converting an electrical signal into pneumatic pressure in many control systems. Models of this device are usually available in direct and reverse action and are field selectable with full or split range inputs or output as the case may be.

The most common application of an I/P transducer is to receive an electrical signal from a controller and produce a proportional pneumatic output for operating a control valve or positioner. The device can be mounted on the wall or a pipe stand or directly on the valve actuator. Where the device can withstand vibrations, they are directly mounted on the valve actuator. In many instances, the device is remotely mounted on instrument pipe stands to reduce vibrations.

Operating Principle
The I/P converter uses an electromagnetic force balance principle to change electrical signals into pneumatic signals. Typically, a 4 – 20mA input is converted into a 3 – 15pisg output. The operating principle of the I/P transducer is shown below:

As shown above, a coil produces the magnetic field. Within the coil is a deflector motor with a flapper valve attached that operates against a precision nozzle to create a backpressure on the servo diaphragm of a booster relay. The input current flows in the coil and produces a force between the coil and the flapper valve, which controls the servo pressure and the output pressure.

Zero adjustment of this device is made by turning a screw that regulates the distance between the flapper valve and the air nozzle. Span adjustment is made by varying a potentiometer, which shunts input current past the coil. The output signal of an I/P transducer easily falls to bleeds pressure upon failure of the electrical signal. This makes it easy to detect electrical failure signal in the device.

Troubleshooting and Calibration Tips
Most I/P converters are supplied instrument air of about 20psig above the required output of the device. An inoperative or poorly performing I/P transducer may just be having problem with its air supply. So to effectively troubleshoot this device, slightly open the air set drain and check that the supply air is dry and oil free. Checks can also be made on external bleed opening to verify that it is clear and free from obstruction and that it is venting properly.

This device is calibrated in much the same way as you calibrate a pressure transmitter. Please see How to Calibrate a Current to Pressure Transducer for a detailed guideline. In calibrating the I/P transducer, you provide an accurate input and check the output with a test gauge or any other test device. Check Zero, Span and Linearity and adjust as necessary. You can use either an accurate current generator or an accurate voltage generator with a compulsory precision resistor of 250 ohms and half watt(1/2W) as power rating.
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