A Differential Pressure Transmitter measures the difference in pressure between two points and converts it into a pneumatic or electrical output signal. The working principle involves the use of a sensing element (e.g., a diaphragm or bellows) that deforms proportionally to the applied pressure difference. This deformation is converted into a measurable signal, which can then be processed and transmitted to control systems or indicators.

• The system comprises two pressure chambers, high-pressure (H) and low-pressure (L), separated by a diaphragm.
• Pressure from two points (H and L) is applied to either side of a flexible diaphragm or bellows within a sealed process chamber. The difference in pressure (ΔP = H - L) causes the diaphragm/bellows to deflect proportionally.
• This deflection is precisely measured by a mechanism, often incorporating a capacitive sensor or LVDT (Linear Variable Differential Transformer).
• The displacement of the diaphragm/bellows is converted into an electrical signal (e.g., 4-20 mA). This often involves a Wheatstone bridge configuration if using a strain gauge or a similar technique based on the chosen sensor.
• This electrical signal is then amplified by a pilot amplifier (see section (c)) and transmitted as the output signal.

As defined in the provided text, zero calibration adjusts the output to correspond to zero differential pressure (H = L). Span calibration adjusts the output range to accurately reflect the full differential pressure range the transmitter is designed to measure. Adjustment screws on the transmitter casing allow for these calibrations, often requiring specialized tools and procedures to ensure accuracy.

A negative feedback bellows is used in some differential pressure transmitters to improve accuracy and stability. It works by counteracting the deflection of the main sensing element. A portion of the output signal is used to generate a counter pressure within this feedback bellows, effectively reducing the deflection from the main sensing element and thus increasing the linearity and stability of the instrument. This reduces the sensitivity to small pressure changes but improves overall accuracy and reduces hysteresis.

The pilot amplifier is essential for converting the weak signal generated by the displacement sensing mechanism into a usable output signal. It amplifies the signal and converts it from a pneumatic signal (in some older designs) or a low-level electrical signal into a standardized 4-20 mA or 0-10 V signal for transmission to a control system. It might use a transducer like a strain gauge to perform this conversion.

When measuring liquid level using a differential pressure transmitter, the transmitter may not be installed precisely at the zero level of the tank. The "zero elevation" concept accounts for this difference in height. The hydrostatic pressure difference due to the elevation difference between the transmitter and the true zero level must be compensated in the output signal calculations. This ensures the accurate measurement of the liquid level even when the transmitter is not located at the tank's bottom. This compensation can involve either adding or subtracting a pressure offset from the raw differential pressure measurement, depending on the transmitter's configuration.