Views: 50 Author: Site Editor Publish Time: 2025-10-17 Origin: Site
1. What is a Pressure Transmitter?
A pressure transmitter is a device that converts a physical pressure value (from a gas, liquid, or vapor) into a standardized, proportional electrical signal (most commonly 4-20 mA or a digital protocol) for transmission to a control system, recorder, or display.
2. Fundamental Working Principle
The core of most modern pressure transmitters is the strain gauge, typically arranged in a Wheatstone bridge configuration, on a diaphragm.
Pressure Application | Process pressure acts on an isolating diaphragm. |
Force Transfer | This force is transferred via a fill fluid (like silicone oil) to the sensing diaphragm inside the sensor. |
Diaphragm Deflection | The sensing diaphragm minutely deflects |
Resistance Change | This deflection causes the strain gauges bonded to the diaphragm to stretch or compress, changing their electrical resistance. |
Signal Generation | The Wheatstone bridge becomes unbalanced, producing a small millivolt (mV) signal proportional to the pressure. |
Signal Conditioning | The transmitter's electronics amplify, linearize, temperature compensate, and convert this tiny mV signal into the standard 4-20 mA or digital output. |
3. Types of Pressure Measured
Absolute Pressure | Measured relative to a perfect vacuum (zero pressure). The reference side of the sensor is sealed under a vacuum. Symbol: PSIA, bar(a) | Application: Barometric pressure, vacuum furnaces, distillation columns. |
Gauge Pressure | Measured relative to the current ambient atmospheric pressure. The transmitter has a vent hole to the atmosphere. Symbol: PSIG, bar(g) | Application: Tank level, blood pressure, pipe pressure. |
Differential Pressure (DP) | The difference between two pressures applied to its two ports. | Application: Flow measurement (across an orifice plate), filter monitoring, liquid level in closed tanks. |
4. Critical Performance Specifications
Range: The span of pressure the transmitter is designed to measure (e.g., 0 to 100 psi). Tip: Select a range where your normal operating pressure is in the upper 50-75% for best accuracy.
Accuracy: The maximum expected error between the measured value and the true value, expressed as a percentage of span (e.g., ±0.1% of URL). It includes effects of non-linearity, hysteresis, and repeatability.
Span (or Zero) Adjustment: The ability to calibrate the transmitter's output at the upper and lower range points to correct for drift or to match the process.
Turndown (or Rangeability): The ratio of the maximum allowable calibrated span to the minimum allowable calibrated span. A high turndown ratio offers more flexibility.
5. Before buying a pressure transmitter, answer these questions:
What type of pressure? (Gauge, Absolute, Differential)
What is the pressure range? (Include overpressure and burst pressure limits.)
Where will it be installed? (Hazardous area? Outdoors? Consider the Enclosure Rating - IP67/NEMA 4X.)
What is the process of media? (Compatible with the wetted materials - Diaphragm, Seal, Body? Is it clogging, corrosive, or abrasive?)
What output and communication is needed? (4-20 mA HART, Foundation Fieldbus, PROFIBUS-PA?)
