What is a transmitter?
A transmitter is a measuring instrument that measures a physical quantity, such as pressure, temperature, flow or level, and converts it into a standardised signal, usually 4-20 mA, that a PLC or DCS can process. A transmitter consists of a sensing element that detects the quantity and electronics that amplify, linearise and convert the signal. Transmitters are the “eyes” of a process installation: without reliable measurements, no control loop can work properly.
🧠 What is the difference between a sensor and a transmitter?
| Sensor | Transmitter | |
|---|---|---|
| Function | Detects a quantity | Measures and converts to a standard signal |
| Output | Often a weak or non-standard signal (mV, Ω) | 4-20 mA, HART, fieldbus or Ethernet |
| Electronics | Minimal or none | Amplification, linearisation, diagnostics |
| Example | PT100 element, strain gauge | Temperature transmitter, pressure transmitter |
In practice the terms are often used interchangeably. Strictly speaking, the sensor is the measuring element and the transmitter is the complete instrument that sends the signal to the controller.
🔧 What types of transmitter are there?
| Type | Measures | Typical measuring principle | Application |
|---|---|---|---|
| Pressure transmitter | Gauge or absolute pressure | Piezoresistive or capacitive diaphragm | Pipes, vessels, pumps |
| Differential pressure transmitter | Pressure difference | Two diaphragms | Flow across an orifice plate, filter fouling, level |
| Temperature transmitter | Temperature | PT100 or thermocouple | Reactors, ovens, cooling |
| Flow transmitter | Flow rate | Electromagnetic, Coriolis, vortex, ultrasonic | Dosing, custody transfer, energy |
| Level transmitter | Fill level | Radar, ultrasonic, hydrostatic | Tanks, silos, wells |
| Analytical transmitter | Composition | pH, conductivity, oxygen | Water, chemicals, food |
🔄 How does a transmitter communicate with the controller?
- 4-20 mA — one analogue signal per cable, simple and robust
- HART — digital signal on top of 4-20 mA for configuration and diagnostics
- Fieldbuses — Foundation Fieldbus and PROFIBUS PA connect several transmitters over one cable
- IO-Link — point-to-point connection for simple sensors in machines
- Wireless — WirelessHART for hard-to-reach measuring points
- Ethernet-APL — Ethernet all the way into the field, including hazardous areas
Smart transmitters with digital communication provide diagnostics, such as a blocked impulse line or a failing sensing element, that remain invisible with a purely analogue signal.
🛠️ How do you select and install a transmitter?
- Determine the measuring range — choose a range in which the normal value sits around 50–75%, not at the limit
- Consider process conditions — temperature, pressure, corrosion and any hazardous area (ATEX)
- Choose the signal — 4-20 mA, HART or a digital variant, matching the controller
- Choose the right mounting — for pressure and differential pressure measurement, the position of the impulse lines determines accuracy
- Calibrate and document — record range, tag number and calibration date in the CMMS
- Plan periodic checks — drift, fouling and ageing make recalibration necessary, often annually
❓ Frequently asked questions
What does a pressure transmitter do?
A pressure transmitter measures the pressure of a gas or liquid with a diaphragm and converts that pressure into a standard signal, usually 4-20 mA. The controller uses that signal to, for example, control a pump or raise an alarm on excessive pressure. Pressure transmitters are the most widely used transmitters in the process industry.
What is the difference between a transmitter and a transducer?
A transducer converts a physical quantity into an electrical signal without standardising that signal. A transmitter does the same, but delivers a standardised signal such as 4-20 mA that can travel a long distance to the controller. In practice the distinction is not always strictly observed.
How often should a transmitter be calibrated?
How often a transmitter should be calibrated depends on the type, the process conditions and the importance of the measurement. A common interval is once a year. Stricter requirements often apply to measurements in safety systems or for custody transfer. Smart transmitters with diagnostics help set the interval based on actual condition.
What is a smart transmitter?
A smart transmitter has a microprocessor and digital communication, for example via HART, PROFIBUS PA or Foundation Fieldbus. It can be configured remotely, provides diagnostic information and can pass on several measured values at once. This reduces the need for on-site maintenance.
Can transmitters be a security risk?
Classic 4-20 mA transmitters are hardly attackable over the network. Smart transmitters with HART, fieldbus or Ethernet, however, can be configured remotely, and therefore also manipulated if an attacker reaches the controller or the network. Access control on configuration tools and write protection on the transmitter limit that risk.
What is the difference between a pressure and a differential pressure transmitter?
A pressure transmitter measures the pressure at one point relative to atmosphere or vacuum. A differential pressure transmitter measures the pressure difference between two points. This lets a differential pressure transmitter also measure flow indirectly across an orifice plate, determine the level in a closed tank or monitor filter fouling.
What does a transmitter’s tag number mean?
A transmitter’s tag number is a unique code, usually following ISA-5.1, that indicates what the instrument measures and where it is located. PT-101, for example, is a pressure transmitter in loop 101, TT-204 a temperature transmitter. The same tag number appears in the P&ID drawing, the PLC and the maintenance system.
📌 In summary
A transmitter measures a process variable such as pressure, temperature, flow or level and converts it into a standard signal, such as 4-20 mA, for the PLC or DCS. The choice of range, type and communication determines how reliable the measurements are on which the entire control system depends.