A sensor-transmitter unit or a sensor working in conjunction with a transmitter serves a crucial role in instrumentation systems: the transmitter's primary function is to convert the raw signal generated by a sensor into a standardized format that can be easily understood and used by other devices.
Understanding the Sensor-Transmitter Function
In industrial and automation settings, sensors are used to detect and measure physical properties like temperature, pressure, flow, level, position, and more. The signal produced by a sensor is often raw, low-power, or specific to the sensor's technology (e.g., a small voltage change, resistance change, or frequency). This raw signal is not always suitable for direct transmission over long distances or for input into control systems or indicators.
This is where the transmitter comes in. As stated in the reference:
The function of a Transmitter is to convert the signal produced by a sensor into a standardized instrumentation signal such as 3-15 PSI air pressure, 4-20 mA DC electric current, Fieldbus digital signal etc., which may then be conveyed to an indicating device, a controlling device, or both.
Essentially, the sensor acts as the "eyes" detecting the physical variable, while the transmitter acts as the "translator" converting the sensor's language into a universally understood signal.
Why Standardize the Signal?
Standardizing the signal is vital for several reasons:
- Compatibility: Ensures that signals from different types of sensors can be connected to the same control systems, displays, or data acquisition units.
- Reliable Transmission: Standard signals, especially current loops like 4-20 mA, are less susceptible to electrical noise and signal degradation over long cable runs compared to raw voltage signals.
- Ease of Calibration and Troubleshooting: Technicians are familiar with standard signal ranges, making installation, calibration, and fault finding simpler.
- Power Efficiency: Some standards, like the 4-20 mA loop, can provide power to the device over the same two wires used for the signal.
Examples of Standardized Instrumentation Signals
As highlighted in the reference, common standard signals include:
- 4-20 mA DC Electric Current: A widely used standard where 4 mA typically represents the minimum measurement value (e.g., 0°C or 0% level) and 20 mA represents the maximum value (e.g., 100°C or 100% level). The signal is linear between these points.
- 3-15 PSI Air Pressure: An older pneumatic standard primarily used in hazardous areas where electrical signals posed a risk. 3 PSI usually represents the minimum, and 15 PSI the maximum value.
- Fieldbus Digital Signals: Modern digital communication protocols (like HART, Foundation Fieldbus, Profibus) that allow for two-way communication, transmitting not just the process variable but also diagnostic information, calibration data, and configuration parameters.
How the Signal is Used
Once the transmitter converts the sensor's signal into a standard format, this signal can be sent to various destinations:
- Indicating Devices: Gauges, displays, or HMIs (Human-Machine Interfaces) that show the current measured value to an operator.
- Controlling Devices: PLCs (Programmable Logic Controllers), DCSs (Distributed Control Systems), or single-loop controllers that use the signal as input to make decisions and control processes (e.g., open/close valves, start/stop pumps).
- Recording Devices: Data loggers or historians that archive the process data for analysis and compliance.
In summary, the function of a sensor-transmitter unit is to detect a physical variable using the sensor and then convert that detection into a robust, standardized signal format via the transmitter, making it usable throughout an instrumentation and control system.
| Component | Primary Role | Output Signal | Processed by | Purpose |
|---|---|---|---|---|
| Sensor | Detects a physical variable (e.g., T, P, Flow) | Raw, low-level signal | Transmitter | Measure physical condition |
| Transmitter | Converts sensor signal | Standardized signal (e.g., 4-20 mA, Fieldbus) | Indicators, Controllers | Transmit usable data to control system |
