As a trusted supplier of gas transmitters, I've had numerous inquiries about the output signals of these devices. Understanding the output signals is crucial as it determines how the gas transmitters communicate the detected gas information to other systems, such as control panels, alarms, or data loggers. In this blog, I'll delve into the different types of output signals commonly found in gas transmitters, their characteristics, and applications.
Analog Output Signals
Analog output signals are continuous signals that represent the measured gas concentration as a varying electrical parameter, typically voltage or current. The most common analog output signals in gas transmitters are 4 - 20 mA and 0 - 5 V or 0 - 10 V.
4 - 20 mA
The 4 - 20 mA output signal is widely used in industrial applications. It has several advantages that make it a popular choice. Firstly, it is a current - loop signal, which means it is less susceptible to electrical interference compared to voltage signals. This makes it suitable for long - distance transmission, often over hundreds of meters without significant signal degradation.
The 4 mA value represents the zero gas concentration, and the 20 mA value represents the full - scale gas concentration. For example, if a gas transmitter is calibrated to detect methane in the range of 0 - 100% LEL (Lower Explosive Limit), 4 mA would correspond to 0% LEL, and 20 mA would correspond to 100% LEL. Any gas concentration between 0 and 100% LEL would result in a current value between 4 and 20 mA.
This type of output signal is commonly used in safety - critical applications, such as in oil and gas refineries, chemical plants, and mines. For instance, in an oil refinery, the 4 - 20 mA output from a gas transmitter can be connected to a central control panel, which can then trigger alarms or shutdown procedures if the gas concentration exceeds a certain threshold.
0 - 5 V or 0 - 10 V
Voltage output signals, such as 0 - 5 V or 0 - 10 V, are also used in gas transmitters. These signals are relatively easy to interface with other electronic devices, such as data acquisition systems or microcontrollers. The voltage value is directly proportional to the gas concentration. For example, with a 0 - 5 V output, 0 V represents 0 gas concentration, and 5 V represents the full - scale gas concentration.
However, voltage signals are more susceptible to electrical interference and signal attenuation over long distances compared to 4 - 20 mA signals. Therefore, they are more commonly used in applications where the distance between the gas transmitter and the receiving device is relatively short, such as in laboratory environments or small - scale industrial setups.
Digital Output Signals
Digital output signals are discrete signals that represent the gas concentration in a digital format. They offer several advantages over analog signals, including higher accuracy, better noise immunity, and the ability to transmit additional information.
Modbus
Modbus is a widely used digital communication protocol in the industrial automation field. Gas transmitters with Modbus output can communicate with other Modbus - compatible devices, such as programmable logic controllers (PLCs) or human - machine interfaces (HMIs).
Modbus uses a master - slave communication model. The gas transmitter acts as a slave device, and the control system or other master device can request data from the transmitter. The data can include the gas concentration, sensor status, and calibration information. Modbus communication can be either serial (e.g., Modbus RTU) or Ethernet - based (e.g., Modbus TCP).
This type of output is beneficial in complex industrial systems where multiple gas transmitters need to be integrated into a single control network. For example, in a large chemical plant, Modbus - enabled gas transmitters can be connected to a central PLC, which can then monitor and control the gas detection system across the entire facility.
HART (Highway Addressable Remote Transducer)
HART is another digital communication protocol that combines analog and digital communication. A gas transmitter with HART output can send a 4 - 20 mA analog signal while simultaneously transmitting digital data over the same two - wire loop.
The digital data can include additional information such as sensor diagnostics, calibration history, and advanced gas concentration data. HART communication allows for remote configuration and monitoring of the gas transmitter. This is particularly useful in applications where it is difficult or dangerous to access the gas transmitter directly, such as in high - altitude or hazardous areas.
Relay Output Signals
Relay output signals are used to control external devices, such as alarms, fans, or shutdown valves. A gas transmitter with relay output can be configured to activate the relay when the gas concentration exceeds a certain setpoint.
There are typically two types of relay outputs: normally open (NO) and normally closed (NC). In a normally open relay, the circuit is open when the gas concentration is below the setpoint and closes when the gas concentration exceeds the setpoint. Conversely, in a normally closed relay, the circuit is closed when the gas concentration is below the setpoint and opens when the gas concentration exceeds the setpoint.
Relay outputs are commonly used for simple on - off control applications. For example, in a confined space, a gas transmitter with a relay output can be connected to a ventilation fan. When the gas concentration exceeds the setpoint, the relay closes, and the fan is turned on to ventilate the space.
Applications and Considerations
The choice of output signal depends on the specific application requirements. For safety - critical applications where long - distance transmission and high reliability are essential, 4 - 20 mA analog output or digital protocols like Modbus are often preferred. In laboratory or small - scale applications where ease of integration and short - distance communication are key, voltage output or HART may be more suitable.
At our company, we offer a wide range of gas transmitters with different output signals to meet various application needs. For example, our Gas Transmitters GWD31E are available with multiple output options, including 4 - 20 mA, Modbus, and relay outputs. Our Point Type Combustible Fixed Gas Detector GWD31 also provides flexible output choices to ensure seamless integration into different systems.
If you're in the process of selecting a gas transmitter for your project, it's important to consider factors such as the type of gas to be detected, the required measurement range, the environmental conditions, and the compatibility with your existing control system. Our team of experts is always ready to assist you in making the right choice.
Conclusion
In summary, the output signals of gas transmitters play a vital role in communicating the detected gas information to other systems. Whether it's analog, digital, or relay output, each type has its own characteristics and applications. Understanding these output signals will help you select the most appropriate gas transmitter for your specific needs.
If you have any questions about our gas transmitters or need further assistance in choosing the right output signal for your application, please feel free to contact us. We look forward to discussing your requirements and providing you with the best gas detection solutions.
References
- "Industrial Gas Detection Handbook", Second Edition, Elsevier
- "Modbus Protocol Specification", Schneider Electric
- "HART Communication Protocol Specification", HART Communication Foundation