A Comparison of Analog and Digital Gas Sensors

Analog gas sensors generate a continuous amperage or voltage that corresponds to the level of gas detected. Digital gas sensors deliver and collect strings of characters that can be interpreted by a computer or a microprocessor.

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Image Credit: GasLab

A COZIR CO2 sensor is seen in the image above, which is linked to an Arduino Mega board using an RS-232.

Introduction

“Analog is old – digital is new,” is what the general public understands about the difference between analog and digital. This has likely been learned through viewing advertisements for DVDs, computers, or digital TVs.

They are correct in that analog gas sensors have been on the market for longer than digital gas sensors. Despite this, each type still has an integral part to play in the detection of gases such as carbon dioxide, methane, oxygen, and more.

All Gas Sensors are Analog

The development of gas detection technology has been the focus of many scientists. Mr. Naoyoshi Taguchi created one of the first and most effective innovations in Japan.

He produced and commercialized an electrochemical gas sensor throughout the 1960s that would output an analog electrical signal corresponding to the amount of propane in air. Devices that use this technology continue to be called ‘TGS’ (Taguchi Gas Sensors).

The benefit of an analog electrochemical gas sensor is that generally, as the gas level becomes greater, the sensor’s electrical output, voltage or amperage, also increases.

The signal can be employed to power a voltmeter or ammeter, which displays the amount of gas detected on a dial gauge once the sensor has been calibrated.

It is important to note that even current non-dispersive infrared (NDIR) gas sensors are analog gas sensors on the inside. NDIR is the infrared absorption by gas molecules in a particular spectrum that is analyzed by a dedicated photo sensor.

The identified amount of infrared light is interpreted as either microvolts (mV) or milliamperes, and is then magnified with an amplifier circuit. An analog-to-digital (A/D, A-to-D, or ADC) converter is introduced to the circuitry of the sensor to read the analog signal.

All gas sensors are essentially analog. It is the introduction of an on-board analog-to-digital converter that makes the sensor digital.

ADC Converters Enable Digital Gas Sensors

Analog to digital converters alter the voltage of the analog or the sensor’s current into a digital output that shows the level of gas.

While the science behind the conversion of an analog signal to a digital one is complicated, the practical output is a string of letters or numbers that signify the level of gas and a computer or a microprocessor can read this.

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Image Credit: GasLab

The ADC converter and logic is included with digital gas sensors, found on a microchip on the printed circuit board.

The short distance between the ADC chip and the analog gas sensor on the board means that it is possible to make gas sensor applications with an efficient response time. The extra electronics is one of the factors why digital gas sensor systems are more costly than analog gas sensors.

Analog Signal Output

One or more of three industry standards are outputted by analog gas sensors: 4-20 milli-amperes, 0-10 volts DC, or 0-3 volts DC.

4-20 mA Analog

4-20 mA is the industry standard for applications in process control where the values of 4 and 20 milli-amperes signify 0 and 100% gas.

The sensor’s signal is transported to process instrumentation, to proportional integral derivative (PID) controllers, programmable logic controllers (PLCs), and SCADA systems. The strength of 4-20 mA is that a 0 mA output represents a fault in the sensor (no signal).

0-10 VDC Analog

0-10 v or 0-10 VDC is one of the most basic control signaling systems where 0 volts signifies no gas and 10 volts signifies a 100% gas level detected by the sensor. The HVAC industry most commonly uses the 0-10 volt standard.

The main limitation is that 0 v can be a result of either no gas or a faulty sensor. Due to the higher voltage required, the analog sensor must have more circuitry and power.

0-3 VDC Analog

Similar to 0-10 VDC, the 0-3 VDC has the same limitations but is utilized in applications where a less powerful output and input is necessary.

Digital Signal Output

RS-232 Serial

The most frequently used digital signal output is RS-232, which is the industry standard for serial data output and input. The RS-232 generates negative and positive voltages through two wires that are translated to binary 0’s and 1’s.

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Image Credit: GasLab

The receive data and transmit data wires (Rx and Tx) along with a ground wire are all that is required to deliver and receive a sensor’s digital signal.

The benefit of the ability to both transmit and receive data means that the sensor can be remotely calibrated or configured from a PC or a microprocessor.

The limitation is that only one electronic device can communicate with another over a serial cable as the data is delivered ‘serially’ from point-to-point.

RS-232 was first produced for telecommunications models and is utilized in COM ports in computers. In present-day personal computers, USB has replaced RS-232, so an additional USB to RS-232 converter is frequently employed.

As RS-232 is an industry standard, it is still commonly utilized where a point-to-point, low-speed, short-range wired data connection is sufficient.

RS-485

RS-485 is an industry standard that enables several sensors (known as slaves) to talk with a supervisory computer (the master) over one set of three wires. Modbus by default can manage up to 32 sensors on one cable, but with extra hardware signal repeaters, it can control up to 256.

The benefit of Modbus over RS-232 is that the greatest distance between the last slave on the cable and the master can be 1200 meters.

I2C

Inter-Integrated Circuit (I2C) is a digital communications technique that is now incorporated by many sensors.

Designed to decrease wiring complexity over short ranges, it is a basic synchronous system that enables several devices to share a signal over two wires similar to RS-232. The wires are known as SDA (data) and SCL (clock).

With I2C, several slaves can be connected to one master, or several masters can direct single or multiple slaves. A beneficial example of this is where sensor data can be delivered to both a memory card to store data and an LCD for instant display.

Modbus

Modbus is a kind of serial communication protocol frequently incorporated in the logic of digital gas sensors. It is an open-source that is normally used for the connection of industrial electronic devices.

Modbus does not use digital output. It performs instead as a covering around alternative types of digital data such as TCP Ethernet, RS-485, or RS-232 Serial.

By delivering and collecting digital signals encoded with Modbus commands and rules, several sensors (known as slaves) can be managed by a supervisory computer (the master) over just one set of wires or on the same network.

Which is Best for Your Application?

It is crucial to understand which analog or digital signal output the device supports before choosing a gas sensor. The majority of electrochemical sensors only output a 4-20 mA signal, but more developed sensors can output both analog and digital outputs.

GasLab recommends from experience that sensors with an RS-232 serial output are the most adaptable because they can communicate with PCs by using a USB or with Raspberry Pi or Arduino microprocessors. RS-485 and Modbus may be included with the sensor output logic or can be introduced later with affordable connectors.

This information has been sourced, reviewed and adapted from materials provided by GasLab.

For more information on this source, please visit GasLab.

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