DYNAMENT Axiom Series Infrared Gas Sensors
Product Information
Specifications
- Manufacturer: Dynament Limited
- Model: Axiom A2L Detector
- Power Supply: 5V to 24V, max 600mW
- Serial Interface: UART
- Communication Protocol: MODBUS
- Logic Levels: RS232(TTL) mode, max 5V
Product Usage Instructions
Detector Mounting
The Axiom detector should be mounted with the gas inlet facing down for normal operation.
Power Supply
The detector operates with a supply voltage between 5V and 24V.
Power is applied on the V+ and GND connections of the USB interface.
Detector Warm-up
The detector takes approximately 45 seconds to warm up after power is supplied. During this time, the output will be held at -250% of full-scale output.
Serial Communications
The detector has a UART interface for serial communication. It allows users to view gas levels, temperature, and calibrate the detector.
MODBUS Gas Reading Protocol
The sensor can be interrogated using the industry-standard MODBUS protocol when the output mode is set to RS232 or RS485 serial communications.
Logic Levels
In RS232(TTL) mode, ensure signal logic levels are under 5V to prevent damage to the detector microcontroller.
FAQ
- What is the recommended power supply for the Axiom A2L Detector?
The Axiom detector is designed to operate with a supply voltage between 5V and 24V. - How long does the detector take to warm up?
The detector takes approximately 45 seconds to warm up after power is supplied.
Dynament Limited
Hermitage Lane Industrial Estate ٠ Kings Mill Way ٠ Mansfield ٠ Nottinghamshire ٠ NG18 5ER ٠ UK. Tel: 44 (0)1623 663636
email: sales@dynament.com
www.dynament.com
Safety Information – Read First
Read and understand this manual in conjunction with the relevant data sheet before using the Axiom detector. This is not an Intrinsically Safe or Ex Flameproof device.
Axiom A2L Detector
Description
Dynament infrared detectors operate by using the NDIR (Non-Dispersive Infra Red) principle to monitor the presence of the target gas. The detector contains a long life tungsten filament infrared light source, an optical cavity into which gas diffuses, temperature compensated pyroelectric infrared detectors, an integral semiconductor temperature sensor and electronics to process the signals from the pyroelectric detector . The detector can be configured to use either digital or analogue outputs.
In analogue mode either 4-20mA or 0-5V output modes may be chosen. An Open Collector output is also available when configured in the either of the analogue modes. In digital mode, the detector can be configured to communicate over Modbus using either RS232(TTL) or RS485 as the physical protocol. For direct communications with instrument electronics.
The R454B variant of Axiom is certified to UL60335-2-40 4th Edition Annex LL.
Power Supply
The Axiom detector is designed to operate with a supply voltage of between 5V and 24V and draws a maximum of 600mW. Power is applied on the V+ and GND connections of the USB interface.
Detector Mounting
Under normal operation Axiom should be mounted with the gas inlet facing down, as shown below:
Detector warm-up
The detector takes approximately 45 seconds to warm up after power is supplied. The detector is held at -250% of the full-scale output during this time. In 4-20mA mode, the output will be forced to 2.5mA and the Open Collector Output will be active during this time. In 0.2-4.0V mode, the output will be 0V and the Open Collector output will be active during this time.
Serial communications
The detector has a UART interface allowing it to communicate with a range of devices in a serial manner. The serial interface allows the user to view gas levels, temperature, and other parameters. It also allows the detector to be calibrated. The communications protocol used is Modbus. A register map can be supplied by Dynament. Axiom can be configured to operate at 4 different baud rates – 38400, 19200, 9600 and 4800. 19200 baud is the default setting.
| Setting | Value |
| Baud Rate | 38400 |
| Data Bits | 8 |
| Stop Bits | 1 |
| Parity | None |
| Flow Control | None |
Logic levels
In RS232(TTL) mode, care must be taken to keep signal logic levels under 5V. Any higher voltage will damage the detector microcontroller.
MODBUS Gas Reading Protocol
The sensor can be interrogated (when the output mode is set to RS232 or RS485 serial communications) using the industry standard MODBUS protocol.
The simplest use of this protocol is to read the gas measurement. The message below is a MODBUS data packet that can be used request this information from a sensor. This is based on the sensor having the default MODBUS address of 1. If this is different, then the first byte of the packet would need to have the required address and the CRC at the end of the packet would need to be re-calculated for the changed data.
| Byte Position in packet | Value (hexadecimal) | Field Name | Description |
| 0 | 0x01 | MODBUS Address | MODBUS address of sensor (factory default of 1) |
| 1 | 0x04 | Function Code | Function type is “Read Input Register” (Code 0x04) |
| 2 | 0x75 | Register Address MSB | The MODBUS address of the first register holding the gas reading information. The gas reading is held in two registers at address 30,058 (0x756A) and 30,0059 (0x756B) |
| 3 | 0x6A | Register Address LSB | |
| 4 | 0x00 | Number of registers to read (MSB) | The gas reading is a 32 bit value in IEEE 754 format, which is stored across two 16 bit registers |
| 5 | 0x02 | Number of registers to read (LSB) | |
| 6 | 0xBC | CRC (LSB) | CRC of frame sent as LSB first unlike the data values within a frame (which are sent MSB first) |
| 7 | 0xDE | CRC (MSB_ |
The sensor will respond with a packet of data containing the requested gas reading. An example response from the sensor for this packet is depicted below.
| Byte Position in packet | Example Value (hexadecimal) | Field Name | Description |
| 0 | 0x01 | MODBUS Address | MODBUS address of sensor responding |
| 1 | 0x04 | Function Code | Response to a read input register request |
| 2 | 0x04 | Number of bytes of data | The total number of data bytes to follow (4) |
| 3 | 0x40 | Gas reading (MSB) | Actual gas reading as a 32 bit value in IEEE 754 floating point number. In this example the number is 0x400B851B which converts to a value of 2.18 %Volume. A value of -250.0 would indicate that the sensor is either still warming up (45 seconds after power on) or has a fault preventing a valid reading from being determined |
| 4 | 0x0B | Gas reading (Second byte) | |
| 5 | 0x85 | Gas Reading (Third byte) | |
| 6 | 0x1F | Gas Reading (LSB) | |
| 7 | 0xBC | CRC (LSB) | CRC of frame sent as LSB first unlike the data values within a frame |
| 8 | 0xDE | CRC (MSB) |
The gas reading is contained in the 4 bytes is packet positions 3, 4, 5 and 6. They are stored in Most Significant Byte (MSB) order and can be combined to form a 32 bit value, which is the gas reading in standard IEEE 754 format.
If the gas reading has a value of -250, this indicates that the sensor is either still warming up (which it typically does for 45 seconds after power on) or is in a fault condition and cannot provide a valid gas reading.
Analogue output
The analogue output of the detector is proportional to the measured gas level. The analogue output can be configured to be a 4-20mA or a 0.2-4.0V configuration. This configuration is carried out during manufacture and cannot be changed afterwards.
4-20mA Mode
The analogue output range is 4mA to 20mA for zero gas to FSD. Fault modes are indicated by a mA output of 2.5mA. The table below shows the analogue output for a detector that is configured in 4-20mA mode.
| Gas Reading (% of detector range) | Analogue output (mA) |
| 0 | 4.00 |
| 10 | 5.60 |
| 20 | 7.20 |
| 30 | 8.80 |
| 40 | 10.40 |
| 50 | 12.00 |
| 60 | 13.60 |
| 70 | 15.20 |
| 80 | 16.80 |
| 90 | 18.40 |
| 100 | 20.00 |
| Warmup / Fault | 2.50 |
0.2-4.0V Mode
The analogue output range is 0.2V to 4V for zero gas to FSD.
The table below shows the analogue output for a detector that is configured in 0.2-4.0V mode.
| Gas Reading (% of detector range) | Analogue output (V) |
| 0 | 0.20 |
| 10 | 0.40 |
| 20 | 0.80 |
| 30 | 1.20 |
| 40 | 1.60 |
| 50 | 2.00 |
| 60 | 2.40 |
| 70 | 2.80 |
| 80 | 3.20 |
| 90 | 3.60 |
| 100 | 4.00 |
| Warmup / Fault | 0V |
Response Times
Axiom conforms to the UL60335-2-4, annex LL specification for response time with a recorded value of <15s
Fault indication
The analogue output is driven to the fault level when an internal fault is detected.
Voltage output
The 0.2-4.0V output will be driven to 0V in the event of a fault.
4-20mA
The 4-20mA output will be driven to 2.5mA in the event of a fault.
Serial Data
The gas readings in the live data are set to -250% of the full-scale during fault conditions. The status flags are set according to the actual fault. Each of the 4 Status flag registers are 16 bits long and work as bit fields. This means that each of the 16 bits indicates a different fault. Status Flag 4 is inverted because during EEPROM initialisation the memory is set to 0xFFFF which would cause all status flags to raise.
| Status Flags 1 | Bit Mask | Fault Description |
| Signal Noise | 0000 0000 0000 0100 | Checks that the raw count is stable. Signal will be considered noisy if noise appears for more than 5 sec. |
| Detector 1 Fault | 0000 0000 0100 0000 | This flag indicated there is a fault with detector1. It gets triggered when a peak cannot be found. This typically happens along with FLAG_SIGNAL_TIMEOUT. |
| Reference Fault | 0000 0000 1000 0000 | This flag indicated there is a fault with reference1. It gets triggered when a peak cannot be found. This typically happens along with FLAG_SIGNAL_TIMEOUT. |
| ADC Overrange | 0000 0010 0000 0000 | This flag is set when the ADC reads a value that is too high; this occurs before it hits the maximum value. This does not just happen because of signal spikes, normally that is filtered out. What typically causes this, is when there is a significant change in the detector environment (for example if you blow on the detector). This increases the dc bias of the signal which pushes the signal to the upper limits of the ADC and clips the signal. |
| Status Flags 2 | Bit Mask | Fault Description |
| Warm Up | 0000 0000 0100 0000 | This flag will tell the user that the detector has warmed up. By default, this is set to 45 seconds, except for an extreme environment, the detector should be warm by then. |
| Max Gas Limit Exceeded | 0000 0001 0000 0000 | This flag indicates that the gas has exceeded the full scale of the detector. This error can be ignored. |
Calibration
Axiom A2L detectors do not require field calibration.
Pressure dependency
NDIR gas detectors exhibit a dependency on the pressure conditions under which they operate. The principle of operation is based upon the absorption of infrared energy by molecules of the target gas within the detector. As the gas pressure in increased, the number of molecules within the detector is increased. This leads to increased absorption, and so the calculated gas reading is increased. The opposite effect is true when the gas pressure is reduced. Axiom detectors are pre-calibrated at nominal atmospheric pressure, and the accuracy of the reading will be adequate in most instances without the need for pressure compensation.
In cases where the detectors are operated at pressures significantly different from their original calibration pressure, for example high altitudes, the detectors should be recalibrated to restore accuracy. Alternatively, a compensation factor can be applied to the reading based on the pressure within the detector. For example, when the detector is used at either elevated or reduced pressures, a pressure transducer can be incorporated into the gas flow and its output value can be used to determine the degree compensation to be applied. Please contact Dynament for more information.
Maintenance
Dynament infrared detectors, like any other detectors, require regular maintenance in terms of inspection to prevent a build-up of dust or dirt or any other contamination that might prevent gas from entering the detector. Maintenance should include cleaning of the housing within which the detector is mounted, a good design will afford some level of protection for the bare detector.
Handling precautions
Axiom incorporates ESD protection circuitry. However, it is recommended that care is taken not to touch the USB-C connector during handling. Axiom should not be exposed to corrosive gases.
Warranty
Infrared detector warranty and lifetime are given in the detector data sheet, a typical statement is as follows:
All Dynament Axiom detectors carry a five-year warranty against defects in materials and workmanship. The warranty is invalidated if the detectors are used under conditions other than those specified in the relevant data sheet.
Particular attention should be paid to the following criteria:
- Observe the correct supply polarity
- Do not exceed the maximum rated supply voltage of 24V
- Do not solder directly to the detector connector
- Do not expose the detector to corrosive gases
- Do not allow condensation to take place within the detector
Decommissioning
Detectors can be returned to Dynament for safe disposal.
Dynament
Hermitage Lane Industrial Estate, Kings Mill Way
Mansfield
Nottinghamshire
NG18 5ER
UK.
www.dynament.com
Appendix
Wiring Guide
Axiom can be factory configured in 4 different output modes:
When a bare ended USB cable used, the contacts follow the colour code below:
| Colour | Axiom Contact |
 Red |
5-24V DC |
 White |
D- |
 Green |
D+ |
 Black |
0V |
| Connection | Function |
| +V | 5.0 – 24V Input |
| D+ | 4-20mA Output with fault indication (2.0mA = Fault) 100R Recommended Load Resistor |
| D- | Open Collector Output Active Low 100mA maximum load 5.5V maximum voltage |
| GND | System 0V |
Recommended Output Circuits
| Connection | Function |
| +V | 5.0 – 24V Input |
| D+ | 0.2-4.0V Output (0.1V minimum) |
| D- | Open Collector Output Active Low 100mA maximum load 5.5V maximum voltage |
| GND | System 0V |
Recommended Output Circuit
| Connection | Function |
| +V | 5.0 – 24V Input |
| D+ | RS485 A |
| D- | RS485 B |
| GND | System 0V |
Recommended Output Circuit
| Connection | Function |
| +V | 5.0 – 24V Input |
| D+ | RS232 Tx 3.3V TTL |
| D- | RS232 Rx 3.3V TTL |
| GND | System 0V |
Recommended Output Circuit
Using the Open Collector Output to Drive a Relay
Axiom’s Open Collector output can be used to drive a relay when connected as shown below. To do this, use the connect the relay coil between the Open Collector output (D-) and a 5V supply. On alarm or fault the Open collector output will be driven low, completing the circuit through the coil, and energising the relay.
Please note that the maximum permitted voltage on the Open Collector contact is 5.5V and the maximum current that can be sunk into the contact is 100mA. For applications that require switching of a 24V load or higher currents, Dynament can provide an Axiom Bridge accessory.
Axiom Bridge
Dynament offer an optional interface board called Axiom Bridge. Axiom Bridge features a USB-A connector and screw terminals. is provided with a USB-C to USB-A cable to simplify integration into existing systems. Furthermore, the board features an Open Collector output capable of switching loads up to 40V and 600mW. Axiom bridge can be provided with an optional on-board 10A relay. The board also features two LED status indicators for power and fault indication.
Documents / Resources
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DYNAMENT Axiom Series Infrared Gas Sensors [pdf] Instruction Manual Axiom Series, Axiom Series Infrared Gas Sensors, Infrared Gas Sensors, Gas Sensors, Sensors |
