CO2 and indoor air quality monitoring with Modbus and MQTT loggers

Indoor air quality monitoring usually starts with CO2, because it shows how well a room is ventilated for the number of people in it. Particles, volatile organic compounds and carbon monoxide add information that CO2 alone does not give. This article explains what each parameter tells you, how to choose between the ICP DAS CL series data loggers, which interfaces and protocols each model offers, and how to bring the values into a building management system (BMS) or SCADA over Modbus or MQTT.

What each parameter tells you

• CO2 rises with occupancy and falls with ventilation, so it is the usual input for demand-controlled ventilation. The CL series uses an NDIR sensor with a range of 0 to 9,999 ppm and an accuracy of ±40 ppm ±3 % of reading.
• PM1, PM2.5 and PM10 show particles from outdoor air, processes or poor filtration. The CL-220 group uses a laser sensor with a range of 0 to 1,000 µg/m³. The CL-210 to CL-213 models measure PM2.5 only, from 0 to 400 µg/m³.
• TVOC is a sum signal from a metal-oxide sensor (0 to 60,000 ppb, ±15 %). It does not identify compounds, but shows trends after renovation, cleaning or changes in use. The CL-204 and CL-207 add an electrochemical formaldehyde (HCHO) sensor.
• CO indicates combustion sources, for example in parking garages and loading areas. The CL series uses an electrochemical sensor with a range of 0 to 1,000 ppm.
• Temperature, humidity and dew point are measured by every CL model and help with comfort and condensation risk.

The logger measures; it does not decide what is acceptable. Set alarm and control thresholds from the guidelines or standards that apply to your building and its use.

Choosing a CL series model

All models below also measure temperature, humidity and dew point. Each comes as an -E version (RS-485 and Ethernet with PoE) and as a -WF version that adds Wi-Fi.

For other gases, the CL-205-E measures NH3, the CL-206-E H2S and the CL-250-E O2.

Interfaces and protocols per model

• -E models have one RS-485 port with DCON and Modbus RTU, and one Ethernet port with Modbus TCP and MQTT. They can be powered by PoE or from a terminal block (+12 to +48 VDC; +10 to +30 VDC for the CL-250-E).
• -WF models add IEEE 802.11b/g/n Wi-Fi in infrastructure or limited AP mode. Modbus TCP works over Ethernet and Wi-Fi, but MQTT is supported on the Ethernet interface only. They have redundant power inputs (PoE, IEEE 802.3af Class 1, and DC) and an IP filter whitelist with web password.
• All models store up to 450,000 records with date and time stamps and have a real-time clock. The iAir Utility configures the loggers, groups them, shows live values and downloads logs as CSV files.
• Each model has one Form A relay output rated 16 A at 30 VDC or 250 VAC, intended for an alarm or for controlling IAQ equipment.

If a location only has Wi-Fi and you need MQTT, poll the -WF logger over Modbus TCP and let a controller publish the data.

Bringing indoor air quality monitoring into BMS and SCADA

• Modbus TCP: SCADA and HMI systems poll the loggers directly over Ethernet or Wi-Fi.
• Modbus RTU: several loggers share one RS-485 line to a PLC, or to a tGW-715 gateway that makes them reachable over Modbus TCP.
• BACnet/IP: the GW-2493M acts as a BACnet/IP server and Modbus TCP client for up to 32 stations, mapping registers to BACnet objects. The GW-2492M does the same with Modbus RTU devices.
• MQTT: the loggers publish to a broker that feeds an IoT platform or historian.
• Edge controller: the WISE-5231 reads CL modules, logs data, runs IF-THEN-ELSE rules, sends e-mail and forwards values over MQTT or to IoTstar.

Worked example: meeting rooms on an office floor

Eight meeting rooms each get a CL-202-E on the ceiling, powered by PoE from the floor switch. The open-plan area gets a CL-222-E for CO2 and particles. The BMS uses BACnet/IP, so a GW-2493M polls the nine loggers over Modbus TCP and maps CO2, temperature and humidity to BACnet objects, well within its limit of 32 stations. The ventilation controller uses the CO2 values for demand control. In each meeting room, the logger relay is wired as a local backup that starts a boost fan if CO2 stays above its set point. Monthly logs are downloaded with the iAir Utility for reporting.

Installation and maintenance

• CL loggers mount on a ceiling or wall, have an IP20 housing and operate from 0 to 50 °C at 10 to 90 % RH, non-condensing. They are for indoor rooms, not wet areas.
• Place them in the occupied zone, away from doors, windows and supply air outlets.
• Allow for warm-up after power-on: 5 minutes for CO2 and CO sensors, 180 seconds for TVOC and HCHO.
• On the CL-220 group, the particle sensor has a stated life of 5 years and the user replaces the filter patch (FLT-C002). FLT-C001 patches are listed as accessories for the CL-210 to CL-213.
• Stated lifetimes on the CL-221 to CL-223 are 15 years for the CO2 sensor and 5 years for the CO sensor.

Which ICP DAS products fit

The CL-200 series loggers cover the measurements, with -E versions for wired PoE installations and -WF versions where Wi-Fi is easier. tGW-700 gateways link RS-485 lines to Modbus TCP, the GW-2492M and GW-2493M connect Modbus devices to BACnet/IP, and WISE edge controllers with IoTstar handle logging, rules and central data storage.

Send us the rooms, parameters and target system you have in mind, and our technical sales team will suggest models and an integration path.