CO2 and AQ values on AQ-N

I’m using several AQ-N via Modbus for projects with my students.
Register 116 (humidity) and 119 (temperature) provide correct values. But register 108 (co2 value (ppm)) always contains the value 0.
Is there something wrong ? Should register 108 contain the CO2 ppm value ?
What are the possible values for register 184 (AQ value) ?
Thank you.

Its possible that the C02 sensor is not populated, there is a flag to check that using a modbus register which you can see using T3000 → tools → registerviewer. It’ll be quicker to just use our T3000 application and have a look from the UI, if the C02 sensor is not present the value box will be greyed out. You could also check your original order, the part number will have the -Co2 option if that was selected when you ordered.

My product is an AQ-N and there is no -Co2 option for AQ-N.
I’m using the Register Viewer tool to see all the register values.

OK, so the C02 option wasn’t selected when you ordered the unit. It’s a fairly expensive little sensor otherwise we would include it by default.


But the Co2 option is not available when ordering the AQ-N unit !

But is there a sensor (Co2 or air quality ?) in the AQ-N model since the modbus 114 register allows you to know the air quality level (good, fair or poor) ?

So, for the AQ-N model :

  • is modbus register 108 (co2 value (ppm)) usable or not ?
  • is modbus register 184 (AQ value) usable or not ? If yes, what are the possible values of this register ?

Thank you.

I don’t know the models and register numbers off hand but you can use the t3000 application to find out, go to tools, register viewer and there you can see all the registers, some descriptive text, the data structure and the current value.

Maurice Duteau

I also have an AQ-N unit and I had similar questions.

  • Modbus register 108 always reads 0 in my case and is unusable. I believe it was meant to report the CO2 sensor reading, if the board had one.
  • Modbus register 184 is usable and contains the decoded AQ value from the AQ sensor. When @maurice replied to my question, he kindly provided a link to the AQ sensor model/datasheet. Please see here: AQ-N-LCD - various questions - #2 by maurice. The datasheet contains a detailed explanation of what the value represents and how it’s determined.


I will get our team to add some notes about this in the documentation. Thanks All.