MHZ19

Project MHZ19
Some research into the MH-Z19 CO2 sensor
Status	Initializing
Contact	bertrik
Last Update	2016-10-26

Introduction

This page is about the MH-Z19 CO₂ sensor and some experiments done with it.

This sensor gives a digital (serial) output of the CO₂ concentration in air, in parts-per-million (ppm). It uses the optical measurement principle of measuring CO₂, which should be much more accurate than the inexpensive electro-chemical sensors you can find. As far as I know, the optical measurement principle uses a broadband light-source to send some light through an air-sample. The sensor then looks at the relative intensity of the light at two different frequencies. The CO₂ gas inside the air absorbs light strongly at very specific wavelengths, allowing a determination of the concentration (ppm) of CO₂. This is then compensated for temperature (and pressure?) for increased accuracy. See also wikipedia for this measurement principle. A new measurement is started every 5 seconds, you can actually see a small amount of light coming out of the sensor while it's measuring.

The MH-Z19 is the cheapest optical CO₂ sensor I could find on AliExpress, about E22,-.

Status

The sensor works as intended.

Next steps:

• play a bit with the various MH-Z19B commands and document what works and what doesn't
• in particular, look at the alarm output (officially this output is not supported) and try to find how to set the alarm limit setting, this output seems to toggle low/high on reboot of the sensor

Hardware and reference data

See the manufacturer MH-Z19 page.

Software

See this github repo for code using this sensor with an ESP8266 board (WeMos D1 mini). It publishes the CO₂ concentration to topic "bertrik/co2" on test.mosquitto.org every 5 seconds.

It seems that support for this sensor was recently added to ESPEasy.

Command 0x86 response frame

Command 0x86 is the command to send to just read out the most recent ppm value.

A response to command 0x86 typically looks like this:

0xFF CM HH LL TT SS Uh Ul CS

where

• CM is the command repeated back
• HH/LL is the CO₂ ppm value, high/low part
• TT is the temperature in degrees Celcius, plus 40. For example, when temperature is 25 deg C, then TT = 0x41
• SS is some kind of status byte, this byte always has only one bit set!
• Uh/Ul is some unknown value, perhaps related to pressure? After booting the sensor, it starts out at 15000 exactly, then typically settles to about 10500.
• Cs is the checksum

Values TT, SS and Uh/Ul are undocumented.

Command set

Normally, this sensor is read out using a command/response sequence over serial (9600,8N1). The following commands are known:

• 0x86: gas concentration reading
• 0x87: calibrate zero point
• 0x88: calibrate span point

The MH-Z19B datasheet additionally mentions to following commands:

• 0x79: ABC logic on/off
• 0x99: Sensor detection range setting, this command can be used to set the measurement range (e.g. 0-2000ppm or 0-5000ppm)

It appears that these commands work on the MH-Z19 too, although with a little different command layout.

command 0x99 (range)

According to the MH-Z19B datasheet, you can configure the measurement range by putting the desired range in byte 3 and 4. However, unlike what the MH-Z19B datasheet says, you can set the range using the following command (in this case 0x07d0 = 2000 ppm in byte 6 and 7):

0xFF 0x01 0x99 0x00 0x00 0x00 0x07 0xD0 0x8F
          <cmd>               ^-range-^

Log of MH-Z19 response at startup

Below is a log of the sensor response to the 0x86 measurement command while starting up. The first couple of measurement seem to be invalid.

     HH LL TT SS U1 U2
RAW: 00 80 47 01 3A 98
RAW: 07 D0 47 01 3A 98 -> 0x7d0 = 2000 decimal, this is the configured measurement range of the sensor with command 0x99
RAW: 00 05 47 01 3A 98
RAW: 01 2D 47 01 3A 98
RAW: 28 97 47 01 3A 98
RAW: 28 97 47 01 3A 98
RAW: 28 97 47 01 3A 98
RAW: 28 97 47 01 3A 98
RAW: 28 97 47 01 3A 98
RAW: 28 97 47 01 3A 98
RAW: 28 97 47 01 3A 98
RAW: 28 97 47 01 3A 98
RAW: 28 97 47 01 3A 98
RAW: 28 97 47 01 3A 98
RAW: 28 97 47 01 3A 98
RAW: 28 97 47 40 3A 98
RAW: 28 97 47 40 3A 98
RAW: 03 84 47 40 2B 43 -> first sample with U different from 0x3a98 (15000 decimal)
RAW: 03 85 47 40 2B 19
RAW: 03 86 47 40 2A F9
RAW: 03 87 47 40 2A E1 -> CO2 = 903 ppm, T = 31 deg C, Status = 0x40, Unknown = 10977

The first measurement shows a ppm value of 128, a temperature of 31 degrees C, a "status" byte of 01 and the "unknown" value of 0x3a98 (= 15000). The second measurement shows a high ppm value of 2000 ppm (the max value within the ppm range). The third measurement shows a low ppm value of 5 ppm. The fourth measurement shows a ppm value of 301 ppm. The fifth measurement shows a very high ppm value of 10391. The final measurement shows a realistic indoors ppm value of 0x387 = 903 ppm. The "unknown value" (byte 4/5) typically settles down to 10500 or so.

So, it takes some time before the measurement stabilizes, proposed heuristic for a valid reading:

• "status byte" has to be 0x40
• "unknown value" has to be lower than 15000