MHZ19: Difference between revisions

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=== Command 0x86 response frame ===
=== 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:
A response to command 0x86 typically looks like this:
<pre>
<pre>
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* TT is the temperature in degrees Celcius, plus 40
* TT is the temperature in degrees Celcius, plus 40
* SS is some kind of status byte, this byte always has only one bit set!
* SS is some kind of status byte, this byte always has only one bit set!
* U1/U2 is some unknown value, perhaps related to pressure?
* U1/U2 is some unknown value, perhaps related to pressure? After booting the sensor, it starts out at 15000 exactly, then typically settles to about 10500.
 
=== Extended commands ===
Normally, this sensor is read out using a command/response sequence over serial.


It appears that there are some additional command beyond just reading the CO<sub>2</sub> concentration, these are mentioned in the MH-Z19B datasheet but not in the MH-Z19 datasheet:
=== Command set ===
* some calibration commands, e.g. to let the sensor know when it's breathing pure nitrogen (0 ppm), or some reference gas with a known CO<sub>2</sub> concentration (e.g. 400 ppm)
Normally, this sensor is read out using a command/response sequence over serial (9600,8N1).
* command 0x79 (ABC logic on/off), to enable/disable the ABC-algorithm (automatic baseline correction)
The following commands are known:
* command 0x99 (sensor detection range setting), to set the measurement range of the sensor in steps of 1000 ppm
* 0x86: gas concentration reading
* 0x87: calibrate zero point
* 0x88: calibrate span point


Some of these commands appear in the MHZ-19<bold>B</bold> datasheet, but appear to work a little differently for the MH-Z19.
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) ====
==== command 0x99 (range) ====
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The 0x07 0xD0 bytes (index 6 and 7) set the range to 2000 ppm.
The 0x07 0xD0 bytes (index 6 and 7) set the range to 2000 ppm.


=== Log of response at startup ===
=== Log of MH-Z19 response at startup ===
Below is a log of the sensor response to the 0x86 measurement command while starting up.
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.
The first couple of measurement seem to be invalid.

Revision as of 17:11, 22 October 2016

Project MHZ19
Mhz19.jpg
Some research into the MH-Z19 CO2 sensor
Status Initializing
Contact bertrik
Last Update 2016-10-22

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,-.

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: 

HH LL TT SS U1 U2

where

  • HH/LL is the CO2 ppm value
  • TT is the temperature in degrees Celcius, plus 40
  • SS is some kind of status byte, this byte always has only one bit set!
  • U1/U2 is some unknown value, perhaps related to pressure? After booting the sensor, it starts out at 15000 exactly, then typically settles to about 10500.

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)

Unlike what the MH-Z19B datasheet says, you can set the range using the following command (2000 ppm in this case): 

0xFF 0x01 0x99 0x00 0x00 0x00 0x07 0xD0 0x8F

The 0x07 0xD0 bytes (index 6 and 7) set the range to 2000 ppm.

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. 

RAW: 00 80 47 01 3A 98
RAW: 07 D0 47 01 3A 98
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
RAW: 03 85 47 40 2B 19
RAW: 03 86 47 40 2A F9
RAW: 03 87 47 40 2A E1

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
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