CO and SO2 sensors giving incorrect reading

nzagorec
Posts: 19
Joined: Tue Feb 20, 2018 9:06 am
Company: Verso Altima Group

Re: CO and SO2 sensors giving incorrect reading

Post by nzagorec » Fri Nov 09, 2018 9:30 am

Here it is:

Code: Select all

H#
Electrochemical gas sensor example
[GASES PRO] ON function
[GASES PRO] Sensor power 3V3 to ON
[GASES PRO] Checksum generated OK
[GASES PRO] ******************************************************
[GASES PRO] SOCKET -> B
[GASES PRO] SERIAL NUMBER: 132590039 018
[GASES PRO] AFE VER: 3
[GASES PRO] SENSOR TYPE: CO-4E
[GASES PRO] SENSITIVITY: 336.700 nA/ppm
[GASES PRO] BASELINE: -48.200 nA
[GASES PRO] O3-NO2 SENSITIVITY: 0.000 nA/ppm
[GASES PRO] AUX BASELINE: 20.200 nA
[GASES PRO] WORK ADJ: 0.000 | 1.0000000000
[GASES PRO] AUXL ADJ: 0.000 | 1.0000000000
[GASES PRO] WORK OFFSET: 0.046
[GASES PRO] AUX OFFSET: 3172.455
[GASES PRO] ZERO STATUS: 3C
[GASES PRO] ******************************************************
[GASES PRO] AFE Configured

nzagorec
Posts: 19
Joined: Tue Feb 20, 2018 9:06 am
Company: Verso Altima Group

Re: CO and SO2 sensors giving incorrect reading

Post by nzagorec » Mon Nov 12, 2018 4:12 pm

Any update on this?

libelium-dev
Posts: 27967
Joined: Mon Sep 28, 2009 1:06 pm

Re: CO and SO2 sensors giving incorrect reading

Post by libelium-dev » Mon Nov 12, 2018 6:01 pm

Hi,

We are going to perform a test generating CO gas but trying not to vary temperature and humidity.

We will let you know as soon as we will have a conclusion.

Regards

libelium-dev
Posts: 27967
Joined: Mon Sep 28, 2009 1:06 pm

Re: CO and SO2 sensors giving incorrect reading

Post by libelium-dev » Thu Nov 15, 2018 9:30 am

Hi,

We continue doing tests on the CO sensor, it seems that there is a bug in the library or calibration parameters, we need a couple of days more to debug the problem. For now we advise you to use the API library v028 which seems to not have this problem.
http://downloads.libelium.com/waspmote-pro-api-v028.zip

We will let you know when we solve the problem. Apologies for the inconvenience.
Regards

libelium-dev
Posts: 27967
Joined: Mon Sep 28, 2009 1:06 pm

Re: CO and SO2 sensors giving incorrect reading

Post by libelium-dev » Fri Nov 16, 2018 11:17 am

Hi,

We have found the problem, there was a bug in the library of the AFE. Please download the following file and replace the MCP3421.cpp file in the API with this one
http://www.libelium.com/forum/libelium_ ... CP3421.zip

Now you can use the latest API version but with the above library.

Apologies for the inconveniences.
Regards

nzagorec
Posts: 19
Joined: Tue Feb 20, 2018 9:06 am
Company: Verso Altima Group

Re: CO and SO2 sensors giving incorrect reading

Post by nzagorec » Fri Nov 16, 2018 2:55 pm

Thank you. We're going to perform tests on our side to confirm that it's indeed working correctly.

libelium-dev
Posts: 27967
Joined: Mon Sep 28, 2009 1:06 pm

Re: CO and SO2 sensors giving incorrect reading

Post by libelium-dev » Mon Nov 19, 2018 8:46 am

Let us know if you find any problem.
Regards

nzagorec
Posts: 19
Joined: Tue Feb 20, 2018 9:06 am
Company: Verso Altima Group

Re: CO and SO2 sensors giving incorrect reading

Post by nzagorec » Tue Nov 27, 2018 3:26 pm

Hi,

Just a follow on up this:

CO sensor now works properly. We tested it in both low and high concentration areas and value goes up and down accordingly.

The problem we're seeing now is in NO2 values. We have 15 of these probes and we tested 5-6 of them. These are the results we're seeing:
  • 3 probes are giving us 0.000000 ug/m3 on every measurement, both after 2 min and after 2 hours, without powering the probe off when going to sleep (ALL_ON is used). We tested them in office, outside and in garage on floor -2 where at the same time CO increased from ~800 ug/m3 to ~2300 ug/m3 (3 times).
  • One probe mounted outside is giving us ~650 ug/m3 (definitely too high, juding by this document this falls into VERY HIGH category).
  • One probe is giving us ~1250 ug/m3 in identical environment as one before.
Air quality stations in our area show that NO2 concentration during the day is usually between 5 and 30 ug/m3 so all these positive values are waaay too high.

PS. SO2 values also look strange. One device (2nd bullet from before) gives us ~300 ug/m3 and other (3rd bullet from before) gives 30-100 ug/m3. Air quality station in our area shows daily values in range 2 to 20 ug/m3.

After fixing a bug in MCP3421.cpp can you confirm that you've tested that all electrochemical sensors work correctly, not just CO?
Do you have any idea why are we getting so inconsistent readings from sensors placed next to each other?
Any idea why some sensors consistently give zero instead of a positive value, where concentrations should increase?

Thank you!

nzagorec
Posts: 19
Joined: Tue Feb 20, 2018 9:06 am
Company: Verso Altima Group

Re: CO and SO2 sensors giving incorrect reading

Post by nzagorec » Wed Nov 28, 2018 1:46 pm

Hi,

Today we performed a test with SE-PRO placed 3 feet from the national air quality station: https://i.imgur.com/SkFsAv4.jpg

Program was run continuously for 1 hour. Sensors were never powered off, nor have we ever entered any kind of sleep. After 1 hour of test these were the measured values:

Code: Select all

Temperature: 3.4400000572 Celsius degrees
Pressure: 101142.0000000000 Pa
RH: 52.8857421875 %
O3: 147.8027496337 ug/m3
CO: 451.0811767578 ug/m3
NO2: 669.8657836914 ug/m3
SO2: 286.8089904785 ug/m3
PM1: 6.7500000000 ug/m3
PM2.5: 7.5899996757 ug/m3
PM10: 13.4299993515 ug/m3
These are the values reported by the air quality station at the same location and at the same time (https://zrak.imi.hr/Zagreb1.aspx):

Code: Select all

CO: 300 ug/m3
NO2: 43.7 ug/m3
SO2: 11.9 ug/m3
PM10: 10.2 ug/m3
As you can see CO and PM10 are similar but NO2 and SO2 are 15/25 times larger!

Even if we take sensor's poor accuracy of ±0.1 ppm into account we're still getting NO2 results that are way off.

O3 is unfortunatelly not measured on this station but another one, located 4.4 km away, measured 51 ug/m3 at that time (3 times less then we did). Daily average for O3 is around 30 ug/m3.

If there's no error in code, how can these sensors be used for monitoring air quality if they're giving values that are 3-25 times larger? We can't use any CAQI calculation with these values because it shows critical air quality on every location.

Thank you.

Code we used for testing: https://pastebin.com/cfg960We
Test results during 1h of operation: https://pastebin.com/1j4sK4DY

libelium-dev
Posts: 27967
Joined: Mon Sep 28, 2009 1:06 pm

Re: CO and SO2 sensors giving incorrect reading

Post by libelium-dev » Thu Nov 29, 2018 12:05 pm

Hi,

Thanks for the detailed explanation and the effort with the tests.

We think that the sensors are working according to their specifications. It isn't possible to get data similar to an official AQI station since the accuracy of the sensors is +-0.1 ppm in ideal conditions. Accuracy values have been calculated at average conditions: 20ºC / 101300 Pa. Accuracy levels may differ with different temperature and pressure levels, aging and in presence of third type of gases which cause cross sensitivity. You can find more information about expected values here (on page 41, point 4)
http://www.libelium.com/downloads/docum ... df#page=41

Regards

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