Ongoing Usage & Tips

Once you have got started, what else can you do with your EDT and AQGateway?

How do I calibrate my EDT?

The gas sensor in the EDT is shipped pre-calibrated and three calibration
constants are stored in the AQ Gateway cloud for your specific EDT. You can view
these calibration constants on the AQ Gateway website: from the Home page
select My EDTs and then select the EDT you want to check. The EDT INFO window
appears and the three calibration constants are in the bottom right corner.

The electrochemical gas sensor in each EDT has one measuring electrode and
one reference electrode, called the Working Electrode (WE) and Auxiliary Electrode
(AE). The three calibration constant are their Zero Baseline currents and the WE’s
sensitivity.

You can modify the WE and AE when in the EDT INFO window: the
Update Baseline Calibrations box is below the Calibration constants.
The AQ Gateway also stores temperature and other correction algorithms, but
they are generic for a gas sensor type and are not specific to your gas sensor.

Please see the FAQs on resetting the zero baseline for details on how to re-calibrate the EDT in various situations

Should I re-calibrate my EDT?

EDT sensors have been calibrated at Alphasense in laboratory conditions.

However, your application may be in environmental conditions that can lead to a shift of the zero baseline. This shift in Zero baseline may lead to negative concentrations or concentrations that are offset from a nearby reference station. You can reset this zero baseline if necessary. Read the Recalibrate Zero baseline FAQ.

When you have a network of EDTs you can use this network advantage to improve the accuracy of the entire network. Read the network calibration FAQ.

WE sensitivity is less sensitive to the local environment. You cannot easily reset WE sensitivity, but if you are concerned then read the Sensitivity calibration FAQ for more information.

How do I calibrate an EDT when next to a reference station? (advanced user) How do I reset zero offset using an Air Quality Reference Station (AQMS)?

Co-locating the EDT next to a AQMS is the ideal way to reset the zero offset.

Co-locate the EDT near a reference station (AQMS), ideally for 24 hours, but for at least 90 minutes and ensure you have access to the readings from the AQMS at that sampling time.
Co-location is defined as sampling the same air space at the same time as the reference site (in this instancean AQMS). You can co-locate next to an air quality network that is not an AQMS, but ensure you understand the uncertainty in the AQ network’s measurements since AQ networks are not designed to be as accurate as an AQMS.

Set the EDT logging interval to between 15 and 60 seconds.When the session is ended, download the zip file for the EDT datalog session and open the [compensated EDT] .csv file.
Then follow these steps:

1 Obtain the AQMS reference concentration at a known time. Also know the averaging period for the reference station: typically 15, 30 or 60 minutes. If the AQMS has reported as μg/m3 then you must convert to ppb by multiplying by:

CO 0.859 NO2 0.523
NO 0.801 O3/NO2 0.511
SO2 0.376 NH3 1.414
H2S 0.707

2 Find the EDT log time that corresponds with the time for the AQMS, remembering that EDT times are recorded as UTC.
3 Working backwards in time, average the EDT readings for the same period as the AQMS averaging period, using Excel or equivalent software.
4 Record the averaged we_count and ae_count readings at that time.
5 we_count is the sum of the zero offset plus gas response. You must now subtract the gas response from the total counts to determine WE0. The AE does not respond to gas, so ae_counts does not need correcting. Convert gas concentration from the AQMS into AD counts with this formula:
AD counts (AQMS) = ppb(AQMS) x WE sensitivity x 0.00528

NOTE:[WE sensitivity]for each EDT can be viewedon the AQGateway. Select My EDT’s, then select the required EDT. WE and AE sensitivities are listed on the [EDT INFO] pageunder [calibrations].

6 Subtract AD counts (AQMS) from averaged WE_counts (from step 4). This is your corrected WE0 (in units of AD counts).
7 Return to AQ Gateway. Select My EDTthen select the relevant EDT. Go to [Update Baseline Calibrations]on the [EDT INFO] page. Enter the new calculated WE0. Then enter the averaged ae_counts into the AE0 field. Select Update.

This new calibration will be applied to any future processing. Also, any historic batches can be reprocessed and will use the new Zero offset calibration constants. The zero offset constants that were applied to any batch are specified in downloaded [Settings YAML file].

How do I reset (calibrate?) the Zero baseline without access to an Air Quality Monitoring Station (AQMS)

If you are not able to collocate with an AQMS or other reference station then the EDT should datalog in a low pollution area during a stable period to record a low gas concentration.

Select a location that is far from roadside and not near any exhausts from heating systems, transport or industrial sites. Also pick a time when the temperature is near 20°C or slightly cooler and stable (not sunrise or if a storm is approaching). The goal is to have the gas concentration to be as close to zero as possible during stable weather.

Set EDT logging interval to 15 seconds. Start logging and leave the EDT to collect data for at least 90 minutes. Then download the session to AQ Gateway.

Going to AQGateway website, select the session and then select Latest Results download. Open the compensated.csv file in the zip folder and follow these steps:

1 Select the column headed we_count. Pick a section of 10 to 20 readings with the lowest absolute value (the we counts may be negative or positive). Determine the average reading in this period of readings. Write down the average we_count and ae_count.
2 If you want to assume that the gas concentration is zero then proceed to step 5. If you want to correct for the estimated low gas concentration then you must first estimate this gas concentration. If the EDT has been in a low pollution area, then for NO2, NO, SO2 NH3 or H2S the gas concentration should be between 0 and 5 ppb. CO background concentration may be between 20 and 200 ppb and O3 concentration normally decreases at night and is dependent on weather conditions, varying between 0 and 20 ppb. You may also be able to estimate the background concentration if the EDT is in a location that also publishes pollution maps. If published concentrations are in μg/m3then convertto ppb: see step 1 in the previous section.
3 You have estimated the background gas concentration. we_count is the sum of the zero offset plus gas response so you must now subtract the gas response from the total counts to determine WE0. The AE does not respond to gas, so ae_counts does not need correcting. Convert gas concentration from the AQMS into AD counts with this formula: AD counts (AQMS) = ppb(AQMS) x WE sensitivity x 0.00528

NOTE: WE sensitivity for each EDT is available on the AQGateway. Select My EDT’s, then select the required EDT. WE and AE sensitivities are listed under[calibrations] on the [EDT INFO] page. See picture above.

4 Subtract AD counts (AQMS) from averaged WE_counts (from step 1). This is your corrected WE0 (in units of AD counts).
5 Return to AQ Gateway and go to [EDT INFO] for the EDT to be calibrated and enter the average WE and AE counts in the Update Baseline Calibrations box (in window). Remember to enter the correct sign: if the average we_count or ae_count is negative then enter it as a negative number.Select UPDATE.

This new calibration will be applied to any future processing. Any historic batches can be reprocessed and will use the new Zero offset constants. The zero offset constants that were applied to any batch in the zip folder for the batch - they are specified in the downloaded [Settings YAML] file.

How do I calibrate a network of EDTs (advanced user)? How do I reset the zero offset for a network of EDTs

If you have a network of EDTs in the same vicinity then you can use the advantage of a network of EDTs (or EDTs mixed with other AQ monitoring nodes) to calibrate an entire EDT network because the background (“far field”) pollution is frequently constant across large areas- this is an advantage of networks that is not possible with single EDTs.

Follow these steps to calibrate the zero offset across a network:

1 Datalog across the network. The EDTs do not need to be recording at exactly the same time, but they should be recording data every 30 or 60 seconds. Record for at least 24 hours, ideally for at least one week to observe diurnal and weekly patterns. Remember that if you have already been data logging at these log intervals than you can review previously recorded data.
2 Download each EDT as a batch file. Select which algorithm gives the best profile that matches a collocated AQMS or is consistent with the reported concentration profile.
3 Overlay the concentration time plots from the different EDTs. Grafana is an easily accessible graphing tool with AQGateway, but you can use other plotting or statistical packages such as Mathematica or MatLab.
4 Ignore any rapid peaks- look at the slowly changing baseline which should be the same pattern across all EDTs. The baseline for each EDT is the zero offset plus the far field gas concentration; the “near field” transient peaks should be ignored when resetting the zero offset.
5 You must choose a reference point and reference concentration. Ideally you select an EDT which is co-located with a reference station (AQMS); you must know at a specific time the reference station measured concentration and its averaging period (see above). If that is not possible then pick a quiet time such as early mornings (2:00AM to 6:00AM) and weekendsand a time when the temperature and humidity are stable;thendecide the concentration by requestingthe reported average city value at that time (eg for NO2 this will be between 0 and 5 ppb in the early morning). You now have either a known reference gas concentration (with co-located AQMS) or an estimated gas concentration at a selected time.
6 The we_count is the sum of the zero baseline plus gas response so you must now subtract the gas response from the total counts to determine WE0 for each EDT. The AE does not respond to gas, so ae_counts does not need correcting.Convert gas concentration from the AQMS or estimated concentration into AD counts:
AD counts (reference) = ppb(AQMSor estimated) x WE sensitivity x 0.00528
NOTE: WE sensitivity for each EDT is available on AQGateway. Select My EDT’s, then select the required EDT. WE and AE sensitivities are listed under[calibrations] on the [EDT INFO] page. See picture above.
7 Subtract AD counts (reference) from averaged we_countsfor each EDT at the selected time. This is your corrected WE0 (in units of AD counts).
8 Return to AQ Gateway and go to [EDT INFO] for the EDT to be calibrated and enter the average WE and AE counts in the Update Baseline Calibrations box (in window). Remember to enter the correct sign: if the average we_count or ae_count is negative then enter it as a negative number. Select UPDATE.
9 Repeat steps 6, 7 and 8 for each EDT.You may need to correct the reference concentration for specific EDTs if they are located at bus terminals or busy intersections where the gas concentration is always higher. You will need to estimate the increased gas concentration or find previous city-wide models which calculate these specific expected higher concentration.

This new calibration will be applied to any future processing. Any historic batches can be reprocessed and will use the new zero offsetcalibration constants. These zero offset calibration constants that were applied to any batch in the zip folder for the batch are specified in the downloaded [Settings YAML]file..

NOTE : To reprocess an existing batch, return to the Home screen, select the required batch and the Batch Detail screen will appear. Select [Queue for Reprocessing] on the Batch detail screen

Do I need to correct for pressure or altitude?

Alphasesne electrochemical gas sensors are almost independent of atmospheric pressure because they measure the molar fraction of the gas, not the gas partial pressure.
You can ignore altitude and pressure correction (unless the pressure is significantly different from 100 kPa), but you may see a spike in the reading if there is a sudden pressure change- the reading will return to the correct reading when the pressure is again stable.

Do I need to correct for temperature?

No- the temperature sensor in the EDT automatically corrects both zero and gain
for temperature from -30°C to 50°C.

What was in the 1.0 March 2020 Release?

See attached

What are the top tips for using my EDT?

When changing batteries, close the AQGateway App before changing batteries. This will ensure that there is not a bluetooth error logged in the EDT firmware. When you re-open the app, you will need to select the EDT as logging is turned off when batteries are removed.
Leaving the app open and connected to the EDT when removing batteries may result in Bluetooth communications errors which may impact on your data and App efficiency.

Can I change the calibration of my EDT?

You can modify the WE and AE by logging into AQGateway and selecting My EDT's from the Menu list on the left of the screen. Select the EDT to be changed. The EDT INFO window opens and the
Update Baseline Calibrations box is below the Calibration constants. The values can be changed within a range of -1000 to +1000. Select Update. This baseline will be applied to all future processing. Any batches that are reprocessed will also have this new value applied.
The AQ Gateway also stores temperature and other correction algorithms, but they are generic for a gas sensor type and are not specific to your gas sensor.

Why is my data taking so long to download? My download seems to be stuck on the last record - why?

Downloading takes approximately 1 second per block. However, it has been noted that, on occasion, when a large download is taking place, the download appears to "hang" on the last block. This can be for a significant period of time. This is related to the size of the download and is due for resolution in future releases.
If this occurs, please be patient and allow the download to complete

I want to do my own graphing of data - how do I do it?

Each batch is available to download. Both the raw data and the compensated data are available in csv files.
Using a tool of your choice, this data can be imported and graphed as you wish
Grafana is only intended to provide a graphical overview and summary of your data.

How can I see an overview of my data?

A graphical overview of your data is available on AQGateway.com.
Log on and select Time Plots. This will bring you to Grafana where the processed data from all EDT's associated with your company is displayed graphically.
There are 6 graphs provided. These are
1. Temperature
2. Relative Humidity
3.Compensated PPB
4. Compensated PPB moving average
And finally, 4 graphs showing the results of each of the 4 algorithms applied to the raw data recorded by the EDT

How safe is my EDT and my data?

To reduce the possibility of your EDT being stolen, add a security wire to the External housing when the EDT is post or wall mounted. Even if stolen, it cannot be accessed without the App which is linked to your account on AQGateway
Your data is only available to those within your organisation who have accounts on AQGateway. Please ensure to keep your password secure.

What data is shown in the PPB grafana graphs?

The data in the ppb and the ppb moving average graphs in Grafana represent the results of the preferred algorithm applied to the EDT data.

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