We have tried to cover some of the most common questions here but please contact us if your question is not answered here.
1 minute video demonstrating how to install AQMesh pods (using post clips)
90 second video demonstrating how to install AQMesh pods (using steel banding)
Full length 10 minute video demonstrating how to install AQMesh pods (with narration)
Scaling is a common tool for air quality instrumentation. It is used across the majority of instruments designed for ambient air quality measurement, including reference equipment, and provides an increased level of accuracy based on the local environmental conditions and how the sensors used react to that environment.
In order for AQMesh to provide the greatest degree of accuracy, it is advised to co-locate the pod with a reference station. This will provide the information necessary for calculating our scaling variables - offset and slope.
AQMesh pods which are not locally scaled will still provide dependable and repeatable performance, as scaling does not affect correlation (R2) values.
Scaling values are calculated over a specific time period and as such are based on the environmental conditions on site over this period. By applying these values for offset and slope we are able to determine the best possible performance results for this pod over the same period, and improve accuracy of the pod moving forward.
Scaling of AQMesh sensors is only required in certain circumstances:
• Following a sensor change, the new sensor will need scaling to its new environment
• Following a large step change in environmental conditions at the site, e.g. a change in average temperature of 10 degrees Celsius or more, compared to when it was scaled
In order to complete the following scaling methodology the user must first select to use advanced settings so that “PreScaled” values for each sensor are available for the downloaded period.
Regression comparison AQMesh PreScaled data versus reference or Gold Pod Scaled data, using the candidate pod on the x axis and the reference on the y axis. Regression analysis can be completed using Microsoft Excel, MathCad or similar mathematical programs.
Removal of potential erroneous data points and find Cartesian equation for line of linear trend line
Apply Offset and slope to full PreScaled data set (including erroneous data points)
Reference ≈ (PreScaled data x Gradient) + Y-Intercept
Gradient = Slope
Y-Intercept = Offset
AQMesh Scaled data = (PreScaled data x Slope) + Offset
Upload slope and offset values to the online application for each gas sensor and particle fraction.
Validate pod accuracy by following the scaling process by either; a second co-location with reference or co-location with Gold pod at the target location, at an interval determined by the users own QAQC process.
For long projects it is advised that data is validated every six months to account for any changes in calibration at the reference station.
For shorter projects, good practice for validation of data would be to run a scaling exercise both pre and post the trial period, providing full confidence in the data received from the trial.
This is often all that a user can identify when carrying out analysis, generally comparison with pods co-located with a reference station. We have been able to identify some common issues which may help users resolve any concerns quickly.
Please first check
- That all the comparisons you are making are based on equipment (either AQMesh pod vs AQMesh pod or AQMesh pod vs reference) located very close together. For example, this photo shows 2 pods co located right next to the inlet of a reference station, imbetween the two:Small distances cause large variations in pollution levels and timing. Anything further than 1 metre apart is not a valid comparison and ideally this distance should be a few centimetres. There is also evidence that air is not well mixed when very close to a roadway, and even if pods are close together they may still show differences over shorter averaging time periods. These will converge better if averaging times are extended, eg; 1 hour, 24 hours. Check your co-located comparison trial is set up correctly with FAQ Setting up a co-location comparison trial with a reference station.
- That you are not trying to assess performance during the intitial period after pod installation. This is a minimum of 7 days (5 days for stabilisation and 2 days for rebasing). This will also apply after a pod has been moved, unless environmental conditions have remained consistent throughout the entire move.
- The stabilisation/rebasing has been manually reset for any pod that has been moved. The 7-day option will be required if environmental conditions were not maintained during the move, and the 2-day option otherwise.
- That the AQMesh data downloaded exlcudes all invalid data, such as when it is flagged as 'stabilising' or 'rebasing' in the downloaded data file. This flag is set automatically when a sensor is changed or when manual stabilisation/rebasing is selected by the user. Readings will not be valid until 7 days after as pod has been moved unless great care is taken to maintain environmental conditions. It is best practise to always re-stabilise the pod after it is moved.
- The offsets are applied to each sensor. With multiple users and changes over time, it is possible that an offset may have been applied to a sensor and forgotten about. Please check that all offsets are zero or take account of any offsets applied when making comparisons. Also note that any offsets applied to O3 will have a direct effect on final NO2 readings.
- That you have not had a sensor failure notification or can see a sensor failure flag.
Other reasons why data may not be optimised
If any sensor has been changed, the whole pod is 're-set' for the sensors to stabilise and re-base. This will show as a period of instability and poor comparison data, either pod vs pod or pod vs reference. See below for an example of an NO sensor changed on 10th June, rebasing complete on 17th June. The CO sensor is clearly temporarily affected during this 7 day period.
NO and NO2 sensors need to re-base to achieve optimal performance. In certain conditions this may take longer than the normal 2 days. We can check whether this is the case with your pod - please ask.
SO2 readings are currently not optimised. There are various reason why we have not given priority to improving SO2 performance:
- As SO2 is affected by other gas readings it makes sense to improve NO, NO2 and O3 first
- We cannot improve the SO2 performance based on universally low SO2 reference levels and we have not been able to secure reliable reference data from an area with higher levels of SO2. We have identified sites with higher SO2 and have pods in place but these authorities are not so willing to share reference data, eg; Taiwan, Hong Kong, Malaysia
- As many developed countries have low SO2 and it is not considered a priority, we consider NO2 improvements more important
A difference in temperature
AQMesh is affected by temperature and corrections are normally effective. However, differences in temperature between co-located pods, such as one pod receiving more direct sunlight than the other, may cause a difference in performance. Please check whether this is the case with your pods location.
An offset may be required
Also note that any offsets applied to O3 will have a direct effect on final NO2 readings. The best practice would be to first adjust the O3 offset and wait for this to take effect, then observe the NO2 final readings and make any necessary offset changes.
O3 readings on v3.0 are based on a rolling average so direct comparisons with reference data may not be valid unless the time basis of the reference data can be matched to that of the AQMesh data. This is not the case with later versions of AQMesh.
Reading interval averaging
When carrying out comparisons with reference equipment, it is important that the reading intervals are averaged in the same way in both AQMesh and the reference equipment.
AQMesh readings are calculated using the 'time ending' convention, ie: a 15-minute reading labelled 12:15 is an average of samples from 12:00:00 to 12:14:50. A 'time beginning' protocol would label the same time interval as 12:00.
If there is a mismatch between 'time beginning' and 'time' ending conventions used in comparison analysis, reading peaks will be misaligned and the correlation will be very poor.
For more information please contact us.
We apologise for the inconvenience of sensor failure. This is usually associated with high (>90%RH) and sustained (>3 days) humidity and conditions where the humidity is able to condense on the sensor membrane. Please note that this is outside the stated AQMesh operating range - see the technical specification.
The sensor needs to be changed and returned to us - we will then investigate the cause of failure. AQMesh, together with the sensor manufacturers, are working to improve the failure rate.
Please follow the procedure for changing a sensor laid out in the AQMesh operating manual, including changing the sensor serial number, which ensures that the individual sensor characteristics are used to calculate optimised readings and also that the sensor is re-set appropriately. Data for a failed sensor or for 7 days after sensor replacement is invalid and will be flagged in the downloaded data file and is shown on the pod graph as stabilising and rebasing (please note, this is not shown when comparing pods on a graph).
The monthly failure rate for sensors is normally well under 1% but we have seen higher rates during periods of sustained, high humidity. Very high humidity levels (c>90%RH) that fall below 70%RH each day or those that do not condense do not normally cause problems.
How do I change a sensor in my AQMesh pod?
This video will demonstrate how to change a sensor:
For more information please contact us.
How will I know if my battery needs replacing?
A low battery alert is sent when an AQMesh battery has approximately a month's power remaining. This should allow plenty of time for a replacement to be sourced and installed without interrupting data. As the power delivery is extremely stable across the battery's life it is not possible to measure remaining battery charge before the point that it declines after approximately 2 years.
This graph shows the typical battery life pattern:
How do I replace my AQMesh battery?
This short video will demonstrate how to replace the battery in an AQMesh pod:
Can I recharge my AQMesh battery?
No, the lithium thionyl chloride batteries are non-rechargeable and must be disposed of according to local regulations.
For more information please contact us.
AQMesh advanced mode is a facility for more experienced air quality management professionals to use the full power of AQMesh and associated data processing techniques to fine-tune performance. Advanced mode primarily allows users to apply scaling (offset and slope), derived from comparison with a co-located reference analyser, to be applied to the AQMesh server. This means that readings (future or scaling can be applied retrospectively) can have scaling applied to ensure that they are 'calibrated' against the reference station.
Not all users have access to a reference station and all AQMesh pods effectively go through the same process when built. Readings generated using this original set up are referred to as 'default' and a high level of accuracy may be achieved without any further processing. See results on the performance blog.
However, local conditions can affect readings and scaling using Advanced mode provides the ability to address this if required and if a reference station is available.
Applying scaling on AQMesh
The v.4.0 AQMesh is characterised when built but further performance improvements may be possible by local scaling (stage 7b in the diagram below). Scaling allows correction for differences in environmental conditions and involves:
- Co-location with a reference station (RTMS)
- Regression analysis to derive offset and slope figures
- Using AQMesh in Advanced mode to adjust settings
We have created a simple guide to this process - but please just ask if you need more support.
To set up an effective trial to compare AQMesh performance against a reference station we recommend following this advice:
- A reliable reference station (maintained and ideally certified) should be chosen as the trial location, in an environment where there is some pollution
- At least 2 AQMesh pods should be used to allow comparison between pods, as well as against reference
- The essential parameters measured by the reference station: NO, NO2, O3, RH and temperature
- Optional parameters measured by the reference station could be: SO2, CO
- All trial pods should be co-located with the reference station inlet, ideally immediately adjacent or at least no more than 1m apart, like shown:
- Pods must be allowed 5 days for environmental stabilisation*
- Readings will reach optimum performance after an additional 4 days when all base lining is completed
- If pods are moved at any point, environmental stabilisation* and base lining will need to be restarted. The 're-stabilisation' button on AQMesh.net should be clicked as soon as the pod is moved.
*Environmental stabilisation is the process by which electrochemical sensors adjust to a change in ambient temperature or relative humidity. Any significant change to these conditions will affect readings until the sensors have stabilised.
Examples of changes in environmental conditions:
- Transportation in an enclosed environment, eg: a car, van or plane where the internal environment is controlled with heating or air conditioning
- When a pod is brought into an office environment, even briefly, for storage or maintenance
Examples where environmental conditions are maintained:
- Transportation of pods by foot from one location to another
- Transportation of pods in an open truck
If your comparison data still doesn't look right, please see FAQ "My data doesn't look right" to identify common issues.
For more information please contact us.
There are two main points to consider when moving or shipping AQMesh:
- The lithium battery must be removed when transported using a carrier, or classed as dangerous goods
- Electrochemical gas sensors require a 5-day stabilisation period when location is changed
For more information see Shipping and moving AQMesh
If your pod doesn't seem to be connecting to the server, please go to the pod and check the LED status before contacting us and we can then diagnose the problem.
LED sequence summary
- Alternate green/red blink, start-up
- Slow green blink (every 2 seconds), super-cap is charging
- Fast green blink (every half second), connecting to the server
- Slow red blink (every 2 seconds), connection failed due to a local issue
- Fast red blink (every half second), connection failed due to a server issue
- Green on for 5 seconds then off, connection succeeded
LED sequence 4 - connection failed due to local issue - possible causes
- The SIM card is locked with a PIN code - the AQMesh pod cannot accept SIM cards which have been configured to include a PIN code. Talk to your service provider if the SIM fitted is not AQMesh standard issue
- The AQMesh pod can't attach to the network - try to change the aerial orientation or the pod location because of bad GPRS coverage
- The AQMesh pod can't connect to the APN - talk to your service provider. The engineer may need to manually enter the correct APN settings via the serial link
- Unexpected error - please contact us
LED sequence 5 - connection failed due to server issue - possible causes
- Can't connect to the server - the server may be offline or the IP and port are not set correctly in the AQMesh pod. The engineer may need to manually enter the correct settings via the serial link
- Response timeout - the server did not reply in time to a command. In the first instance contact us as it may be possible to adjust settings remotely
- Response error - the AQMesh pod received data from the server, but the format of the response is not correct. Contact us.
- The pod has not been enabled in AQMesh (tick box)
Please note that when AQMesh has successfully connected, all lights are OFF
This design is to minimise power use and to avoid the pods attracting unwanted attention when installed.