Hi All
I’ve a basic question – if I purchase an AQE will it be able to measure specific gas concentrations that can be used in an Air Quality matrix as defined by Air Quality Standards.
I need to ask this basic question because the AQE description lack meaningful specifications. The AQE description – in my opinion - is using flowery language that is not helpful in defining exactly what the sensors are designed to do.
From my experience data processing is in layers – the fundamental physical conversion layer needs to be validated first and verified. If its not specified it almost certainly is likely to be flawed. If the physical data collection layer is flawed the integrity of upper processing layers are likely to be highly suspect.
Ed Bordens statements that calibration is the wrong question suggest he just doesn’t understand real world sensors. I’m happy to be educated if I have missed something. But repeated referencing the same statement is not persuasive.
I’m an electronic/software design engineer who has designed other transducers to use physical sensors to the limit of their specification.
Designing a circuit to transform a physical quantity (NO2 CO O3) needs to follow some simple rules.
There needs to be a physical sensor that transforms the concentration of gas to a measurable electrical value.
This is then converted through an electronic circuit to the digital world (Analog to Digital Conversion)
An algorithm running on the processor then typically “digitally signal processes” into a useable trace-able values.
I’m not an Air Quality expert, so I’m using thresholds from a Sonoma Technologies document called “Desired Characteristics and Information about Major Air Pollutants for Device Manufacturers to Use in Creating Instruments for Non-regulatory Monitoring (e.g., Citizens, Schools, NGOs).”
If someone has other specifications please share them.
The following thresholds fall out of the table
CO to 9ppm with an accuracy of 1ppm. Normal atmospheric ranges are up to 20ppm
NO2 to 100ppb with an accuracy of 1ppb. Normal atmospheric ranges are up to 400ppb.
As of a couple of days ago the NO2 sensor was specified as the MiCS-2710. So as I read this data sheet the sensor is not capable of monitoring to the above specification. It has a Sensor response specified as from 0.2ppm to 1.5ppm (200ppb to 1500ppb)
The MiCS-2714 does seem to have the physical range of NO2 concentrations. Its sensitivity is 10ppb to 900ppb
Since this is an Open Hardware Design – I’m offering my input to get a sensor that is understood – if I’m missing something – please lets have a system design document so its is clear.
The CO sensor MiCS-5525 does seem like it can meet the above specification. The sensor characteristics show the sensor capable 1-400ppm
However it’s a non-linear relationship, and the lower sensitivity of 1 to 10ppm is on the flattest part of the curve – which could be the most challenging to measure.
All the sensor specifies that the active measurement be compared against a synthetic air measurement (presumably down at the factory).
All the sensors define the range they measure could be different for each sensor – so is it measuring 1 unit or 100units – has to be determined for each individual sensor.
Look forward to hearing sensible responses to these basic questions
--
Cesar García - @elsatch
Ando con encolamiento para responder correos y los proceso lunes, miércoles y viernes. Si es algo urgente/rápido contáctame por Twitter. Gracias!
Hmm Yes interesting, thankyou for looking at the data sheet, – I was only looking at the MiCS-2710/4 graphs. (I’m on vacation at the moment, and don’t have ability to print and compare data sheets carefully)
So looking at the differences between MiCS2710 and 2714 they are different in package types, but otherwise appear incredibly similar.
What is intriguing is that the graph of Rs/Ro cover different ranges. However since the 2714 is so similar it suggests that at the more sensitive end the range – that of interest in Air Quality thresholds - is non-linear. The Application Note datasheet MiCS-AN2 FAQ states
“As the characteristic curves of the sensors are non-linear, e2v recommend the use of multi-point calibrations, covering the whole range of the application in terms of temperature, humidity and gas concentration. Once the set of calibration parameters is obtained, the data can be interpreted using polynomials or curve fitting techniques. “
Also it seems to me the “NO2 Detection Range” of FS min=0.05ppm to max=5ppm needs some careful analysis. I hate to do too much of a casual analysis on it – but it seems to me that for some individual sensors may only measure up to 50ppb and then max out – flat line – and then other sensors could measure up to 5000ppb before flat lining.
So the meaningfulness of the measurements “accuracy” comes at a cost – and the question is can a sweet spot be found that is MINIMAL calibration for a defined range that is sufficiently accurate for Air Quality thresholds.
The data sheet doesn't state any long term drift that I can see of.
Its very painful that the designers are not stating what is the range they are aiming at.
Martin what sort of information do you think you need to be pull any “trend” information out of the production sensors.
What assumption do you make about geographically linked sensors - and how close do sensors have to be to be able to make any assumptions that they are measuring something similar. Or another way of asking how well does air mix up pollutants and distribute them?
Thanks!
m.