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ESDK-FDH module - formaldehyde sensor

ESDK-FDH module gives ESDK a sense of smell for formaldehyde.

In this article, we’ll take a look at what formaldehyde is, why detection is important, and the new ESDK-FDH module design and features.

What is formaldehyde?

Formaldehyde (chemical formula CH2O) is a naturally occurring organic compound that happens to be highly toxic and a known carcinogen. Used mainly in the production of industrial resins that then go on to be used in products such as particle boards (MDF etc) and coatings, formaldehyde is produced on an industrial scale — in 1996 the total annual production capacity worldwide was up to 8.7 million tonnes.

Formaldehyde also has a number of more niche uses outside of industry, including use as a disinfectant and biocide in cosmetics at low concentrations, as a tissue fixative and embalming agent (although using formaldehyde for this purpose has been banned within the European Union owing to its carcinogenic properties), drugs testing when combined with concentrated sulphuric acid, and in some photographic film development processes in low concentrations.

A number of worldwide agencies have classified formaldehyde as a known carcinogen, with restrictions placed on usage and permissible limits in the environment. In 1998 a Canadian study was undertaken to look at houses insulated with urea-formaldehyde foams and discovered that formaldehyde levels as low as 0.046ppm had a positive correlation with eye and nasal irritation, and a further 2009 review of studies showed a strong association between formaldehyde exposure and childhood asthma development.

Further studies conducted in 2009 and 2010 showed a positive correlation between formaldehyde exposure and the development of leukaemia (a group of blood cancers), and some evidence also suggests a link between workplace exposure and the development of sinonasal cancers.

Indoor exposure to formaldehyde commonly arises from treated wood products including plywood and particle boards but also from paints, varnishes, floor finishes, and a further major contributor is cigarette smoke. To reduce formaldehyde exposure, the US EPA has recommended the use of “exterior-grade” wood products such as plywood that utilise a phenol-based resin.

Despite the toxicity, mammals and humans alike need formaldehyde as an intermediate in cellular metabolism — this occurs naturally within the body.

Given the low concentrations of formaldehyde necessary to pose quite a significant health risk, we decided to build a sensor board capable of detecting the gas.

The FDH Board

The FDH Sensor Board

The ESDK-FDH module features the usual affair of ecosystem connectors, plus a Sensirion SFA-30 module that provides a convenient I2C interface.

Board features a Sensirion SFA-30 module

Utilising a COTS Sensirion module to handle the sensing element interface means we do not have to deal with tricky analogue circuitry as the sensor utilises another electrochemical element (similar to what is used in the ESDK-NO2 module).

Electrochemical sensing elements exploit a chemical reaction (either an oxidation or reduction reaction) to produce a measurable current that corresponds to the concentration of the target gas. To reduce cross-sensitivity to other chemicals such as ethanol, Sensirion has fitted their sensing element with a filter that allows only formaldehyde to diffuse through.

The sensor features a measurement range of 0-1000 parts-per-billion with a saturation point of 5000ppb — well above the US NIOSH recommended exposure limit of 0.1ppm for 15 minutes. Accuracy of the formaldehyde sensor comes in at ±20 ppb or ±20% of the measured value depending on which value is larger, with a repeatability figure of ±5 ppb or ±5%, again whichever value is larger.

Sensirion quotes the lifetime of the sensor as exceeding six years provided the environmental conditions are kept within 10-95% humidity, and 0-50°C operating temperature.

A matching Molex Micro-Lock Plus connector is present on the ESDK-FDH board, which enables the use of pre-terminated leads that Molex provide — the 1.25mm pitch of the connectors makes for rather small terminals which necessitates the use of a crimping tool to ensure a proper termination.

Also featured on the board is the usual ESDK power LED that can be disabled by cutting the trace linking “CON1” — this is an 0603 footprint to allow the power LED to be re-enabled by fitting a zero ohm resistor.

Testing

Graph showing results of sensor test

To test that our sensor and the software interface worked we placed a square of laser-cut MDF over the top of the sensor and then watched the readings on the yet-unreleased ESDK desktop UI (an article covering the new UI will be published soon). As expected, the readings began to sharply increase as formaldehyde released by the MDF diffused through the sensor. A slow decrease back to ambient formaldehyde levels was observed as excess gas was consumed in the chemical reactions taking place in the sensing element.

Software Support

At present, the Sensirion documentation for the I2C interface present on the SFA30 describes the interface as “preliminary” and that it is only suitable for R&D usage, but the datasheet for the I2C interface has not been updated since January 2021. If any changes to the I2C interface happen then the DesignSpark.ESDK library will be updated to match.

Software support for the ESDK-FDH board will be included in the latest release of the DesignSpark.ESDK library, followed by support in the air quality application.

To Finish

The ESDK-FDH board expands the ESDK ecosystem to enable the detection of formaldehyde concentrations up to 2000 ppb.

Engineer of mechanical and electronic things by day, and a designer of rather amusing, rather terrible electric "vehicles" by night.
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