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Building on 40 years experience with alcohol fuel cell sensors

General

How do the sensors work?

Dart chemical sensors operate by means of the fuel cell principle. A fuel cell is a chemical battery to which the fuel and oxidant are continuously supplied, so in principle it is everlasting.

We should explain here what a “battery” is (strictly speaking, an electrochemical cell). Many chemical reactions consist of a transfer of electrons from one reactant to another. A simple example is the reaction of zinc with manganese dioxide. If heated together, the following reaction can take place:

Zn + MnO2 = ZnO + MnO

Underlying this change, is an oxidation (loss of two electrons) from each zinc atom, and a reduction (gain of two electrons) by the manganese atom.

If instead of mixing the two ingredients together, we keep them separate and force the electrons to make their way along a wire to reach their destination, we have the basis of the familiar AA, AAA, PP3 batteries.

To complete the electrical circuit, the zinc and manganese dioxide “electrodes” also need a connecting liquid pathway, the “electrolyte”, in the above example commonly an alkali (so the battery is known as alkaline manganese).

What is the difference between sensors and wafers?

We describe the electrode component and reservoir as a wafer set. They do not form a complete working product on their own.

Sensors are complete products ready to be integrated into your device as sold.

Some of your competitors use a pair of single sided wafers. What difference does that make?

When the electrode coatings are on separate wafers, there is an imperfect electrical connection between the two. This gives rise to an internal resistance which can vary from one sensor to another, and it can change from time to time in each sensor as the electrolyte volume varies with changes in temperature and humidity. This means that there can be a wide difference in properties between sensors in a batch, and variations in properties within a sensor during its lifetime.

With our double coating, and patented biporous components, the geometry of the sensor electrodes is fixed, and the electrolyte variations are largely confined to the reservoir wafer, an uncoated component, which gives good batch consistency and long term calibration stability.

What are the sensors made of?

The electrodes are made of platinum catalyst, which is manufactured on-site at our UK building.

The electrolyte is a dilute acid.

The contact wires are made of high purity platinum.

The case material can be PP, ABS, or other types that we currently will not disclose.

How should I dispose of end of life sensors?

Please return them to us for recycling. Depending on the quantity we will credit you for recovered platinum. Contact us for more information.

Otherwise, you may find that a refinery near you can recover the precious metals and credit you for the value reclaimed.

How long do the sensors last?

There is no simple answer to this. In theory they are everlasting (short of gross contamination) as nothing is consumed during use. In practice, they get slower and slower owing to some combination of poisoning and slow surface area loss of the platinum catalyst. Sensor size is not an important factor.

For unheated alcohol sensors, service lifetimes of ten years are known, but the answer really is “until, in your opinion, they get too slow to be of practical use, or can no longer be calibrated” (but even then a component change to raise the gain may bring them back into service). Five years is typical. And you cannot really over-use them, if fact they seem to last longer when well-used than when hardly used at all.

When alcohol sensors are permanently heated, outputs may drop more with time. For example, in an evidential unit introduced in 1998 which has sensors maintained at 38C, outputs are sustained by increasing amplifier gain as required. In 2005-2007 a rolling programme of replacements was carried out as a precautionary measure. Many of the sensors replaced were those originally installed in 1998. They had slowed somewhat in early years, after which little further change was observed.

For formaldehyde sensors, please refer to the datasheet.

Both alcohol and formaldehyde sensors will suffer short sensor life if subjected to prolonged exposure to very low humidity.

What power supply does the sensor need?

There is no power requirement for the sensors, only for the supporting electronics. Heating may be advisable (see below).

The sensor sometimes gives a negative voltage after purging a test gas. Why is this?

There are two common causes of this behaviour:

  1. Your test gas contains no moisture.
  2. The concentration of analyte is excessively high.

Why does my sensor give erratic readings, or sometimes zero when I am expecting a positive response?

The most common cause of this is when a single rail amplifier has been used with one of our sensors. The amplifier interfaced with the sensor requires a dual rail power supply as a single rail amplifier cannot deal with near zero or negative inputs that may develop over time.

What is the colour coding standard for your connector wires?

Blue is the sensing electrode which should be connected to the input labelled as such, or in the absence of this it should be connected to the negative input. Green is the counter electrode, which will either have a matching labelled connection point similar to the sensing electrode, or should be connected to the positive input.

A liquid has leaked from one of your sensors. What is it and what should I do about it?

This is most likely electrolyte that has leaked from the sensor, this may occur after prolonged exposure to high humidity or after flooding the sensor with liquid. The sensor is likely to under-perform after such a leakage and replacement is advised. The liquid is a dilute acid, and should be wiped up with a wet cloth as soon as possible. If the acid was allowed to dry on a circuit board, be sure to clean the affected area thoroughly before attempting any rework.

Formaldehyde Sensors

Please contact us if your question is not answered either here or by the datasheet.

Does the formaldehyde sensor need a pump?

Some manufacturers use one but it works fine as a passive device.

What is the maximum concentration that can be measured?

The maximum HCHO concentration would be defined as the concentration where the response ceases to be linear, unfortunately we currently cannot generate a sample gas concentration high enough to achieve that. The highest concentration we have tested at is currently 1ppm. 

What is the maximum pressure that the sensor can be used at?

Similarly constructed sensors of our manufacture have been tested in an external laboratory up to 10 atmospheres without ill effects. Being an aperture-controlled device, there is of course no coefficient of pressure, unlike membrane controlled devices.

What is the minimum concentration that can be measured?

0.03ppm is the lower limit of what is practical to measure as currently variations in zero offset can be of that order.

What effect does humidity have?

There is no response to humidity as such, but sharp changes cause a short-lived transient.

How stable is the calibration of the HCHO sensor?

Long term data we have suggest good calibration stability for at least three years. Under non-extreme exposures, five years is a reasonable expectation but we are still collecting data.

What is the repeatability of the formaldehyde sensor?

In the range we can currently test, repeatability is currently less than 2% of signal.

What substances are the sensors also sensitive to?

Please refer to the datasheet, listed in the “Product Documents” section of the formaldehyde sensor shop pages.

What is the part number of the Molex connector used to connect to your circuit boards?

50-37-5023 – you will also need the appropriate crimp tool if you plan to make your own connectors.

Calibration Standards

When do you restock the certified standards?

Since the shelf life is 2 years (agreed with UKAS) we make a new certified batch just before the expiration of the existing batch. The cost to certify is high so it is generally not viable to make batches more often; the point at which the cost spread across a batch becomes reasonable is around 150 units.

Please contact us to find out the current expiration dates of stock.

What is the partition ratio? 2300:1, 2100:1, what does that refer to?

Body alcohol levels were originally measured from blood samples. When breath analysis began, it was necessary to find a relationship between blood and breath.

A retired former forensic scientist who had worked for the UK service told us of a survey carried out on a fairly large random sample of people, and the relationship

alcohol concentration in blood / alcohol concentration in breath

was found to vary between 1,600 and 2,900. Choosing a fixed point is therefore somewhat arbitrary, and the two ratios most used are around the median value, 2300/1 in the UK and 2100/1 in the USA.

It is important to realise that the breath values are absolute. In the UK the limit value is 35 micrograms of alcohol in 100 cm3 breath, which is 191.4 ppm ethanol, and this equates to 80mg/100 ml blood at 2300/1. In the US, an 80mg blood value is held to equate to 38 micrograms.

Some examples of national blood/breath standards:

  • Australia 2300:1
  • China 2200:1
  • France 2000:1
  • Germany 2100:1
  • Italy 2100:1
  • Poland 2100:1
  • UK 2300:1
  • USA 2100:1

Please consult with your country’s relevant regulatory body for the partition ratio used in your locality.

How many uses can I get out of a liquid standard before depletion becomes a problem?

It depends on the level of accuracy you want. With a Guth type simulator with a 500ml bath, ten to twenty units might be the limit. If you use two simulators in tandem, then 50 to 100 is possible. Then discard the upstream contents, transfer the liquid from downstream to upstream, and finally replace the downstream standard.

Alcohol Sensors

What effect does humidity have?

Electrochemical sensors of most types usually have a limited recommended humidity range for operation, for example 30% to 95%. At or near 100% they are liable to absorb moisture and flood.

At very low humidity the electrolyte volume will shrink enough to produce low results. This first became apparent many years ago when screeners put into Arizona were found to have a very short sensor life. There is however another more subtle situation. When the temperature falls below zero, the water content of the atmosphere drops sharply. The worst situation is where the outside temperature is low and the sensors/units are stored unprotected in a heated building, where the relative humidity is close to zero. Please note that prolonged exposure to such low humidity invalidates the warranty.

In some instances, sensors appear to recover when exposed to high humidity again, but data are as yet incomplete and inconsistent.

Meanwhile, as for fixes where continuous low humidity might be a problem, either store sensors/units in airtight containers or in a medium/high humidity environment; or design units so that the sensor is sealed off when not in use. It seems that it is storage which is critical, if they are being used frequently with wet gas/breath the problem does not seem so critical.

Do I need to heat the sensor?

Sensors are heated in the following applications.

  • Operation in a cold climate (police hand-helds).
  • Interlocks, where they must operate from a start at very low temperatures.
  • Fixed installations (e.g. coin-ops) where there is no power penalty and the system can benefit from greater speed of operation, needs no temperature compensation, and also not be potentially subject to condensation from breath.

For battery powered units, heating is unusual owing to the power drain.

For -20°C operation it is usual to heat the sensor. It is very slow at that temperature and gives about 10% of the ambient value. Also there are likely to be problems with condensation from breath, which leads to unreliable results. So it is good to get some heat into the sensor and also into the breath pathway to prevent condensation. Worth mentioning at this point, in your design, keep the sample pathway from mouthpiece to sensor as short as possible, or condensation can occur there leading to low results.

Why can I not buy a calibrated alcohol sensor?

As indicated earlier, the calibration will depend on variables in your system – electronics, software, and sampling system.

What effect does temperature have?

First, the speed of response increases with temperature.

Secondly, there is a temperature coefficient to the response. The measured output (peak or integrated) rises with increase in temperature until somewhere in the region 20C to 40C, then gently falls. We are often asked to provide temperature coefficient data for sensors, but they depend on the details of the installation and anyway vary a little from sensor to sensor, so for most accurate set-up (for police use at least) you should calibrate each unit individually at least at three temperatures – 5°C, 20°C, 40°C are commonly chosen – generating a compensation algorithm.

Finally, it may shorten sensor life to some extent, particularly if accompanied by very low humidity. Formulations designed for interlock use have the highest resistance to temperature.

How do I calibrate an alcohol sensor?

There are two commonly used methods, a “wet” gas and a “dry” gas.

A wet gas uses a liquid calibration standard in a breath alcohol “simulator”. The most popular simulator now is probably the range made by Guth Labs in the USA. We supply certified liquid standards, mostly for the UK market, as export is expensive owing to the large weight of the glass bottle and the water content. They are used in some countries as a traceable reference for locally made standards.

A dry gas uses a compressed alcohol/air mixture in a gas cylinder. Note that the fuel cell sensor reads dry and wet gases differently, the difference typically being about 6% higher for the wet, and so a correction – the “wet/dry factor” – must be made to adjust the dry result up. Also with a dry gas, a correction must be made for variation from standard barometric pressure as the dry gas will expand more into a low pressure atmosphere than a high pressure and so have a lower concentration.

The output is linear through zero so single point calibration is normal, the chosen value usually being the legal limit in the jurisdiction of the purchaser. For additional accuracy (for police use) calibration at three temperatures is usual.

What size of sensor do I need for my application?

For consumer units the 11mm sensor is suitable. We recommend the 16mm sensor for evidential and police use. The 32 mm sensor is only needed for old instruments taking that size.

Do I need a premium or economy 11mm sensor?

The physical difference is that the premium carries a higher load of platinum (and so it costs more). This gives it greater calibration stability over repeat samples.

In practical terms, if you want to meet approval standards such as NF, you choose the premium.

If low cost is your priority, choose the economy and be more careful with the thinner contact wires.

Some interlock manufacturers are satisfied with the performance of the premium 11 mm sensor.

How do I get a useful signal from the sensor to my device?

A rigorous treatment would be full integration of the response curve, but this has not been found necessary for adequate accuracy, repeatability and calibration stability. Use a zero load transimpedance circuit, and terminate the integration at some fixed fraction of the peak value (for example, 5%). An example circuit is available on request. Your electronics must be designed to be low-noise for this method to be effective.

What are the characteristics of the sensor’s output?

There is a sharp peak and its magnitude depends on (for example) sample volume, alcohol content, age of sensor. It would not normally exceed around 1 mA. Diagnostic values are peak height, peak time, integration time and integral value. Peak values do decline in the early weeks and shelving during this period is common, but integral values are very stable over prolonged periods.

How do I get a sample into the sensor?

Breath alcohol sensors are almost always operated with a snap sample taken near the end of an exhalation, and signal processing may be classified according to whether it is by peak voltage or a current integration method. The course of breath expiration is generally detected with the aid of a pressure switch, a heated thermistor, or a microphone.

Recent work suggests that the best way to operate the pump is to take the sample, hold for about 200 mS, then reset. Alcohol vapour is taken up very rapidly in to the sensor wafer. Drawing in first, then blowing out, is the preferred method.

A suitable pump can be bought directly from our office in Shenzhen, small quantities can be purchased on this website and shipped from our UK factory.

Make sure the tube connecting the pump to the sensor is relatively long with a loop to trap moisture if possible. If the sensor should flood this may prevent damage to the pump. Similarly the sensor should be mounted away from circuitry if possible in case of an electrolyte leak, which may happen if the sensor is misused.

The environmental alcohol sensor works by diffusion, so it just needs to be exposed to the environment to be sampled.

What are the requirements for the mouthpiece?

The mouthpiece design is not critical, there are many patterns. Basically it consists of a simple tube with a hole in the side to accommodate the inlet to the sensor. If a pressure switch is used to detect breath flow, then a constriction is placed downstream to build up back-pressure.

What is the minimum output of the sensor?

The minimum current is zero for a sample containing no alcohol.

What is the lowest ethanol detection limit?

Under practical conditions the sensor has no difficulty reading down to 5 µg/100 ml, 0.005 BAC, which is a commonly chosen cut-off point.

Under ideal conditions less than 1 ppm is possible, but the humidity of human breath and possible low level CO in smokers give transients which become significant at low levels.

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