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Building on 40 years experience with alcohol fuel cell sensors
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?

Green 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 positive input . Blue is the reference electrode, which will either have a matching labelled connection point similar to the sensing electrode, or should be connected to the negative 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.

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