Dart Sensors - Frequently Asked Questions and Information

The design and manufacture of chemical sensors. The manufacture of reference standards. The supply of chemical sensors, catalysts, fuel cells, reference standards and associated components.

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Alcohol Sensor Application Notes

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. To take a simple example, the reaction of zinc with manganese dioxide. If heated together, the following reaction can take place:

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).

The electrodes are of platinum, high quality, high surface area, made by our own processing method developed over 25 years to achieve high performance, calibration stability and long life; the electrolyte is an acid. Contact wires preferably are platinum too, not too thin (fragile), not too thick (expensive).

Breath alcohol sensors are almost always operated with a snap sample (see section 8 below) 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 detected with the aid of a pressure switch, a heated thermistor, or a microphone.

Originally, breath alcohol sensors were operated only in �peak mode�, that is to say the sensor was connected to a shunt resistor, initially of 1000 ohms, the peak voltage response across the resistor being measured. Sensors then were slow in operation, response times of around 40 seconds were commonplace, and clearing time was up to five minutes (today, we can obtain peak times of as little as 200 mS and clearing times well below 5 seconds). Peak measurement is still used for some applications as it is less affected by adverse conditions (see 7 below) but has two disadvantages compared with integration and is avoided where possible:

Repeatability improves with higher load resistances, but the response time is extended, and so a compromise is sought which is generally between about 220R and 390R. Once the peak value is passed, it is usual to short the sensor to speed up clearing. A JFET is normally used for this purpose.

The development of much faster sensors alongside cheap microprocessors has enabled more complex algorithms to be used in signal processing. 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.

The sensor must either have a low load resistor value (typically 10 ohms or less), if a V/V amplifier configuration used, or (as we favour) an I/V (transimpedance) configuration which gives effectively a zero resistance. Either provides fast response and clearing times. Integration of the response down to 10% of peak value or less is adequate to give very stable short and long term calibration stability. But your electronics must be low-noise (more on this in 16 below).

There is no simple answer to this. Old fuel cells never die, they only fade away. In theory they are everlasting (short of gross contamination) as nothing is consumed during use (although we are aware of competitor products which generally last much less than a year). 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 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 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.

Prolonged exposure to very low humidity leads to short sensor life (more below, Q 13).

Type 5 is the standard formulation used universally since about 1982. We consider it useful in three instances:

Type 4 has been used by us since about 1993. It gives a very fast response and clearing time, and has very good low temperature properties, but is unsuitable for applications where the sensor is likely to see frequent or prolonged exposures above 50C. Integration is best.

Types 1, 2 and 3 are various formulations introduced in about 2004, principally for interlock products where resistance to short term exposures at high temperatures (up to 85C) is essential. There is as yet no consensus on which is the best. Peak measurement gives best results.

The original 32mm diameter size used a sample volume of about 1.2 cm3. All other things being equal, and with a sample pump capable of precise delivery, the sensor area can be reduced in proportion to the sample volume. So a 16mm diameter disc works just as effectively with a sample volume of around 0.3cm3 , the 11 mm with 0.25cm3 . In practice, a precise sample volume in the range 0.25 cm3 to 0.50 cm3 works well for these sizes. The smaller the better, as long as the volume is precise and the electronics can cope with the lower signals.

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.

We are often asked where to source the pump. Most manufacturers make their own with a bought-in solenoid and their own design of diaphragm parts. Sensatronic manufacture a very high quality sample pump available in a variety of sample volumes.

In units using the integration method, the calibration should be very stable. Calibration checks are carried out at 6 monthly intervals in the case of the UK police evidential equipment, but that is a statutory requirement, and few would need a recalibration at this interval. There are instances known of hand-held units containing our sensors which have held calibration (35 +/- 2) for up to six years. So the best advice is to check as often as you think necessary.

A sensor made with freshly-prepared components displays hyperactivity. Peaks will drop and response time slow a bit in the first months, then they steady up. Calibration may fall initially (peak mode is more affected).

A wet gas uses a liquid calibration standard in a breath alcohol �simulator�. The most popular simulator now is probably the Guth 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. Our Dry Alcohol Standard (DAS) is a way of reducing weight, the end user simply adding water to obtain the liquid standard, but please note, as they are individually made they cannot be certified.

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 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�see 12 below.

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 me of a survey carried out on a fairly large random sample of people, and the relationship

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 microgrammes 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 microgrammes.

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 � 5C, 20C, 40C 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.

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 humidities 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 situations. 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 I guess 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.

Yes, but these �interfering� substances are unlikely, except in very extreme circumstances, to cause a problem. One of the reasons why the fuel cell sensor is ousting the semiconductor type is its greater specificity � it is, for example, not sensitive to acetone, which can occur in substantial qualities in the breath of diabetics.

The fuel cell sensor routinely passes the specificity requirement for breath alcohol equipment approvals - for example, all the tests required by the authorities in USA and the UK for evidential use. But to give actual examples:

It has no sensitivity to the following substances: acetone and other ketones; toluene, benzene and other aromatic hydrocarbons; ethyl acetate and similar esters; methane, ethane and other saturated hydrocarbons; carbon dioxide; water vapour (brief transients may occur).

There is sensitivity to the following within the limits specified by the UK Home Office: methyl alcohol, isopropanol, carbon monoxide, diethyl ether.

It is also sensitive in varying degrees to: hydrogen sulphide, sulphur dioxide, aldehydes including formaldehyde, phenol, hydrogen, alkenes. NOx and ozone give a negative response.

For -20C 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.

The type 4 formulation is better at low temperatures, but is not suited to a high temperature exposure. If you want an interlock sensor to come to working temperature quickly, it can be operated at just 20C, or anything down to 5C, or even lower. So the heater device can be set to operate when the temperature falls below the temperature of your choice. Then you may need to use some temperature compensation for the expected upper temperature range.

The usual way to heat the sensor is with a heated metal (usually aluminium) block. The sensor is attached to the block. The heat is supplied by a power transistor and controlled by means of a thermistor next to it � it needs to be as close as possible to avoid hysteresis swings of temperature. Watch out for the thermal offset which occurs during the heating-up stage.

16. Some of your competitors use a pair of single sided wafers. Why? 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.

Are you getting a build-up of condensation near the sensor inlet (keep gas pathway short and heat if necessary to 35C minimum).

We have seen a few examples of this. Don't automatically blame the sensor, it may be a matter of signal/noise. When new, the sensor (particularly type 4) has a very sharp spike response followed by a quick clean-up, and even quite indifferent electronics will not hamper accurate determination of the integration cut-off point. But the fuel cell profile changes shape with age, the sharp spike drops away and the tail grows longer. If the output is noisy, the software grabs at the first dip to find the cut-off point, and takes it too early. It will be more of a problem at low alcohol levels, results will be low. Linearity is therefore affected.

Incidentally, you can anticipate the ageing of the peak spike by allowing the sensor output to hit the rail on high values when new: integration carries on just the same (but the cut-off point is slightly altered).

Under practical conditions the sensor has no diffficulty reading down to 5 mg/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.

Accurate results depend on a combination of five ingredients: a good sensor, a precise and reliable sampling system, quality electronics, sound software and correct calibration (to which we can add, realistic expectations on the part of the user�do not expect evidential quality from a $30 breathalyser). So when something goes wrong, don�t automatically blame the sensor. Usually (but not always!) the fault lies somewhere else.

Incorrect or missing goods

Faulty or damaged goods

We will then contact you to advise of a course of action.

Remember that the more detail you put into the fault desctription, the easier it will be for our engineers to reproduce the alleged fault.

In order to expedite your returns please ensure you have marked your RMA number clearly on the outside of the package.
Ensure the goods are securely and carefully packed. Items that are not packed correctly will be rejected and your entitlement to a replacement or refund will be void.

Please contact our Customer Services Department if you require advice on how to properly pack your item.

Include any manuals, datasheets and accessories that came with the product.

Please note that returned items are tested for the faults that are indicated within your fault description and for basic operation of the product. If these faults are not found, then the product will be returned to you and a carriage/handling fee will be charged.

Returned items confirmed as faulty will be replaced by an identical product (or a product of similar specification), sent for repair or credited depending on the age, condition and status of the product and the availability of an appropriate replacement.

Electronic Interfacing

Dart Sensors diffusion sensor products are best operated in conjunction with a high gain transimpedance (current to voltage) op-amp circuit. Gain resistors of the order of 1M are typical. A high gain, low input current offset op amp is essential.

The sensor will pick up an offset if left unpowered. They are shorted out for delivery. They should have a means of keeping shorted in operation. A shunt resistor of about 470 ohms will suffice, but a jfet is a more elegant solution.

If you want to achieve a high degree of accuracy, then you should apply a temperature-dependent compensation. Temperature/response curves will follow shortly.

Dart Sensors gives full technical support to users of its products. If you have any queries which have not been addressed above please contact us at customer.service@dart-sensors.com

Dispatch and Delivery

Orders will be shipped fully insured with DHL. Prices can range from around �30 for western Europe to �50+ for USA and Asia. This service usually delivers within 2 days of dispatch for Europe and within 4 for the rest of the world. Please let us know if you have a DHL account you wish to use for payment of shipping fees. We can also ship with FedEx, UPS and TNT if required.

Quotation/Ordering process

Please provide a purchase order if you have a credit arrangement with us, or we will not know that you have placed an order. If you are paying in advance of dispatch, receipt of payment will be taken as an order being placed.

If you currently have a credit arrangement with us, the goods will be dispatched as soon as they are ready. If not, we will wait to confirm receipt of payment and then dispatch when the goods are available.

All orders are accepted by Dart Sensors Ltd. on these conditions of sale which supersede any other terms appearing in this or any previous publication or elsewhere, and, unless otherwise specifically agreed in writing on Dart Sensors documentation by a Director of Dart Sensors, these conditions of sale override and exclude any other terms stipulated or incorporated or referred to by the customer, whether in the order or during any negotiations or any course of dealing established between Dart Sensors and the customer, and constitute the entire understanding between Dart Sensors and the customer for the sale of products.

All descriptions and illustrations contained in Dart Sensors literature or web pages or any price list or otherwise communicated to the customer are intended merely to present a general idea of the products, and nothing contained in any of them shall form any part of the contract between Dart Sensors and the customer.

Prices of products will be as set out in our quotation. For repeat business, if no new quotation is served, prices will be as agreed in the most recent quotation.

Orders may be placed by email, fax or post. A written quotation will be supplied to the customer in respect of new business or repeat business in which the previous quotation no longer applies. The customer must sign and return the quotation before the order will be accepted.

Dart Sensors reserves the right to decline to trade with any company or person. In addition, and not withstanding any other provisions of these conditions of sale, Dart Sensors may decline to accept any order, whether or not payment has been received, by giving notice of non-acceptance to the Customer by email, facsimile or telephone within 24 hours (excluding weekends and public holidays) of receipt by Dart Sensors of an order. In the event that Dart Sensors declines to accept an order in respect of which payment has been received, the full amount of such payment will be refunded.

All products are delivered by UPS insured and carriage paid unless otherwise agreed in advance. Import duties etc. are the responsibility of the customer.

Delivery will normally only be made to the customer's usual business address as notified to Dart Sensors.

The customer must inspect the products as soon as is reasonably practicable after delivery and shall within 30 days of delivery give notice to Dart Sensors in detail of any defect in the products or of any other complaint which the customer may have in relation to the products. If the customer fails to give such notice, the products shall be conclusively presumed to be in all respects in accordance with the contract and free from any defect which would be apparent on reasonable examination, and the customer shall be deemed to have accepted the products accordingly.

In the event that the customer establishes to Dart Sensors� reasonable satisfaction that the products are not in accordance with the contract or are so defective, the customer's sole remedy in respect of such non-accordance or defects shall be limited as Dart Sensors may elect to the replacement of the products or refund of the purchase price or, where sums are owed by the customer to Dart Sensors, the issue of a credit note against return of the products.

Queries regarding shortages of products must be made within 10 days of the despatch date and must be accompanied by the despatch note. Queries regarding products invoiced but not delivered must be made within 10 days of invoice date and the invoice number must be quoted.

Except as set out above in no circumstances shall Dart Sensors be liable to compensate the customer in damages or otherwise for non-delivery or late delivery of the products or any of them for whatever reason or for any loss consequential or otherwise arising from non-delivery or late delivery.

Risk of loss of or damage to the products shall pass to the customer at the time of delivery. The property in the products shall not pass to the customer until all sums due or owing to Dart Sensors by the customer on any account have been paid, and until payment the following provisions of this paragraph shall apply. Except where full payment is made in cash, the whole of the price shall not be treated as paid until any cheque or other instrument of payment given by the customer has been met on presentation or otherwise honoured in accordance with its terms. If the customer defaults in the punctual payments of any sum owing to Dart Sensors, then Dart Sensors shall be entitled to the immediate return of all products sold by Dart Sensors to the customer in which the property has not passed to the customer. Demand for or recovery of the products by Dart Sensors shall not of itself discharge either the customer's liability to pay the whole of the price and take delivery of the products or Dart Sensors� right to sue for the whole of the price.

Dart Sensors warrants that it will, at its option, replace products supplied by Dart Sensors in which under proper use defects appear, or repair the same, or refund the purchase price thereof, subject to the claim being made in writing to Dart Sensors within 12 months after despatch, or such other period as may be indicated by Dart Sensors for specific products from time to time, provided that products or parts to which the claim relates are returned to Dart Sensors within that period suitably packaged and carriage paid and, where relevant, in accordance with any particular instructions which Dart Sensors may have notified to the customer at the time of supply. Returned products or parts must be accompanied by an advice note stating the original invoice number in respect of the products and the nature of any claimed defect, together with such further information as Dart Sensors may at the time of supply have stipulated. Any products or parts which are replaced by Dart Sensors shall become the property of Dart Sensors. This warranty shall be in lieu of any warranty or condition implied by law as to the quality or fitness for any particular purpose of the products except any implied by law which by law cannot be excluded.

Dimensions and other physical characteristics are subject to normal commercial tolerances. The customer shall be responsible for ensuring the fitness of the product for the customer's application.

Dart Sensors reserves the right without prior notice to discontinue any product or to make design changes as part of its continuous programme of product improvement, or to assist product availability.

Dart Sensors will be pleased to advise the customer in these and other matters related to the use of the products.

The information contained in Dart Sensors literature and web pages is, as far as Dart Sensors was aware, correct at the time of publication.

Where the customer intends to supply the products to any other person, the customer shall all ensure that all warnings, labels, instructions, manuals and other information in respect of the products or their use which are supplied with the products are not lost or damaged in any way while the products are in his possession or under his control and that they are supplied with the products when he releases them from his possession or control.

A statement of quality conformance is available for all products. Where these fall within the scope of Dart Sensors� registration to BS EN ISO 9002, the statement will be in accordance with the conditions of registration and incorporated in the customer's despatch note. Liability

Dart Sensors shall not be under any liability for damage, loss or expense resulting from the failure to give advice or information or the giving of incorrect advice or information whether or not due to Dart Sensors� negligence or that of its employees, agents or sub-contractors.

These conditions set out Dart Sensors� entire liability in respect of the products and Dart Sensors� liability under these conditions shall be in lieu and to the exclusion of all other warranties, conditions, terms and liabilities express or implied statutory or otherwise in respect of the quality or the fitness for any particular purpose of the products or otherwise howsoever except any implied by law which by law cannot be excluded. Save as provided in these conditions and except as aforesaid Dart Sensors shall not be under any liability, whether in contract, tort or otherwise, in respect of defects in the products or failure to correspond to specification or sample or for any injury, damage or loss resulting from such defects or failure.

In no event shall any breach of contract on the part of Dart Sensors or tort (including negligence) or failure of any kind on the part of Dart Sensors or that of its employees, agents or sub-contractors give rise to any liability for loss of revenue or any consequential or indirect loss or damage arising from any cause whatsoever. Dart Sensors� liability (if any) whether in contract, tort or otherwise in respect of any defect in the products or of any duty owed to the Customer in connection herewith shall be further limited in the aggregate to �500,000.

1. Credit terms (subject to satisfactory references) are available. Payment is due within 30 days of receipt of goods. If any sum is not paid on the due date for payment, all sums then outstanding from the customer will immediately become due and payable notwithstanding that such sums would not otherwise be due until a later date.

We understand and will exercise our statutory right to claim interest and compensation for debt recovery costs under the late payment legislation if we are not paid according to agreed credit terms.

2. Cheque with order. Please ensure that cheques are made out to Dart Sensors Ltd. and are crossed. All payments must be made without any set-off, deduction or counterclaim.

Unless otherwise confirmed, nothing in Dart Sensors� literature or web pages is to be taken as a representation of the source of origin, manufacture, or production of the products or any part thereof.

Products must be returned to Dart Sensors in their original condition and in their original packaging within 30 days of delivery (as on the delivery documentation) stating relevant despatch number in order for the customer to be eligible for any refund.

Where products are returned to Dart Sensors after expiry of this 30 day period then Dart Sensors reserves the right to levy a handling charge. Products accepted for return will be credited at invoice value and the appropriate handling charge (if any) applied. If invoice details are not quoted the credit will be based upon the lowest sales price.

All products are returned at the customer's risk and Dart Sensors accepts no responsibility for any loss or damage thereto.

Cancellations or part cancellation of an order can only be accepted after prior negotiation and agreement to terms which will indemnify us against any expense incurred. In the event of part cancellation we reserve the right to invoice for any difference in selling price applicable to the quantity dispatched up to the time of cancellation.

Products are offered for sale subject to any patent, trade mark, registered design, copyright, topography right or other right of any person.

If Dart Sensors is hindered or prevented from performing any contract owing to any cause beyond the reasonable control of Dart Sensors or by its inability to procure services, materials or articles required for the performance of the contract except at enhanced prices, Dart Sensors may at its sole option delay the performance of, or cancel the whole or any part of the contract, and Dart Sensors shall not be held responsible for such delay or cancellation or any inability to deliver. In particular, although Dart Sensors will use all reasonable endeavours to deliver back orders by the date stated by Dart Sensors, Dart Sensors shall not be held responsible for any delay in the delivery of, or inability to deliver, such orders.

All contracts between Dart Sensors and the customer shall be governed by and interpreted in accordance with English Law and the customer submits to the jurisdiction of the High Court of Justice in England, but Dart Sensors may enforce any such contract in any court of competent jurisdiction.

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Table of Contents

Quality