GAS DETECTION Cost of Ownership In today's competitive marketplace the initial cost of any capital equipment is important to the end user but of equal importance is the ongoing cost of ownership. In many cases gas detectors are purchased for an immediate safety, operational or legislative need, but little thought is given to keeping them operational and within performance requirements for their life. There are 3 main costs. a) Calibration or performance testing b) Maintenance and replacement costs c) Loss of product or replacement when units are being serviced All of these are factors that need to be considered by the user. If an instrument is to be used to make measurements upon which some action is to be taken, then it is important that it operates with certain accuracy limits. If, for example, an operator has to evacuate a confined space when the Oxygen level is below 16% or 18% then it is important that this measurement is as accurate as possible, otherwise life may be at risk. Similarly if the flammability of the atmosphere is above a particular level then there is a danger to the user. There is, therefore, a need to ensure that the instrument performance remains consistent between checks.
Although the level in some cases may not be the absolute judge since the change in reading may be equally important, ie falling Oxygen levels indicate a worsening situation as does increasing flammability levels. There has to be some absolute level to give guidance to the user. Instruments have to be calibrated or tested at regular intervals. If you operate an ISO9000 quality system, test instruments which perform measurements upon which judgements are made have to be calibrated, normally traceable to national standards. Gas detectors are similar. The simplest way is to test the unit regularly in the field with a certified test gas to establish a basic performance level. This can be done before each test, daily, weekly, monthly etc depending upon the risk involved and the performance of the instrument. The simplest test is just a reaction test to see if the instrument works. This could be breathing into an Oxygen instrument to ensure that the O2 level falls or similarly with a CO2 instrument to ensure that the CO2 level rises. We breath out around 4% CO2 so both CO2 and O2 readings are affected. With a flammable instrument the application of the gas from the mains or a lighter will give some indication that the basic unit functions. Apart from time there is little cost and it provides a basic function test but it does not constitute a performance test and is certainly not a calibration. The next level up is to perform a 'bump' test or basic level check. This can be done by the user with some basic instruction and requires the use of a test gas or test gases. Gas is applied to the instrument under controlled conditions and the level of response noted and compared against some agreed criterion. Test gases are available from a number of suppliers with different accuracies. In a bump test the accuracy of the gas is probably the least important since other factors affect the
instrument performance, such as temperature, pressure, skill of the operator etc. Hence in bump testing wide parameters are used. A typical gas mixture may have an accuracy of say 2% and could be applied to an instrument with an accuracy of say 3% of Full Scale. This may vary depending upon the ambient temperature and other factors. If the gas is around the mid point of the instrument scale, then depending upon the actual specification of the instrument you may get a variation similar to that illustrated below. Gas = 50%LEL ± 2% ie 48%LEL to 52%LEL Instrument accuracy ± 3%LEL ie 48-3 to 52 + 3 = 45% to 55%LEL With no other factors involved this may increase to around 40% - 60%LEL in a practical situation. Provided reasonable limits are used and there are clear instructions, a bump test is a useful tool to ensure calibration but it does add cost in providing the gas and having basic training. If the instrument is calibrated in the field then the same gas can be used. You have to rely on the operator to carry out the calibration and do it regularly and with some skill. Modern gas detectors can indicate to the operator when a calibration is due either by flagging due date or by indicating when a calibration fault has occurred. Many gas detectors have built-in field calibration which automate the process and have built-in safeguards against gross operator error. In many cases they can be pre-programmed to specific gas mixtures and provide internal records of the calibration. Multimix gas reduce the number of cylinders that are required and help reduce cost. Automatic field calibration systems operated by computer based systems are also available to ensure the correct calibration of instruments in the field and can be used to extend the calibration period based on real data. If you find that with, say, monthly calibration checking in a particular application there is no basic change in instrument performance this can be extended to 3 months and if the same performance levels occur they can be extended to 6 months, 12 months etc, hence reducing costs based on real
data in your application, rather than some fixed figure decided on by the manufacturer. Many manufacturers have automatic calibration systems with either local computers and data storage or networked systems for large users. The idea is to keep the instruments in use, in the field, whilst still maintaining calibration records and avoiding the return to a specialised repair facility. There are some maintenance costs associated with gas detectors depending upon the application and type of sensors. Flammable detectors using semiconductors may have a life of around 1-5 years, depending on the application and exposure to gas or other compounds. Pellistor based catalytic sensors typically have a life of 5 years+ depending on application. Infra-red sensors have a much longer life typically but the source which could be a lamp type device may have around 3000 hours life. Solid state sources and detectors have a much longer life but still require maintenance if not replacement. Simple modular assemblies with self calibration reduce the skill and cost of replacement. Electrochemical sensors used for Oxygen and Toxic gas measurement such as CO, H2S etc have a typical life of 2-3 years and have to be replaced after this period.
There is increasing use of disposable Oxygen and Toxic Gas detectors which have a life of 2-3 years including batteries and are basically maintenance free but they will need to be tested if the risk warrants it. There is no difference in the need to test, irrespective if the unit is disposable or not. The disposable units do not require batteries and hence this cost is avoided. In other units which do have batteries there is the cost of replacement of either dry cells, usually alkaline, or rechargeable battery packs. The re-chargeable batteries have improved in performance and failure mechanisms with new type batteries, such as Nickel Metal Hydride and Lithium Ion batteries, but there is still the capital cost of chargers and the usually hidden cost of power for these, albeit small. In addition to the cost of replacement sensors there are the costs of skilled calibration and maintenance staff who check the product on a regular basis. This is an overhead cost which applies to all gas detectors. The disposable units are affected in the same way, although there is no repair capability, calibration checking may still be required. In the event of failure then the capital cost may be incurred again but you have a new detector presumably with the manufacturer's warranty. All manufacturers are striving to bring down the cost of maintenance and cost of ownership in many ways. With certified equipment there is a requirement to have the equipment checked for compliance with the certification requirements on a regular basis by a skilled and competent person. This is defined in BS EN 60079 and is typically in the order of 2 yearly. CSA, the Canadian Standards Association, insist that any flammable detector approved by them has a statement in the handbook indicating that the flammable detector has to be tested daily or before use, this is a blanket statement irrespective of any performance tests they carry out. The same criterion does not apply to an Oxygen or toxic detector approved by them. This means that there are costs associated with the maintenance and safety of all hazardous area equipment not just gas detectors. If instruments are returned to a competent repairer then they are no longer available in the field for users, hence replacements have to be made available.
If an instrument is sent to a central repair facility on a 6 monthly basis and the turnaround time is say 5 working days in 250, eg 50 weeks x 5 days, the instrument is out of service for routine servicing for around 4% of its working year. If you include for failures this figure may increase to 5% to 10% depending upon application. Thus in the capital cost you may have to include a larger number of units for spares. Thus the ability to keep instruments in the field by providing improved reliability, field calibration and maintenance free units can be a vital availability issue as well as replacement parts costs. Many manufacturers will offer complete packages that ensure users always have a working calibrated unit either by means of replacement units, leasing arrangement or imaginative repair operations such as replacements or field service units. The use of automatic calibration and fault diagnosis on fixed systems using computer based systems linked into SCADA or BMS (Building Management Systems) is increasing and helping to reduce the cost of ownership. The use of gas detectors in many industries is increasing but the costs are reducing in an ever more competitive market. Buyers are no longer just looking at a price but are assessing the total cost of ownership as well as technical features. The world gas detection market is addressing the needs of its customers by ever more innovative ways. In the UK CoGDEM members are working together to ensure there are technical standards that enable users to have a minimum requirement for safe gas detection equipment both portable and fixed, whilst at the same time competing in the marketplace to give individual customers the best options for their particular market at the lowest possible cost without compromising safety.