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    The sense and nonsense on Emission testers

    This article aims to dispell some of the myths that seem to be around emission testers.



    Article No. 33 Page 3/3

    		AVL 4000 CITROEN OEM testerAVL 4000 RENAULT OEM tester click for more info
    ...Sales people

    A short warm-up time is for most sales people a major selling point, so needs attention. If one tester warms up faster than the other it only means that that machine is made less precise or in some cases that the electronics around the bench are less stabilised, so it will never reach the stable readings like the more expensive tester. Don't worry about my bias as AECS is selling both ends of the spectrum and has had ample opportunity to compare and test many different brands of testers. For AECS a long warm-up time is even a selling feature!

    The tester is meant to be switched on first thing in the morning and switched of at night, the tester's pump will switch it self of after a timed period of non use so it won't wear unnecessary. The heat of the tester's internals will help keep the printed circuit boards clean and dry, so no destructive aluminium or copper corrosion will occur inside the machine.

    O2 and NOx cells

    You might have noticed that with the infra red test bench only CO, HC and CO2 are being measured, you are correct.
    Oxygen and NOx are measured at the outlet of the gas bench by 2 separate galvanic cells.
    A galvanic cell works extremely simplified like this:
    As soon as for example Oxygen enters the O2 cell two metal plates start to 'corrode' this chemical reaction creates a voltage which is representative of the amount of oxygen entering the cell. The 'corrosion' will eat up the two metal plates (Anode and Cathode) as soon as the sensor has been taken out of its package, during testing, when the machine is switched of and when the open package is lying on the shelf. Think of it as a battery. A normal O2 cell has a life span of between 12 and 36 months. Replacing the O2 cell is part of normal ongoing maintenance (availability and cost of the cell are important). An average O2 cell is around $200.

    Schematic illustration of an Oxygen Sensor as used in almost all emission testers.

    The same applies roughly for the NOx cell, with as difference it has a life span between 6 and 12 months and they are much more expensive than O2 cells (around $1200 !).
    A NOx measurement is only useful with the car on a chassis dynamometer, and than it will still only really reveal if the EGR is working or not.

    Diesel contamination

    Diesel molecules are large and circular, they are almost made to cling to anything. They will be clinging on to the internals of the IR gas bench if a Diesel vehicle is measured with a petrol tester.
    The internals of the gas bench are not cleanable, so if a whiff of Diesel fumes enters the IR gas bench the lenses will become opaque and there for permanently affect the petrol emission measurements. The gas bench can now only be replaced. In many cases is one error enough! A replacement IR gas bench will cost around $5,000 (!) of course plus the cost of installing and calibrating!

    Periodical value calibration

    As discussed the Zero calibration is performed by the machine itself every time you start up and during measuring, to keep its accuracy.
    A value (span and offset) calibration needs to be performed every 12 months which is in most countries a legal requirement. The calibration needs to be preformed with dedicated certified calibration gas with known values. The calibration costs are variable from equipment supplier to supplier and are a recurring cost. Some 'equipment suppliers' do not have the capability to perform the calibration in house.
    On average is the cost of a 4 gas calibration around $300, depending on what type of machine you have. A modern digital controlled machine is easy, some older analogue machines take a lot longer twisting and turning potentio meters.
    Please note that it is impossible to tell from the outside of the machine if it is digital or analogue!
    All machines have a dedicated method of calibrating. The calibration method is only available through the equipment supplier or manufacturer.
    The supplier will often receive service news letters on where to watch out for and what has been changed in the method. Non recognised or generic calibration outfits often miss out on documentation like that at the customer's expense.

    Diesel emission testing is a whole different issue, but with similar pitfalls. More on Diesel emission testers in the next article.
    AECS equipment supplies and services AVL and MIAC emission testers.

    AECS is a nation wide automotive diagnostic training provider (click for traininjg information).


    AVL emission tester in dyno lab set-up.


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