BS EN 15751 Automotive fuels - Fatty acid methyl ester (FAME) fuel and blends with diesel fuel - Determination of oxidation stability by accelerated oxidation method
9 Measurement
9.1 Set up the apparatus as shown in Figure 1. If commercially available equipment is used, follow the manufacturer's instructions.

9.2 Attach the membrane pump (6.1.2) and adjust the air flow to exactly (10 +/- 1) l/h. Switch off the pump. Dedicated instruments are usually equipped with automatic flow control.

9.3 Bring the heating block (6.1.8) to a temperature such that the required test temperature (usually 110 °C, but see 8.2.2) is reached in the test tube(s), using the thyristor and the contact thermometer (6.1.7) or by using an electronic temperature controller. The temperature shall be kept constant (+/- 0.1 °C) during the test period (see also 8.2.2).

If a heating bath (6.1.8) is used, heat to the desired temperature and control the temperature according to 8.2.2.

9.4 Fill the measurement cells (6.1.4) with 60 ml of distilled or demineralised water using a measuring pipette (6.3).

9.5 Check the electrodes (6.1.5) and adjust their signals to the zero axis of the recorder paper, using a calibration potentiometer.

Set the paper feed to 10 mm/h and the measuring frequency to one acquisition per 30 s. Set the measuring value of 200 µS/cm at the maximum result of 100 %.

If it is not possible to adjust the paper feed to 10 mm/h, use 20 mm/h. This shall be reported on the recorder paper.

NOTE Automatic oxidation stability analysers might be able to collect the data via a computer system.

9.6 Weigh (7.5 +/- 0.1) g of the conditioned sample (see 8.1 ) into a reaction vessel using a pipette (6.3).

9.7 When the test temperature is reached, switch on the membrane pump (6.1.2) and set the air flow to exactly (10 +/- 1) l/h. Connect the air inlet tubes and outlet tubes to the reaction vessels and the measurement cells, using the connecting hoses (6.5).

9.8 Place the reaction vessel with the sealing cap (6.1.3) into the corresponding hole in the heating block or into the heating bath (6.1.8).

The preparation steps 9.7 and 9.8 shall be carried out as fast as possible. Then immediately start the automatic data recording or note the start time on the recorder paper.

9.9 The measurement may be terminated:
- when the signal has reached 100 % of the recorded scale, usually 200 µS/cm, or
- when the curve levels after reaching the inflexion point (see Figure 3), or after 48 h of testing time.

Care should be taken to not stop the test too early to ensure the calculation of an accurate second tangent.

9.10 During the determination, check the following parameters:
a) The setting of the flow meter. Adjust where necessary in order to ensure a constant flow;
b) The colour of the molecular sieve (6.6) of the air filter. Repeat measurements when the molecular sieve changes colour during the test. It is recommended to exchange the molecular sieve prior to each run.

NOTE 1 At temperatures above approximately 25 °C, volatile carboxylic acids can evaporate from the measurement cell. This may lead to a decrease of the conductivity of the aqueous solution, thus causing significant deviations of the conductivity curve (see [6]).

NOTE 2 A rapid conductivity increase immediately after starting the test and before reaching the induction period may indicate insufficient cleaning of the sealing caps or connecting hoses (evaporation of residual volatile compounds from the elastomers) (see Figure 4). The cleanliness can be validated according to the procedure given in Note 1 in 8.2.1. Also fuels that contain volatile acids can unexpectedly show a rapid initial conductivity increase (see Figure 5).

10 Calculation and evaluation
10.1 Automatic evaluation
The automatic evaluation as given by the equipment manufacturers may be used if the second derivative of the conductivity curve shows a clear maximum. This is generally the case if pure FAME and diesel/FAME blends with a FAME content equals or higher than 10 % (V/V) are investigated (see Figure 6, upper diagram).

If the second derivative of the conductivity curve is noisy and no clear maximum can be recognised, the manual evaluation (10.2) of the conductivity curve itself shall be applied (see Figure 6, lower diagram).

NOTE Software settings are recommended that permit simultaneous display of the conductivity curve and its second derivative in order to enable the operator to check the automatically calculated value for the induction period.

10.2 Manual evaluation
Set the first tangent to the flattest part of the slowly increasing conductivity curve. Great care shall be taken to fit the best possible tangent line, e.g. by using an enlarged presentation of the original graph. Some instruments supply a zoom-function to accomplish this. The second tangent is set after exceeding the inflexion point at the steepest part of the conductivity curve (see Figure 3).

The induction period is obtained from the intersection point of both tangents.