BS EN 15751 Automotive fuels - Fatty acid methyl ester (FAME) fuel and blends with diesel fuel - Determination of oxidation stability by accelerated oxidation method
7 Sampling
Unless otherwise specified, sampling shall be conducted according to EN ISO 3170 or EN ISO 3171 and/or in accordance with the requirements of national standards or regulations for the sampling.

It is important that the laboratory receives a sample which is truly representative and has not been damaged or changed during transport and storage.

Store the sample in the dark at about 4 °C and measure it as soon as possible after receipt.

8 Preparation of measurement
8.1 Preparation of test sample
In order to ensure a consistent test condition, all samples shall be treated in the way described below:
- Take the required quantity from the centre of the carefully homogenised sample using a pipette.
- Analyse the samples immediately after sample preparation.

8.2 Preparation of apparatus
8.2.1 Cleaning procedure
NOTE 1 The use of new disposable reaction vessels, air inlet tubes and connecting hoses is recommended in order to save the cleaning procedure.

Sealing caps, measuring cells and electrodes shall be cleaned with 2-Propanol in order to remove organic residues. The connecting hoses should also be washed in the same manner if not replaced.

Rinse with tap water and finally with demineralised or distilled water. Dry the cleaned parts in an oven at 80 °C for at least 2 h. The temperature may not exceed 80 °C due to elastomer stability.

NOTE 2 The drying time of at least 2 h assures that solvent adsorbed by the elastomers is removed completely.

In case of reuse, purge the empty reaction vessels and the air inlet tubes at least three times with ternary solvent mixture (5.1) in order to remove residual fuel and adherent organic ageing residues. The last solvent portion should remain colourless.

Rinse with 2-Propanol and tap water. Put the inlet tube into the reaction vessel and fill completely with an aqueous alkaline laboratory cleaning solution.

Store the vessels at room temperature overnight.

Rinse the purified vessels and their inlet tubes thoroughly with tap water and finally with demineralised or distilled water. Dry them in an oven for at least 2 h at 80 °C.

In case of doubt, the cleanliness of the sealing caps and connecting hoses can be checked by running a blank sample under standard test conditions. In this case the conductivity increase shall not exceed 10 µS/cm within 5 h.

8.2.2 Temperature correction
8.2.2.1 General
Any deviation from the test temperature in the test vessel has a significant impact on the result. In order to ensure that the correct measurement temperature is used, the difference between the temperature of the sample and the temperature of the heating block, ΔT, needs to be determined. For this determination, a calibrated external temperature sensor is used.

The temperature correction always needs to be conducted when the test is carried out at a different temperature than before.

8.2.2.2 Procedure
Switch on the heating block and wait until the target temperature is reached and is stable.

Fill one reaction vessel with 5 g thermo-stable oil. Insert the temperature sensor through the cap into the reaction vessel. Use distance clips to keep the sensor away from the air inlet. The sensor should touch the bottom of the vessel.

Insert the complete vessel into the heating block and connect the air supply.

If the value of the measured temperature is constant, calculate ΔT:
ΔT =Tblock - Tsensor
where
ΔT is the temperature difference between heating block and sample;
Tblock is the temperature of the heating block;
Tsensor is the measured temperature in the reaction vessel.

Adjust the temperature of the heating block according to Formula (2):
Tblock = Ttarget + ΔT
where
Ttarget is the intended measurement temperature.

EXAMPLE Ttarget is 110 °C. If a ΔT of +2 °C is determined, the temperature of the heating block has to be set to 112 °C.

After this temperature correction, the measured temperature in the reaction vessel should be equal to the target temperature.