Stability Measurement – FAQs

The induction time measured using the Rancimat cannot be used to directly calculate shelf life.

An important reason for this is that the experiments are structured differently. Measurements using the Rancimat are generally made at temperatures between 100 °C and 130 °C. In addition to accelerating the oxidation process, these temperatures can cause side reactions which do not occur when samples are stored under normal conditions. For example, volatile antioxidants may be driven out or antioxidant compounds may form (e.g., Maillard reaction). Furthermore, in measurements with the Rancimat, air is continuously fed through the sample, whereas it is generally stored in closed containers.

Consequently, the induction time determined using the Rancimat only gives an indication which must be correlated with the actual shelf life of a sample using storage tests.

The relative standard deviation of determinations with the Rancimat tends to be between 5 and 10%. The following points should be observed to keep scattering of the measurement results to a minimum:

• The sample must be homogeneous.
• Only use new reaction vessels and air tubes.
• Always position the air tube in the same way in the reaction vessel. See What do I need to keep in mind when positioning the air tube in the reaction vessel?
• Before weighing in the sample, remove any particles that may have adhered to the interior and exterior of the reaction vessel. See Preparation of the measuring vessel.
• The reaction vessel cover must form a seal with the reaction vessel and must not show any cracks.
• The reusable accessories must be cleaned thoroughly. Furthermore, the reaction vessel cover and connection tubing must be heated in the oven. See Cleaning after the determination.

A loud pump noise indicates that a large pump capacity is required to achieve the set gas flow. This can be caused by either a leak or a blockage in the feed lines to the pump. You should keep track of the cause, as it increases wear and therefore reduces the pump service life in the long run.

Possible leaks:

• Tubing connections on the rear of the instrument are not screwed tightly enough. Tighten connectors securely.
• One of the tubing connections on the rear of the instrument is defective. Replace tubing connection.
• The cover of the drying flask is not tightly screwed. Screw the cover on tightly.

Possible blockages:

• The dust filter on the rear of the instrument is clogged. Replace filter, see also Regular maintenance – Dust filter (6.2724.010).
• The filter tube (6.1821.040) in the drying flask is clogged due to dust from the molecular sieve. Remove dust by tapping out the filter tube. Replace the aspiration filter (6.2821.090), if necessary.

If the sample amount ranges between the minimum and maximum amount possible (see What is the smallest and what is the largest sample amount that I can use for the determination?), an error of ±10% of the sample weight is irrelevant.

There should be at least enough sample in the reaction vessel so that the air tube is still submerged in the sample at the end of the measurement. That is the case for a volume of approximately 2 mL and above. There should only be as much sample in the reaction vessel so that the entire sample fits in the heated section of the heating block. That is the case for a volume of up to approximately 12 mL. In most applications, between 3 and 7.5 g of the sample are used.

If the sample cannot be weighted directly into the reaction vessel, it must be melted carefully in the water bath beforehand.

The temperature of the water bath should only be slightly above the melting point of the sample. If the sample is heated too much, it may change in ways that could affect the end result. The sample amount corresponds to the amount that is also used for liquid samples.

Solid samples must be broken up before they are weighed into the reaction vessel. The particle size should be as uniform as possible. Samples of this type are not mixed by the air flow during the determination. Only a small sample amount should be used to avoid a nonhomogeneous temperature distribution within the sample. We recommend selecting the sample amount in such a way that the base of the reaction vessel is covered evenly. A sample amount of 0.5 g is frequently used. We also recommend using shorter air tubes for PVC (140 mm, order number 6.2418.120) with solid samples.

The exact volume of the measuring solution is less significant when determining the induction time, as only the relative change in conductivity is evaluated.

Accessories
The following accessories are generally used to determine the oxidation stability of plant oil and other plant and animal fats and oils:

• 6.1429.040 Reaction vessel short (150 mm)
• 6.2418.100 Air tube (148 mm)
• 6.1428.107 Measuring vessel (polystyrene)
• 6.1816.010 Silicone tubing

All the accessories required for more than 100 determinations are included in the scope of delivery of the 892 Professional Rancimat.

Standard method parameters
Sample amount	3 g
Measuring solution (water)	60 mL
Sample temperature	120 °C
Gas flow (air)	2 L/h
Evaluation	Induction time Automatic (2nd derivative) Optional (manual): tangential method

Accessories

The following accessories are generally used to determine the oxidation stability of biodiesel (B100) and biodiesel blends:

• 6.1429.050 Reaction vessel long (250 mm)
• 6.2418.130 Air tube long (248 mm)
• 6.1428.030 Measuring vessel (glass)
• 6.1839.000 Iso-Versinic tubing

All the accessories required for 100 determinations are included in the scope of delivery of the 893 Professional Biodiesel Rancimat.

Standard method parameters
Sample amount	7.5 g
Measuring solution (water)	60 mL
Sample temperature	110 °C
Gas flow (air)	10 L/h
Evaluation	Induction time Automatic (2nd derivative) Optional (manual): tangential method

Accessories

The following accessories are generally used to determine the thermostability of PVC:

• 6.1429.040 Reaction vessel short (150 mm)
• 6.2418.120 Air tube PVC (140 mm)
• 6.1428.107 Measuring vessel (polystyrene)
• 6.1816.010 Silicone tubing

All the accessories required for more than 100 determinations are included in the scope of delivery of the 895 Professional PVC Thermomat. 

Standard method parameters
Sample amount	0.5 g
Measuring solution (water)	60 mL
Sample temperature	200 °C
Gas flow (air)	7 L/h
Evaluation	Stability time Conductivity change 50 µS/cm

The temperature is the measuring parameter that has the greatest effect on the end result (induction time, stability time). If the temperature is not prescribed by a standard, the sample temperature should be selected in such a way that the result lies between 4 and 10 hours.
As a general rule:

• If the sample temperature is increased by 10 °C, the induction time halves.
• If the sample temperature is decreased by 10 °C, the induction time doubles.

Temperature correction serves to compensate for cooling caused by the gas flow and to offset heat lost when heat is transferred between the heating block and the reaction vessel. 

This value can either be taken from a table or can be determined using the software wizard «Determine temperature correction ...» using the optional accessory for determining temperature correction (6.5616.100 or 6.5616.110). 

We recommend that you determine the precise temperature correction value, especially if results from different instruments are to be compared. Detailed information on determining the temperature correction can be found in the software help and the instrument manual.

The gas flow does not have a significant effect on the end result as long as it is ensured that the sample can be mixed well and the cooling effect is compensated for by correctly determining the temperature correction.

• AOCS Cd 12b-92 «Sampling and analysis of commercial fats and oils: Oil Stability Index»
• ISO 6886 «Animal and vegetable fats and oils – determination of oxidation stability (accelerated oxidation test)»

• EN 14112 «Fatty acid methyl esters (FAME) – Determination of oxidation stability (accelerated oxidation test)»
• EN 15751 «Automotive fuels – Fatty acid methyl ester (FAME) fuel and blends with diesel fuel»
• EN 16568 «Automotive fuels – Fatty acid methyl esters (FAME) fuel and blends with diesel fuel – Determination of oxidation stability by rapidly accelerated oxidation method at 120 °C»

• ISO 182 part 3 «Tendency of PVC to evolve HCl at elevated temperatures – Part 3: Conductometric method»
• DIN 53381 «Thermostability of PVC (dehydrochlorination method)»

The reaction vessels and air tubes are disposable and should not be cleaned and reused. This is because:

• Removing sample residue is extremely time-consuming and therefore costly.
• Cleaning procedures that are typically applied damage the glass. The altered glass surface impairs the reproducibility and accuracy of the results.
• Poorly cleaned reaction vessels lead to scattered and, in some cases, even incorrect results.

The reaction vessel cover can be reused but must be cleaned after each use (see Cleaning after the determination – Reaction vessel cover (6.2753.107)).

The cover must be replaced if it has cracks or no longer forms a seal with the reaction vessel when closed.

During measurement, sample components may be absorbed by the plastic. This occurs in particular when measuring biodiesel and can cause the cover to expand. An important measure for counteracting this is to heat the cover after cleaning (see Cleaning after the determination – Reaction vessel cover (6.2753.107)). If heating the cover still does not enable a seal to be formed with the reaction vessel when it is closed, the cover must be replaced.

The conductivity of the deionized water should not exceed 20 µS/cm. If the value measured at the start of the measurement is higher than this, it is indicative of a problem with the conductivity measuring cell. This is generally caused by the electrode being contaminated with sample residue. To avoid this, the measuring vessel and the conductivity measuring cell should be cleaned thoroughly after each determination (see Cleaning after the determination –  Measuring vessel (6.1428.107, 6.1428.030) and Cleaning after the determination – Measuring vessel cover (6.0913.130)).

The cell constant of the conductivity measuring cell used in the Rancimat lies between 1.0 and 1.2 cm^-1. For measurements in which the induction time is to be determined, more precise calibration is not required, as only the relative change in conductivity is evaluated in the case of the induction time.

If, however, the stability time is to be determined, in which the absolute change in conductivity is evaluated, we recommend that you determine the cell constants. You can find more detailed information on working with calibrated conductivity measuring cells and determining the cell constants in the manuals for your instrument and the software used.

You must ensure that the PTFE tubing does not feed the air directly onto the conductivity measuring cell. If gas bubbles are released onto the electrode, then this results in noisy measurement curves.

If the air tube is always positioned in the same way, the reproducibility of the results improves.

	Fitting without foam barrier If you do not use a foam barrier, we recommend that you position the air tube in such a way that the bottom end does touch the glass wall of the reaction vessel.
	Fitting with foam barrier (6.1451.010) If you use a foam barrier, the position of the air tube is fixed. Note! You must ensure that the foam barrier is fitted in the upper half. The foam barrier can melt if it is too deep in the heating block during the measurement.