An all-levels sample can also be taken, with the same proviso that it is between 70% and 85% full when withdrawn from the liquid. Spot samples are those samples taken from a specific location in a tank as shown in the below diagram or from a pipeline.
Under no circumstances is it allowable to decant a Vapour pressure sample from one container to another under field conditions. The sample must be taken in the container that it will be transported to the laboratory in, and because of this, certain types of sampler, such as beaker samplers, zone samplers, bottom samples, flow-through samplers, etc. must not be used. It is also extremely important that the cap used makes a Vapour-tight seal with the bottle (or can). Plastic caps with polyethylene inserts are preferred. Sampling equipment must first of all be clean; and, properly maintained to obtain good samples. In addition, sample cords must not contain residue from incompatible material.
11.1 Sampling for Vapour Pressure Analysis
One of the most common types of vapour pressure test is the Reid Method and this is often referred to as taking Reid vapour pressure [Rvp] samples. There are other vapour pressure test methods; however, the sampling requirements are the same Sample handling is critical when taking vapour pressure samples; mishandling of samples is the most common cause of non-representative samples when testing for Vapour pressure. Most of the requirements for vapour pressure sampling are directed toward preventing the loss of vapours, which will result in erroneous low analytical results.
11.1.2 Types of Samples
Spot samples will be full when pulled from the liquid. Quickly pour out approximately 20% of the liquid, cap the bottle and invert it. Composite spot samples must not be used as it is not allowable under any circumstances to decant a vapour pressure sample from one container to another under field conditions.
11.1.1 Equipment
The most common type of Vapour pressure sample is a running sample; however, spot samples can also be taken. A running sample is obtained by lowering an open (un-stoppered) sampling device to the bottom of the outlet suction level, but always above any free water, and returning it to the top of the product at a uniform rate such that the sampling device is between 70% and 85% full when withdrawn from the product.
Vapour pressure samples are usually taken in glass bottles using a cage sampler or a lead weight and a cord, as shown in the diagram.
11.1.3 Closed System Sampling
Cork detail
Closed system sampling can be problematic for vapour pressure samples because most of these systems require some form of decanting in the field, which is not allowed.
The sample must be removed from the liquid between 70% and 85% full or discarded.
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