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Unbiased sampling, a matter of precision and maintenance
There are no miracle solutions to produce unbiased and precise samples. Good maintenance and the use of multiple vezins are the keys to success. Here are some valuable tips about sampling and maintenance to increase sample precision.
Principles of operation using a two-in-one sampler
Let’s start with a basic explanation of how slurry sampling works using this two-in-one sampler.
The slurry is sampled first by the cutter arm, which collects between 20 and 50 litres of slurry.
As a second step, you can always add an extra vezin to reduce the sample by 4% to 16%, resulting in an ergonomically sized sample suitable to your handling capabilities.
Rejects from the first cut flow back into the stream, avoiding sampling loss. For high volume applications, a third vezin stage is included to ensure adequate sampling of the process.
Specifications for precise and unbiased sampling equipment
According to the theory of sampling introduced and developed by Pierre Gy in the 1970s and 1980s, there are five specifications to ensure each particle has the same probability of being sampled. These can be summarized as follows:
- Cut the near-vertical stream entirely and perpendicularly.
- Provide adequate and constant cutter speed.
- Prevent particles from bouncing from one blade to another by ensuring cutter blades are sharp.
- Have the capacity to capture all samples without loss or contamination from previous samples.
- Ensure that the cutter blade distance is equivalent to at least three times the size of the largest particles and is constant for linear cuts or tapered for radial cuts.
Tips and tricks for sampler maintenance
A weekly checklist
Ensure that your operators inspect the sampler once a week to ensure that cutter blades are:
- Free of obstructions
- In place
- Sharp
Gearbox oil levels should also be checked once a month.
Parking positions of the cutter
The parking positions of the cutter’s primary slurry sampler must be set properly and checked periodically. Otherwise, the sampler can damage the equipment and lead to sample contamination. For example, a small amount of slurry could be introduced into the cutter arm if it is set at a parking position that is too close to the main slurry flow entry pipe.
Also, make sure your two parking positions are set 5 to 10mm from the side of the sampler housing. Parking positions are set by adjusting the cams of the proxy cam box.
Avoid contamination
One of the keys to obtaining representative samples is to avoid contamination between sampling intervals. By sample, we mean the cumulative increments (cuts) that make up the sample at the end of a time period (e.g., an 8-hour shift).
Be sure to wash all the cutters between sample bucket change-outs. This can be done by activating the wash water system if your auto sampler is equipped with one, or simply by using a hose to wash the cutters through the grill of the breather doors.
If you need more information about sampling maintenance or processes, contact us!