Sampling can help determine product purity and makeup, whether a product meets customer requirements or specified regulations, whether moisture content is appropriate, or whether chemicals are below their recommended level of volatility. This requires using a sampler that will enter the material stream, capture a quantity of that material, and extract a representative sample for analysis.
Free-flowing solid and powder materials such as pellets, granules, grains, seeds, friable materials (any material that is easily broken apart), and powders must be sampled. For example, for powders used in pharmaceutical products as well as other processes, it is important to know the particle size distribution. If this is not controlled in the production and process, high product rejection rates and significant production losses can result. This particle size distribution can be determined only by taking a representative sample from the process stream.
For all solids and powders, the sampler chosen is determined by the following: material flow rate, the sample amount required, and the material characteristics such as flow behavior, the shape and makeup of the particles, and the tendency of the solid or powder to segregate or group together. In many cases, sampler location constraints can determine which model should be used.
In solid and powder sampling, segregation is an issue, and can get worse as the particle size distribution increases. This segregation is dependent on the flow characteristics of the material and can be significant for free-flowing materials. Here’s how it works: During storage of materials, fine particles vibrate and move into the spaces between larger particles, forcing them to rise over time to the top of the container. This is why samples should not be taken from the surface area of any stored material and why it is important to thoroughly mix stored material and then take a strip sample of the free-flowing materials as they are moving or pouring.
The goal of any sampling program is to obtain representative samples as well as avoid any bias in the selection of that sample or samples. The only way to obtain a representative sample is by sampling the solid or powder while it is in motion.
Ways NOT to sample
Based on the information above, ways NOT to obtain a representative sample include:
Scooping product from the top of a bin or pile. The top layer of a bulk solids process stream generally contains larger particles, while finer particles tend to migrate towards the bottom. This particle segregation or bias causes the scooping process to yield an inaccurate sample and skews the results.
Using a sample thief. This is a device consisting of two tubes, with one fitting tightly inside the other and with oblong holes cut through the tubes in corresponding positions. One end of the outer tube is pointed so it can be inserted into a material. The inner tube is rotated to close the holes, the device is inserted into the material to be sampled, the inner tube is rotated to open the holes so the material can enter the device, and then the inner tube is rotated to close the holes before removing the device from the material. Sounds complicated, right? Although this method is slightly better than simply scooping material off the top of a powder, it still is an inferior sampling technique, because the act of plunging the device through the bulk powder disturbs and biases the sample. Because large particles flow more easily than small particles, the device is likely to be filled with only larger particles.
Coning and quartering. The two above methods take the sample from a static rather than flowing sample, so the sample is not representative. The coning and quartering method attempts to address this, but poorly. A cone of material is poured and is then flattened out, and the material is divided into quarters. Two opposite quarters are discarded, and the remaining two quarters are recombined and poured into another conical pile, which is then again flattened and divided. The quartering process is continued until an amount of material that is the desired sample size remains. This process is inaccurate and still segregates particles by size – with the poorest flowability, finer particles collect at the center of the cone and the coarser particles move toward its edges.
The best sampling method
So, how does one obtain a representative sample of solid and powder materials? The only way is to take a sample from free-flowing material in motion within a process. Additionally, the whole of the stream of powder should be taken for several short increments of time. This type of sample can be taken using an automatic strip sampler, which takes a sample of a strip across the stream of a free-flowing material. Products sampled can range from the previously mentioned pharmaceutical powder to dog food, bird seed, grains and flours, cereal flakes, cement dust and much more.
The wide range of Sentry® solid & powder samplers includes the Sentry RX automatic strip sampler. It can take a representative sample of free-flowing materials from drop chutes, bins, hoppers, pipes, or air slides, as well as from positive or negative, horizontal or vertical dilute phase pneumatic conveying systems. The RX takes a sample when a solenoid-controlled air cylinder extends a sampling tube into the process line. While in line, the tube is continually taking sample and simultaneously discharging it to the collection point. The sample is then analyzed.
The RX sampler is designed to be compact and have low maintenance requirements. The unique design helps eliminate product segregation and degradation through the reduction of moving parts during the sample collection process. This means samples are both consistently drawn and are representative of your product.
Learn more about taking a representative sample and about Sentry solid & powder samplers here.