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When to Use Sample Probes

When to Use Sample Probes(Written By Randy Grater, Service Manager Climet Instruments Company)

It has been called the 'horn', the 'megaphone', the 'funnel', and even the 'big sucky thing'. Occasionally it is called a sample probe.
Officially, it is an isokinetic sample probe.

Isokinetic means that in a laminar flow area, the air flows in and around the probe without creating any turbulence.

To understand why this is important, we need to understand a fundamental difference between micro and macro particles. While 5 m particles may seem infinitely small when we consider that our hair is 70 m to 100 m in diameter, 5 m particles exhibit surprising inertia compared to submicron particles. If one were to sample without a sample probe, 5 m and larger particles would have to be very close to the inlet before they would be pulled over to the inlet. However, the inlet would be surrounded by turbulence, and the turbulent air would repel particles that were being pulled over to the inlet. So, without an isokinetic sample probe, we lose 5 m particles that should be counted, whether we are sampling in laminar flow or turbulent flow.

The isokinetic sample probe has a wide opening that captures air that belongs to the sample without creating turbulence. The 5 micron particles flow into the opening and are then funneled down into the tubing that connects the probe to the particle counter's inlet. Particles belonging to the sample are thus drawn in.

However, due to the inertia of 5 m and larger particles, there is a significant loss of 5 m particles in the transport tubing. Some have tried sampling without a probe, but they are just exchanging a loss of 5 m particles in the transport tubing for a loss of 5 m particles for a lack of a wide enough opening to entrain the 5 m particles; only 5 m particles in a narrow cone above the inlet will enter the particle counter.

Some have used a couple of inches of tubing to place the probe directly over the inlet. While this entrains particles into the sensor, it raises another, more noticeable problem. Whether sampling with a bare inlet or sample with only a couple of inches of tubing to support the sample probe, light has a direct path to enter the sensor cavity if the sensor is directly under a fluorescent light fixture while sampling. This is a problem, because modern ballasts turn the light off and on in the kilohertz range, producing a pulse with a width that is about the same that a particle produces when it passes through the laser beam. This makes it impossible to filter electronically. But we have a solution for that.

Climet gives the customer the choice between the default standard stainless steel probe with transport tubing or a light blocking isokinetic probe that fits directly over the inlet. The probe has an aerodynamic light blocking element that allows air to flow around it, and has been verified against a probe without a light blocking element to verify that the light blocking element does not take out 5 m particles by impaction. The probe with transport tubing is needed in applications where the particle counter cannot go, such as sampling in a clean hood (The particle counter's exhaust would produce turbulence, resulting in high counts, if placed in the hood). For applications where sampling is made from a cart, the light blocking probe is a better choice. For customers with both applications, a second probe can be purchased at a lesser price when the particle counter is purchased.

Probes with transport tubing should be pointed towards the filter when in a laminar flow area. In not laminar flow areas, the probe should be placed at work height and pointed upwards. Pointing the probe upwards allows the particles to float down into the probe. If the probe were in a horizontal position, 5 m and larger particles will float past the probe due to their inertia and never become part of the sample.

For more information, please don't hesitate to contact us today!
Created On  22 Nov 2017 8:30  -  Permalink


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