How to Choose the Right Cleanroom Particle Counter
Cleanroom particle counters differ widely based upon particle threshold, cost, and features. How to do you determine if you need a particle counter or what model of particle counter to purchase? In this article we will go over the basics of particle counters and give you the facts to help you determine which one is best for your cleanroom application.
What is A Particle Counter?
A particle counter is an instrument that detects and counts particles one at a time. There are three main categories of particle counters, aerosol, liquid and solid, but only aerosol particle counters directly relate to cleanrooms, so we will be focusing on them. Aerosol particle counters determine the air quality by counting and sizing the particles in the air. They are mainly used in cleanroom applications. Since cleanrooms have defined particle count limits, aerosol particle counters are used to test and classify the cleanroom to ensure it is up to standard.
Types of Particle Counters
There are two types of aerosol particle counters, optical and condensation. With optical particle counters, the particle passes through a high energy light source, then the amplitude of the light scattered or blocked is measured and the particle is counted. Optical particle counters have several different ways to detect and measure particles. They are light blocking, light scattering, and direct imaging. The light blocking method can detect particles larger than a micrometer in size and is based on the amount of light the particle blocks when passing through the light source. The light scattering method is capable of detecting smaller size particles and is based on the amount of light that is redirected by the particle passing through the light source. Direct imaging uses light emitted by a laser as a source to illuminate the particles passing through and measure the area of the particles, like an automated microscope.
What is a Condensation Particle Counter (CPC)?
A condensation particle counter counts aerosol particles by first enlarging them and using them as nucleation centers to create droplets in a super saturated gas. Condensation particle counters can detect particles as small as 2nm, which cannot be detected with optical particle counters. Particle counters come in many different sizes. There is a hand-held form, which is a small self-contained model that is easily transported and used. Hand-held particle counters tend to have a lower flow rate, so they are ideal for cleanroom spot checking and certification of lower volumes of air. Larger portable units have a fixed location and continuously monitor the air 24/7. These portable units have a higher flow rate and are ideal for sampling larger volumes of air.
Things to Consider When Selecting a Particle Counter
The first is the size of the particles that you need to monitor. This is determined by the ISO class of your cleanroom. Make sure the particle counters you are looking at can measure all the particle sizes that you need to monitor. For example a particle counter can have a size range of 0.3/0.5/5.0 µm.
Particle Counter Flow Rate & Location Sampling
The second factor is the flow rate capacity of the particle counter. To do this you will need this formula: Vs = 20/Cnm) x 1000. Vs is the minimum single sample volume per location. Cnm is the class limit for the largest considered particle size specified for the class of your cleanroom. 20 is the defined number of particles that could be counted for class of the cleanroom. This calculates the number of liters of air that need to be sampled in your cleanroom. So for example if you want to have a class 4 cleanroom, the largest size particle is 1.0 µm and can have a maximum concentration of 83 particles/ m3 according to the ISO 14644-1 cleanroom standards. Plug it into the formula for Vs=(20/83) x 1000, and Vs=240.96. So we would need to sample 240.96 liters of air. So if we chose a small hand-held particle counter that had a flow rate of 2 liters per meter that would take about 120 minutes or 2 hours to sample the air.
So for this example I would recommend finding a larger portable particle counter with a higher flow rate.
