Lab Experiment: Build Simple Coulter Counters to Count Particles
More than 50 years ago, brothers and engineers Wallace and Joe Coulter designed an automated method to count particles in solution. Today, many industries take advantage of their invention, such as this scientist on a ship who uses a Coulter counter instrument to count the phytoplankton in ocean water samples.Copyright Kathryn Hansen, NASA http://www.nasa.gov/topics/earth/features/icescape2010_plankton.html
Students build and use a very basic Coulter electric sensing zone particle counter to count an unknown number of particles in a sample of "paint" to determine if enough particles per ml of "paint" exist to meet a quality standard. In a lab experiment, student teams each build an apparatus and circuit, set up data acquisition equipment, make a salt-soap solution, test liquid flow in the apparatus, take data, and make graphs to count particles.
The electrical sensing zone method of particle characterization, also known as the Coulter principle, was proposed by electrical engineers and brothers Wallace and Joseph Coulter in the 1940s. Their Coulter Counter Model A instrument automated the counting of red blood cells and ushered in modern hematology. Today, many industries use modern versions of their instrument to analyze and measure samples of particulate material that can be suspended in an electrolyte solution, such as medicines, pigments, fillers, toners, foods, abrasives, explosives, clay, minerals, construction materials, coating materials, metals, and filter materials.
After this activity, students should be able to:
- Describe qualitatively how a particle changes the resistance between two electrodes as it moves through an aperture and how that resistance is measured.
- Describe the basic functioning of a Coulter counter, how the original Coulter counters improved blood cell counts, and how these improvements benefited patients and medical staff.
- Explain a number of ways that the Coulter counter is used in science and industry today.