| Particle Number Density Meter: This meter measures the number density of gas particles. You must attach the number density meter to an orange hotspot on the Ultrascope (see Ultrascope View (Diffusion)). The number the meter displays roughly equals the number of gas particles of the type being measured that are visible in the Ultrascope. In the Properties Box of the number density meter, you can choose to measure the numbers of small, medium, or large particles, or particles that are colored blue. See the container help page for details. You must place an number density meter to plot number density using the graph tool.