The *Sigma* value is the count of all particles from that specific size channel, and greater, combined. The *Delta* readings are the values for the size channel named, up to but not including the next largest size channel.

For example, if your channels are set to 0.3, 0.5, 1.0µm, etc. the cumulative (sigma) value for the 0.3 µm channel represents the count of all particles 0.3um and larger. The differential (delta) value for the 0.3um channel represents the count of all particles larger than or equal to 0.3um but smaller than 0.5um. Likewise, the sigma value for the 0.5um channel represents the count of all particles 0.5um and larger, while the delta value for the 0.5um channel represents the count of all particles larger than or equal to 0.5um and smaller than 1.0um.

For example, if the display for the 0.5µm particle channel reads 1.23 E 7 this value would mean to move the decimal point 7 places to the right for the value of the measurement equal to 12,300,000. This is typically only seen in the /M3 (counter per cubic meter) measured value mode.

**P311 Particle Counter**

Allows for the additional indication of standard deviation, average, and 95% UCL. The user must calculate and determine the minimum number of sampling locations and time of sample at each location.

**P611 Particle Counter**

Allows for the same indication as the P311 with standard deviation, average, and 95% UCL. However, the user can input the area of the cleanroom or critical environment to be measured, and the meter will calculate the minimum number of sampling locations and the amount of time for each sample point.