If properly configured, the remote particle counters in SmartScan can simultaneously provide measurements in both particles/ft³ and particles/m³. Particle counters whose sample flow rate is internally regulated to one cubic foot per minute (1 CFM) should be configure to continuously take one minute samples to produce measurements in particles per cubic foot. During operation, both channels report these measurements to the SmartScan server once every minute.
Since a one cubic meter sample contains 35.3 cubic feet, the count values from a little more than 35 one cubic foot samples must be summed to produce a measurement in particles per cubic meter. This will require over 35 minutes at a 1 CFM sample rate. To speed things up, SmartScan handles the summing in a simple but every effective manner producing a particles per cubic meter value every minute.
All SmartScan DA-06/07 Remote Monitoring Stations have two special 36 element arrays for each remote device. These can be used to accumulate a count of the particles per cubic meter for both channels of a two channel remote particle counters. The diagram below shows the structure of one of the arrays along with three other variables that will be used to control the count accumulation.
The “Index” identifies which of the 36 array elements that will receive the next count information from the particle counter. The “Sum” is used to accumulate the particles per cubic meter value and the “Count” is incremented each sample until it reaches “36” indicating that the array is full. (See why a “36” element array was chosen rather than “35” later in this article.)
During the count up to 36, the accumulation operation proceeds as shown in the diagram below.
The value of the “Index” says that array element number 3 will be processed next. As the next cubic foot measurement is received from the particle counter, it is stored in the array according to the “Index” and added to the “Sum” value (operations #1 and #2). The “Count” is also incremented (#3) if it is less than 36. This continues until the “Count” reaches 36 which is an indication that the array is full and the “Sum” then contains a valid Particles per Cubic Meter value.
Once the array is full (“Count reaches 36), the accumulation operation starts following the diagram shown below.
As before, the “Index” points to the array element to be processed next. When a new measurement is available from the particle counter, the old measurement from the array is subtracted from the “Sum” (#1 in the diagram) before the incoming value is stored in the array (#2) and added to the “Sum” (#3). The “Index” is then incremented to the next array element (#4). (Note that when the “Index” reaches the end of the array, it automatically moves back to the beginning for the next measurement. This is called a “circular buffer.”)
Once the array has been filled, the Particles per Cubic Foot measurements are reported to the server every time the particle counter provides a new set of measurements (or once a minute).
As mentioned at the beginning of this article, one cubic meter contains 35.3 cubic feet. Thirty six (36) one cubic foot samples means that slightly over one meter will be sampled for each Particles per Cubic Meter value. This was chosen to be conservative and provide a worst case output. (Newer versions of the DA-06/07 allow a user to select that 35 or 36 samples be accumulated for the cubic meter values.)