BTS reports particle counts as per two principle standards: ISO 4406 and NAS 1638
ISO Codes Allocation of Scale Numbers
Number of particles per millilitre |
Scale number | |
| More Than | Up to and Including | |
| 80,000 | 160,000 | 24 |
| 40,000 | 80,000 | 23 |
| 20,000 | 40,000 | 22 |
| 10,000 | 20,000 | 21 |
| 5,000 | 10,000 | 20 |
| 2,500 | 5,000 | 19 |
| 1,300 | 2,500 | 18 |
| 640 | 1,300 | 17 |
| 320 | 640 | 16 |
| 160 | 320 | 15 |
| 80 | 160 | 14 |
| 40 | 80 | 13 |
| 20 | 40 | 12 |
| 10 | 20 | 11 |
| 5 | 10 | 10 |
| 2.5 | 5 | 9 |
| 1.3 | 2.5 | 8 |
| 0.64 | 1.3 | 7 |
| 0.32 | 0.64 | 6 |
| 0.16 | 0.32 | 5 |
| 0.08 | 0.16 | 4 |
| 0.04 | 0.08 | 3 |
| 0.02 | 0.04 | 2 |
| 0.01 | 0.02 | 1 |
| 0.005 | 0.01 | 0 |
| 0.0025 | 0.005 | 00 |
| ISO 4406 specifies two size ranges. The scale number refers to the number of particles per 1 ml, in each size range. Both scale numbers are reported with a backslash between them, with the >5 micron scale number always first. | Example: |
|||
| >5 | 3600 | 19 | ||
| >15 | 28 | 12 | ||
| ISO 4406 Code = 19/12 | ||||
NAS 1638 specifies five size ranges, and the counts refer to the number of particles in each range per 100 ml. Each range is assigned a class number based on the number of paricles present, and the overall class is the range which has the highest count. |
Example: |
|||
| >5 | 9700 | 6 | ||
| >15 | 1200 | 5 | ||
| >25 | 300 | 6 | ||
| >50 | 50 | 6 | ||
| >100 | 0 | 0 | ||
| NAS 1638 Class = 6 | ||||
NAS Classes of Maximum Contamination Limits (Based on a 100 ml Sample Size)
| Classes | Particle Classes Size Range (Microns) |
||||
| 5 to 15 | 15 to 25 | 25 to 50 | 50 to 100 | Over 100 | |
| 00 | 125 | 22 | 4 | 1 | 0 |
| 0 | 250 | 44 | 8 | 2 | 0 |
| 1 | 500 | 89 | 16 | 3 | 1 |
| 2 | 1,000 | 178 | 32 | 6 | 1 |
| 3 | 2,000 | 356 | 63 | 11 | 2 |
| 4 | 4,000 | 712 | 126 | 22 | 4 |
| 5 | 8,000 | 1,425 | 253 | 45 | 8 |
| 6 | 16,000 | 2,850 | 506 | 90 | 16 |
| 7 | 32,000 | 5,700 | 1,012 | 180 | 32 |
| 8 | 64,000 | 11,400 | 2,025 | 360 | 64 |
| 9 | 128,000 | 22,800 | 4,050 | 720 | 128 |
| 10 | 256,000 | 45,600 | 8,100 | 1,440 | 256 |
| 11 | 512,000 | 91,200 | 16,200 | 2,880 | 512 |
| 12 | 1,024,000 | 182,400 | 32,400 | 5,760 | 1024 |