TRIR
(N × 200,000) / EH
Compare recordable rate across sites or years.
Trap: Using headcount or 100,000 hours instead of 200,000.
4 recordables, 250,000 hours → 3.2.
Formula desk
Name it before you punch keys.
DART
(DART cases × 200,000) / EH
Severity-flavored rate when TRIR hides restricted duty.
Trap: Counting first aid as DART.
2 DART cases, 250,000 hours → 1.6.
TWA
Σ(Ci × Ti) / 8
Combine partial-shift samples into an 8-hour TWA.
Trap: Dividing by elapsed time when the standard is an 8-hour TWA.
80 ppm × 3 h + 40 ppm × 5 h → 55 ppm.
OSHA 5 vs NIOSH 3
OSHA T = 8 / 2^((L−90)/5) · NIOSH T = 8 / 2^((L−85)/3)
Allowed duration at a constant A-weighted level.
Trap: Mixing 5 dB exchange with an 85 dB criterion.
OSHA 95 dBA → T = 4 h.
Q = VA
Q = V × A
Duct, capture, or dilution flow.
Trap: Using diameter as radius. Inches left as inches.
12-inch round at 2000 fpm → about 1,571 cfm.
NIOSH RWL
RWL = 51 × HM × VM × DM × AM × FM × CM
Two-handed lifting recommended weight limit.
Trap: Treating 51 lb as the RWL when a multiplier is open.
HM = 0.50, others 1 → RWL = 25.5 lb.
ppm ↔ mg/m³
mg/m³ = (ppm × MW) / 24.45 at 25 °C
Gas or vapor unit conversion at NTP.
Trap: Using 22.4 when the stem is 25 °C.
CO MW 28 at 50 ppm → 57.3 mg/m³.
ROI
ROI = (Gain − Cost) / Cost
Simple return on a control spend.
Trap: Calling payback years an ROI percent.
Avoided 80k on 20k spend → 300%.
FMEA RPN
S × O × D
Rank FMEA rows. Not an FTA probability.
Trap: Using RPN on a fault tree.
8 × 4 × 5 = 160.
Dike volume
L × W × H, then × 7.48 for gallons
Secondary containment capacity.
Trap: Ignoring freeboard or tank displacement in the last sentence.
40 × 30 × 1.5 ft → 1,800 ft³.