今日目标 | Learning objectives
完成本课后，你应能熟练进行分数、百分比、比值与科学计数法运算，用换算系数法完成 Pa/kPa/MPa、m/cm、mg/kg、ppm↔mg/m³ 等常用单位换算，并算出 OSHA 事故率。
By the end, work fluently with fractions, percentages, ratios and scientific notation, convert common units (Pa/kPa/MPa, m/cm, mg/kg, ppm↔mg/m³) with the factor-label method, and compute an OSHA incidence rate.
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核心概念 | Key concepts
百分比 = 部分 ÷ 整体 × 100%。例：180 次检查中发现 45 项隐患，隐患率 = 45 ÷ 180 × 100% = 25%。比值常写成 a:b，如风险矩阵的"可能性：严重度"权重。
Percentage = part ÷ whole × 100%. Example: 45 findings in 180 inspections → 45 ÷ 180 × 100% = 25%. Ratios are written a:b, e.g. likelihood-to-severity weightings in a risk matrix.
写成 a × 10ⁿ（1 ≤ a < 10）。例：1 MPa = 1 × 10⁶ Pa；0.00045 g = 4.5 × 10⁻⁴ g。考试中浓度、压力、剂量常用科学计数法表示，务必会心算移位。
Written as a × 10ⁿ (1 ≤ a < 10). Example: 1 MPa = 1 × 10⁶ Pa; 0.00045 g = 4.5 × 10⁻⁴ g. Exam concentrations, pressures and doses often use scientific notation — practice shifting exponents mentally.
压力：1 kPa = 1,000 Pa，1 MPa = 1,000 kPa = 1 × 10⁶ Pa，1 atm ≈ 101.3 kPa。长度：1 m = 100 cm。质量：1 kg = 1,000 mg = 1 × 10⁶ µg。体积：1 m³ = 1,000 L。用"换算系数法"：把系数写成分数，让原单位上下相消，留下的就是目标单位。
Pressure: 1 kPa = 1,000 Pa, 1 MPa = 1,000 kPa = 1 × 10⁶ Pa, 1 atm ≈ 101.3 kPa. Length: 1 m = 100 cm. Mass: 1 kg = 1,000 mg = 1 × 10⁶ µg. Volume: 1 m³ = 1,000 L. Use the factor-label method: write each factor as a fraction so unwanted units cancel, leaving the target unit.
公式：mg/m³ = ppm × MW ÷ 24.45（25 °C、1 atm 下）。变量：ppm = 体积比百万分数（无单位）；MW = 摩尔质量，g/mol；24.45 = 25 °C 时气体摩尔体积，L/mol。算例：苯（MW = 78.11 g/mol）25 ppm → 25 × 78.11 ÷ 24.45 ≈ 79.9 mg/m³。反向换算：ppm = mg/m³ × 24.45 ÷ MW。
Formula: mg/m³ = ppm × MW ÷ 24.45 (at 25 °C, 1 atm). Variables: ppm = parts per million by volume (dimensionless); MW = molecular weight, g/mol; 24.45 = molar volume at 25 °C, L/mol. Worked example: benzene (MW = 78.11 g/mol) at 25 ppm → 25 × 78.11 ÷ 24.45 ≈ 79.9 mg/m³. Reverse: ppm = mg/m³ × 24.45 ÷ MW.
记忆句 / Memory aid: 想要的单位放分子，原单位斜对角相消 / Put the wanted unit on top; cancel old units diagonally.
术语起步 | Starter glossary
- ppm (parts per million) — 百万分数：气体或液体中某成分的体积浓度。/ Concentration by volume in air or liquid.
- Conversion factor — 换算系数：等于 1 的分数（如 1,000 Pa / 1 kPa），用于单位换算。/ A fraction equal to 1 (e.g. 1,000 Pa / 1 kPa) used to convert units.
- Scientific notation — 科学计数法：a × 10ⁿ 的紧凑记数方式。/ Compact form a × 10ⁿ for very large or small numbers.
- Molecular weight (MW) — 摩尔质量：单位 g/mol，连接 ppm 与 mg/m³ 的桥梁。/ g/mol; the bridge between ppm and mg/m³.
- Incidence rate — 事故发生率：每 200,000 工时的可记录事故数（OSHA 口径）。/ Recordable cases per 200,000 hours worked (OSHA basis).
把以上术语加入你的英文术语表；每课结束补充 3–5 个新术语。/ Add these to your glossary; extend it with 3–5 new terms after every lesson.
备考情景 | Study scenario
空气采样显示苯（MW = 78.11 g/mol）浓度为 25 ppm。在 25 °C、1 atm 下，它等于多少 mg/m³？
An air sample shows benzene (MW = 78.11 g/mol) at 25 ppm. At 25 °C and 1 atm, what is this in mg/m³?
自编练习题，不是 BCSP 官方真题。 / Original practice question, not an official BCSP exam item.
巩固练习 | Quick review
1. 250 kPa 等于多少 MPa？压力表读数 0.35 MPa 又等于多少 kPa？写出换算步骤。/ Convert 250 kPa to MPa, and 0.35 MPa to kPa. Show your steps.
2. 某项目去年累计 1,200,000 工时，发生 6 起可记录事故。用 OSHA 公式 TRIR =（可记录事故数 × 200,000）÷ 总工时，求事故率并解释含义。/ A project logged 1,200,000 hours with 6 recordable cases. Using TRIR = (cases × 200,000) ÷ hours, compute the rate and explain it.
学习打卡与笔记 | Progress & notes
已完成 0/4 项 / 0/4 tasks complete
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可靠来源 | Official references
NIOSH — Pocket Guide to Chemical Hazards（IDLH、ppm↔mg/m³）
BCSP — CSP Examination Blueprint (CSP11)
OSHA — Recordkeeping & incidence rates 事故记录与发生率
本课为一般性学习资料，不替代现场专业评估、法规要求或组织程序。/ Educational content only.
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