一、两种原理
膜法（电化学法）：电极内的阴阳极与电解液被一层透气膜与被测水隔开。氧气透过膜在阴极被还原，产生的电流与氧分压成正比。测量过程会消耗氧，因此需要水样流动。
荧光法（光学法）：传感器前端的荧光物质被蓝光激发后发出红光，水中氧气会猝灭这一荧光。通过测量荧光寿命或相位差换算氧浓度。不消耗氧，不需要流速。
二、核心差异
- 流速要求：膜法需要一定流速（常见经验约 0.1 至 0.3 m/s 以上，以说明书为准）；荧光法可在静止水中测量。
- 启动准备：膜法新装或更换电解液后需要极化（通常数十分钟到数小时）；荧光法通电即可用。
- 维护量：膜法需定期更换电解液与膜帽，并清洁阳极；荧光法只需定期更换荧光帽，周期更长（常见约一年以上，视工况而定）。
- 漂移：膜法随电解液消耗与膜污染漂移较明显，需更频繁标定；荧光法漂移小。
- 初期成本：膜法传感器通常价格更低；荧光法单价较高但综合维护成本低。
- 干扰：膜法受硫化氢、二氧化碳等气体透过膜影响；荧光法不受这些气体影响，但荧光帽划伤、强光直射、附着生物膜会影响测量。
三、怎么选
- 污水曝气池、好氧池：污染重、维护不便，优先荧光法。
- 清水、养殖、地表水监测：两种均可，预算敏感且能保证定期维护时可选膜法。
- 静止水样、实验室或低流速点位：荧光法更合适。
- 高温或含强氧化剂的特殊工况：需核对传感器材质与温度范围后再定。
四、标定方法
- 空气标定（单点）：将探头置于饱和湿空气中（如装有少量水的密闭容器上方），待温度平衡后做满度标定，这是最常用的方法。
- 零点标定：用亚硫酸钠配制的无氧水做零点，通常在精度要求高或初次安装时做。
- 比对验证：用便携式溶解氧仪或碘量法在现场做比对，判断在线仪表是否可信。
标定时必须等待温度平衡，且要注意大气压对空气标定的影响；部分仪表支持自动气压补偿。
五、常见故障
- 读数偏高：膜法探头表面附着气泡、膜片破损；荧光法荧光帽污染。
- 读数偏低：膜法电解液干涸或阳极污染；荧光法荧光帽老化。
- 响应变慢：膜法膜片污染或电解液失效；荧光法荧光帽表面结垢。
- 数据跳变：曝气气泡直接冲击探头、电缆进水、接地不良。
选型时把介质、流速、维护人力与预算一起考虑，通常能得到更合适的方案。提供工况参数后我们可以帮您匹配传感器类型与安装附件。
1. Two principles
Membrane (electrochemical): the cathode, anode and electrolyte are separated from the sample by a gas-permeable membrane. Oxygen diffuses through and is reduced at the cathode; the resulting current is proportional to oxygen partial pressure. Oxygen is consumed, so the sample must flow.
Optical (fluorescence): a fluorescent dye at the sensor tip is excited by blue light and emits red light; dissolved oxygen quenches that fluorescence. Oxygen concentration is derived from fluorescence lifetime or phase shift. No oxygen is consumed and no flow is required.
2. Key differences
- Flow requirement: membrane sensors need flow (commonly above roughly 0.1 to 0.3 m/s - follow the manual); optical sensors work in still water.
- Start-up: membrane sensors need polarisation after filling or replacing electrolyte, from tens of minutes to several hours; optical sensors work as soon as power is applied.
- Maintenance: membrane sensors need periodic electrolyte and membrane cap changes plus anode cleaning; optical sensors only need periodic cap replacement, at longer intervals (often over a year, depending on duty).
- Drift: membrane sensors drift more as electrolyte depletes and the membrane fouls, so they need more frequent calibration; optical sensors drift little.
- Initial cost: membrane sensors are usually cheaper to buy; optical sensors cost more but have lower whole-life cost.
- Interference: membrane sensors are affected by gases such as hydrogen sulphide and carbon dioxide crossing the membrane; optical sensors are not, but are affected by cap scratches, direct sunlight and biofilm.
3. How to choose
- Aeration tanks, aerobic tanks: heavy fouling and poor access - prefer optical.
- Clean water, aquaculture, surface water: both work; membrane is viable if budget is tight and maintenance is reliable.
- Still samples, laboratory or low-flow points: optical is more suitable.
- High temperature or strong oxidants: confirm sensor materials and temperature range first.
4. Calibration
- Air calibration (single point): place the probe in saturated humid air (for example above a small amount of water in a closed container), allow temperature to equalise, then calibrate the span. This is the most common method.
- Zero calibration: use oxygen-free water prepared with sodium sulphite when accuracy requirements are high or on first installation.
- Comparison check: compare against a portable dissolved oxygen meter or the iodometric method on site to judge whether the online reading is trustworthy.
Always allow temperature to equalise before calibrating, and remember that air calibration is affected by barometric pressure; some transmitters compensate automatically.
5. Troubleshooting
- Reading too high: bubbles on the membrane or a damaged membrane; fouled optical cap.
- Reading too low: dried-out electrolyte or fouled anode; aged optical cap.
- Sluggish response: fouled membrane or exhausted electrolyte; scaling on the optical cap.
- Jumping data: aeration bubbles hitting the probe directly, water in the cable, poor earthing.
Consider medium, flow, maintenance resources and budget together and you usually reach a better answer. Send us the duty data and we will match the sensor type and mounting accessories.