Measuring humidity in condensing environments
在相对湿度高（超过90％）的环境中进行测量始终是一个挑战，因为环境非常接近饱和。考虑在60°C和95％RH的过程环境。相应的露点温度为59°C - 意味着水蒸气在等于或小于59°C的任何表面上都会凝结 - 在凝结之前仅提供很小的边缘。下面我们描述了通过测量可能导致的技术问题。
- Water can condense on an unheated probe, caused by process conditions changing rapidly due to pressure spikes or temperature variation, and the probe may then stay wet for several minutes. This leads to a situation where there is no valid measurement data available when it’s most needed – when conditions are changing.
- 在一半探针头位于过程环境中和外部另一半的装置中，如果外部温度条件比过程环境凉爽，则探针也会在过程侧冷却，并且可能会产生干扰测量。
Variations on these two problems occur in many high-humidity applications. For example, outdoor measurements can be disrupted by fog, mist, rain, or heavy dew. The challenge is to obtain valid measurement data and maximum sensor uptime, even if the environment is at, or close to, saturation.
Vaisalahas developed warmed-probe and sensor-heating technologies to address these problems.
加热技术
加热技术enables reliable and accurate humidity measurement even in environments with 100% relative humidity (RH), where unheated humidity sensors are unable to measure. It is available in Vaisala products such as the smart probeHMP7and fixed-mount transmitterHMT337, which are specifically designed for high humidity environments. These products feature a composite humidity sensor with a bonded temperature sensor. With the composite sensor, the relative humidity and the temperature of the sensor are known at all times. The humidity probe containing the composite sensor is equipped with a heating element. This keeps not only the sensor, but also the whole probe – including the filter – at a temperature several degrees above the ambient temperature, ensuring that water will not condense on the sensor, even when the measurement environment is at the dew point temperature (100% RH). The dew point temperature of the composite sensor can be calculated using the measured relative humidity and temperature values, and can be used directly as the output parameter.
如果相对湿度或其他湿度参数输出是首选的，则需要在与湿度探针相同的测量环境中配备单独的温度探针。这是必要的，因为相对湿度是一个依赖温度的参数，探测加热会破坏相对湿度测量。相对湿度可以使用露点值和来自附加温度探针的温度值可靠地计算。
Sensor-heating technology
Sensor heating is another type of heating function available in selected Vaisala humidity measurement products. By default, the sensor-heating function rapidly heats the sensor to 100°C for about 30 seconds; users can define both the heating temperature and the duration of the heating period. Unlike a warmed probe, heating is achieved by directly warming the temperature sensor bonded to the humidity sensor. Although this is much faster than heating the whole probe, it does mean that the measurement will be offline while the sensor is heated and then cooled back down to its operating temperature; this typically takes about one to two minutes. During that time the last valid measurement is held in the output and display of the transmitter. Users can configure the sensor-heating function to activate automatically at a specified humidity level.
Maximum protection from condensation can be achieved by using the sensor-heating function in combination with a warmed probe. Sensor heating is considered as a defense mechanism against rapidly increasing humidity levels that might wet the sensor. It also speeds up recovery from condensation.
Chemical-purge technology
除了加热的探针和传感器加热技术外，在选定的Vaisala湿度测量产品中还有第三种加热方法。化学清洗有助于纠正可能的长期漂移，并在存在气态化学杂质的环境中扩展所需的校准间隔。碳氢化合物溶剂，清洁化学物质和灭菌剂都是具有穿透传感器趋势的物质的示例，并且可以使用化学净化功能将其除去。化学清除通过使用传感器的温度元素作为加热元件将传感器迅速加热至160-180°C，从而蒸发了干扰化学物质。化学吹扫循环持续约六分钟，包括加热和沉降阶段。当传感器温度再次在purge水平上再次稳定时，传感器在其正常测量模式下继续进行。在化学清除期间，发射器的输出值锁定。
Summary of heating methods
| Warmed probe | Sensor heating | Chemical purge | |
|---|---|---|---|
| Environment | High humidity | For high humidity | For applications where interfering chemicals in the measuring environment pose a risk |
| Heated element | Whole probe | Sensor | Sensor |
| 加热激活 | Probe heated continuously | When a specified RH value is reached | Manually, automatically (interval), or during power up |
| Duration and heating temp. | 连续的;整个探针在环境温度以上的2…3ºC温暖 | Configurable; by default PT100 is heated to +100ºC (0…200ºC) for 30 seconds (0…255s), RH 95% | Humidity sensor is heated to approximately +160°C for several minutes |
| Output value | 露点值始终可用；RH值始终可提供其他温度探针 | Output is locked during heating; duration: 1–2 minutes | Function locks output values for about 6 minutes |
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