Measuring Humidity in Condensing Environments
Taking measurements in environments with high relative humidity (over 90%) is always a challenge because the environment is so close to saturation. Consider a process environment at 60°C and 95% RH. The corresponding dew point temperature is 59°C – meaning that water vapor will condense on any surface that is at a temperature equal to or less than 59°C – giving only a very small margin before condensation occurs. Below we describe the technical problems that this can cause with measurements.
- 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.
- In installations where half of the probe head sits in the process environment and the other half outside, if the temperature conditions outside are cooler than in the process environment the probe will cool down on the process side as well, and may potentially create condensation that disrupts measurement.
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.
Vaisala has developed warmed-probe and sensor-heating technologies to address these problems.
Warmed-probe technology 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 probe HMP7 and fixed-mount transmitter HMT337, 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.
If relative humidity or another humidity parameter output is preferred, the HMT337 transmitter needs to be equipped with a separate temperature probe installed in the same measurement environment as the humidity probe. This is necessary as relative humidity is a temperature-dependent parameter and probe heating disturbs the relative humidity measurement. Relative humidity can be reliably calculated using the dew point value and the temperature value from the additional temperature probe.
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.
In addition to warmed probe and sensor-heating technologies, there is also a third heating method available in selected Vaisala humidity measurement products. Chemical purge helps to correct possible long-term drift and extends the required calibration interval in environments where gaseous chemical impurities are present. Hydrocarbon-based solvents, cleaning chemicals, and sterilizing agents are all examples of substances with a tendency to penetrate into sensors, and they can all be removed using the chemical purge function. The chemical purge evaporates the interfering chemicals by rapidly heating the sensor up to 160–180°C using the temperature element of the sensor as a heating element. The chemical purge cycle lasts about six minutes and includes a heating and a settling stage. When the sensor temperature stabilizes again at the pre-purge level the sensor continues in its normal measurement mode. The output value of the transmitter is locked during the chemical purge.
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|
|Heating activated||Probe heated continuously||When a specified RH value is reached||Manually, automatically (interval), or during power up|
|Duration and heating temp.||Continuous; entire probe is warmed 2…3ºC above ambient temperature||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||Dew point value always available; RH value always available with additional temperature probe||Output is locked during heating; duration: 1–2 minutes||Function locks output values for about 6 minutes|