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Dew point sensing technologies and their performance differences in compressed air applications
until condensation forms. A gas sample passes over a metallic mirror surface which is chilled by a cooler. Light is then directed at the mirror allowing an optical sensor to measure the amount of reflected light. When the mirror is cooled to the point at which condensation begins to form on its surface (i.e. the dew point has been reached), the amount of light reflected by the mirror diminishes which is in turn detected by the optical sensor. The rate of cooling is then carefully regulated by a temperature sensor on the mirror. Once a state of equilibrium has been reached between the rate of evaporation and condensation, Moisture is a constant issue in Different sensing the mirror temperature is equal to compressed air systems. When dew technologies the dew point. Due to the chilled point sensors are working optimally, mirror’s optical measurement measures can be taken to avoid The three most common types of principle, the sensor is highly malfunctions, inefficient operations sensors for measuring dew point sensitive to the presence of dirt, or poor quality end product. are chilled mirrors, metal oxide, oil, dust, and other contaminants However, the measurement of dew and polymer sensors. on the mirror surface. Similarly, point in compressed air systems accurate chilled mirror devices can present many challenges that Chilled mirror technology can tend to be expensive and are lead to erroneous readings, poor offer the highest accuracy over often employed when absolute stability, and even sensor failures. a wide range of dew points. The accuracy is essential and frequent The most common issues with dew operating principle is based on maintenance and cleaning can be point sensors in compressed air are the fundamental definition of dew performed. usually centered on the following: point – cooling a volume of air
• Response time • Reliability of the reading Sensor Wide High Immune Immune to Long- Reason- • Recovery from water spikes or technology measure- accuracy to dust conden- term able condensation ment range and dirt sation stability price • Exposure to compressor oil Chilled +++ +++ Mirror To better understand these Capacitive ++ ++ ++ + + ++ challenges, it is worth first exploring Metal oxide the performance differences Capacitive between the most common sensor ++ ++ +++ +++ +++ ++ Polymer technologies.
VIM-GLO-IND-HUM-Dewpoint-sensing-technologies-App-note-B211092EN-B.indd 1 09/06/2020 9.05 Next are capacitive metal oxide layer (polymer vs. metal oxide), a During auto-calibration, the sensor sensors, including aluminum capacitive polymer sensor is also is heated and allowed to cool oxide technology, which are bonded together with a resistive while the humidity and monitored designed for very low dew temperature sensor. The polymer readings of the sensor are point measurement in industrial sensor measures the humidity monitoring and plotted. This data processes. While the types of (amount of water molecules in is analyzed and used to adjust the materials used in construction the measured gas) in terms of reading of the humidity sensor. can vary, the sensor structure relative humidity (RH) while the and operating principle generally temperature sensor measures The key to this accurate calibration remain the same. These capacitive the temperature of the polymer is that the sensor’s output is sensors are built in a layered sensor. From these two values, the equivalent to relative humidity structure sandwiching together microprocessor in the transmitter (RH), which changes in relation to a substrate base layer, a lower electronics calculates the dew the temperature. This well-known electrode, a hygroscopic metal- point temperature. An auto- physical dependence allows the oxide middle layer, and a water calibration feature, also invented auto-calibration to evaluate if the permeable upper electrode. The by Vaisala, is used to measure low humidity reading at 0% RH is capacitance across the upper and accurate dew point values in very correct. Any possible drift is then lower electrode changes based dry conditions with the polymer automatically corrected by the on the amount of water vapor sensor. When relative humidity microprocessor. This results in an absorbed by the metal oxide layer approaches zero, rather small accuracy of better than ±2ºC even (the dielectric of the capacitor), changes in humidity will result at low dew points. which is a function of dew point. in quite large changes in dew While providing excellent low point readings. For example, dew Polymer technology, which is the dew point measurement accuracy points of -40°C and -50°C at room result of years of testing and careful to -100°C and lower, they tend temperature correspond to relative material selection, combined with to offer poor long-term stability humidities of 0.8% RH and 0.3% intelligent electronics offer a high- in processes with varying dew RH, respectively. With the typical performance solution in applications points at the higher ranges (e.g. ±2% RH accuracy specification of where minimal maintenance refrigerant dried systems). Metal polymer sensors, an accuracy of is required for the dew point oxide sensors can also be easily ±2°C dew point can be achieved transmitter. damaged by high humidity levels down to -9°C. Auto-calibration and condensation. This drift in the extends this accuracy from ±2ºC output reading means frequent down to -80°C. calibration, which can typically be done only at the manufacturer’s calibration lab.
Rounding out the sensor types, capacitive polymer sensors measure accurately over a wide humidity range in addition to H²O offering excellent long-term stability. Since January 1997,
when Vaisala launched the first 1 ever polymer sensor for dew point measurements, DRYCAP® 2 5 technology has been used in a wide variety of industrial and 3 meteorological applications. New innovations have enabled polymer 4 sensors to be utilized in low dew point applications as well. While the capacitive operating principle is similar to that of metal oxide, there are a few key differences. Layer construction of the DRYCAP® sensor. 1. Water vapor permeable Beyond the obvious material upper electrode 2. Humidity sensitive polymer layer 3. Bottom electrode difference in the hygroscopic 4. Sensor substrate 5. Connection pins
VIM-GLO-IND-HUM-Dewpoint-sensing-technologies-App-note-B211092EN-B.indd 2 09/06/2020 9.05 Issue #1: How can I ensure fast response time?
When installing a dew point probe that has acclimated to ambient dew points into a -40°C compressed air line, the response time required for traditional sensors to stabilize at this -40°C value can often take from several hours to several days for equilibrium to be reached. This is due to the fact that other capacitive sensor technologies must rely on the relatively slow process of using the dry process air to passively dry the hygroscopic (water absorbing) layer of the sensor.
A better solution is to use a capacitive polymer sensor with a purge function. Vaisala's DRYCAP® sensor reacts immediately when it senses a decrease in dew point of The graphs above are derived from an ongoing test in which two Vaisala 10°C or more by initiating a sensor DRYCAP® DMT242 dewpoint transmitters were installed in a compressed air line seven years ago and have not been recalibrated or adjusted. The purge cycle that applies heat to the line conditions are representative of instrument air. x-axis represents sensor. This expels water molecules years, y-axis represents difference to the reference value at periodic from the polymer layer, drying the checks. sensor, and providing a stabilized reading within 5-6 minutes.
Issue #2: How can I ensure installed at the same installation number of variables that can affect the reading is correct? point but they’re all reading the reading: process conditions, different values – how do I know installation method, how the signal The most frequently asked which one is correct?” is being read, accuracy of sensors question about dew point from installed, and length of time since compressed air technicians is – “I This question is usually the most last calibration. have several dew point instruments difficult to answer due to the While it is widely known that every dew point sensor drifts, the critical questions are – by how much and how quickly?
Well-established high quality polymer sensor with auto- calibration provides a high level of accuracy due to a self-calibration method that activates once every hour in a stable environment (more frequently in varying conditions) to ensure the absolute minimum sensor drift – providing years of maintenance-free, reliable measurements you can rely on. Vaisala DRYCAP® Capacitive polymer sensor structure
VIM-GLO-IND-HUM-Dewpoint-sensing-technologies-App-note-B211092EN-B.indd 3 09/06/2020 9.05 Issue #3: How does the Issue #4: Can the In brief sensor recover from water sensor withstand ▪ Chilled mirror technology spikes or condensation? exposure to entrained offers the highest accuracy compressor oils? over a wide range of dew Occasional high-humidity water points, but its performance can spikes or exposure to water Trace amounts of compressor be limited by the presence of droplets are an unavoidable oil suspended in compressed contaminants. phenomenon in the normal air can spell disaster for some ▪ Capacitive metal oxide sensors operation of a compressed air sensor technologies. Fortunately, provide very low dew point system. Whether your dew point the structure of some polymer measurement, but they can sensor can survive and recover sensors, like Vaisala DRYCAP®, be damaged by high humidity from these events depends on the have been specifically designed levels and condensation. type of sensor installed. to be selective to only water ▪ Capacitive polymer sensors molecules. This is achieved by with auto-calibration Metal oxide sensors continue to a specially designed permeable functionality operate over oxidize in the presence of water, upper electrode with a pore size a wide humidity range, are resulting in a structural change allowing only water vapor to pass unaffected by condensation in the porous oxide layer. This through it. By comparison, much and ensure long term stability. translates into measurement larger hydrocarbon molecules (i.e. inaccuracy and sensor drift. oils) are unable to pass through Polymer sensors, on the other these pores, thereby eliminating hand, are immune to water due to any cross-sensitivity to oils. Clearly their inherent inert characteristics. by design, chilled mirror optics When a polymer sensor with auto- and reflective surfaces need to calibration functionality senses a remain clean in order to maintain water spike, it initiates an auto- their performance – and thus calibration cycle as normal dry have a minimal tolerance for oil air returns to the line, returning to contamination. normal operating values within a few minutes.
Auto-calibration function.
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