Pressure dewpoints must be monitored

Here's a quick overview of the dewpoint monitors commonly found in manufacturing plants

The drier the air, the more expensive it is. Many clients assert that they require a -40F pressure dewpoint, yet few bother to monitor the variable. The reason is a perception of problems with dewpoint monitors and probes.

Modern probes are offer solid performance with easier maintenance and reduced sensitivity to installation conditions.

Most production facilities require dewpoint measurement of either refrigerated dryers (+40F class) or desiccant dryer (-40F class or better). Some require both. Key measurement points include:

  Discharge air from each dryer.
  The lines to production areas.
  Air entering critical users.

Monitor types
Not every monitor is suited for every application. The four most common sensor types are:
  Chilled mirror.
  Oxide type.
  Thin film polymer.
  Polystyrene core.

If pressure dewpoint is critical, measuring and monitoring it upstream of critical users often can alert operators to a malfunction before the process deteriorates. Monitoring also pinpoints the source of the problem.

Historical perspective
Most dewpoint sensors used through the late 1980s and early 1990s featured a polystyrene core coated with hygroscopic film and were wrapped with a bifilar winding of two palladium wires. A vacuum pump pulls an air sample across the probe. The sensor's electrical resistance responds to changes in humidity. The sensor usually includes a thermistor, which also changes resistance with temperature. Coupling the two devices results in a sensor that exhibits a linear relationship between current and dewpoint. Maintenance is low because the probe can be changed in the field and requires no recalibration. Generally, these are not recommended for continuous use in an industrial environment. If this type ever becomes saturated, it must be replaced.


Figure 1. Schematic of a chilled mirror pressure dewpoint monitor.

Early aluminum oxide
Almost every desiccant dryer manufacturer used an early style of aluminum oxide probe to monitor pressure dewpoint in the range of -40F to -100F, but the sensors were sensitive to contamination from transient moisture loads. They couldn't clear themselves and had to be removed for drying and recalibration. Many users simply quit repairing the probes and stopped using the controllers and monitors. Today, the use of the old-style oxide-based probes is limited to light-duty, clean-environment laboratory conditions.

Contemporary aluminum oxide probes and sensors, however, can dry themselves in situ. They are smaller, more sensitive and manufactured using better production methods. Plants with disabled pressure dewpoint monitors should review this option.

Chilled mirror
This sensor uses a Peltier-cooling module to chill a polished rhodium or stainless steel mirror (see Figure 1). A light-emitting diode illuminates the mirror and a photodiode monitors the reflected light. Water vapor condensing on the cold mirror scatters some of the light and, in response, the servo controller reduces the current to the Peltier module, which warms the mirror. The control system thus maintains the mirror at the temperature of incipient condensation the dewpoint.

Several companies offer chilled mirror dewpoint monitors. They are accurate with high repeatability, but require diligent maintenance. They're sensitive to dirt and air line contamination. Although most units include self-compensating electronics to correct for a dirty mirror, eventually enough dirt accumulates and triggers an alarm indicating the mirror needs cleaning. Some line contaminants may etch the reflective surface and the mirror will have to be replaced some models have field-replaceable mirrors. The high initial and ongoing maintenance costs argue against using this monitor in production facilities. They're best for laboratories or if moisture contact in the product could be catastrophic.

Aluminum oxide
Modern aluminum oxide probes measure water vapor pressure, which is proportional to dewpoint. Overplating a thin film of gold on the oxidized surface of an anodized aluminum strip forms a capacitor (see Figure 2). Water vapor diffuses through the gold into pores in the oxide to alter its conductivity, which is proportional to water vapor pressure. A ceramic probe works the same way, but uses a ceramic substrate instead of aluminum.
These probes are relatively sturdy and insensitive to dirt and debris. The initial cost is lower than the chilled mirror type. The calibration and maintenance cost may be somewhat lower. This probe offers an accurate, fast response. Saturated air won't damage them because they dry quickly, but drying time is determined by relative humidity. Continued saturation, however, leads to deterioration. Initial cost, durability, accuracy and repeatability recommend aluminum oxide and ceramic sensors for production facilities.

Thin film polymer
A thin film polymer sensor measures relative humidity with a
capacitive polymer bonded to a temperature sensor. An onboard microprocessor converts the two readings to dewpoint.

Polymer sensors have a high tolerance for corrosive chemicals and are immune to condensation. They offer long-term stability, accuracy and a fast response. Some manufacturers offer an auto-calibration feature to compensate for widely varying dewpoints and for very low dewpoints. The initial cost is in the low-to-moderate range. They're best suited for refrigerated dryer systems because their performance won't degrade in extended saturation situations. On the other hand, they have marginal accuracy in the lower pressure dewpoint ranges.

Calibration
Sensor manufacturers recommend factory calibration at least once a year. We recommend having a spare probe to install until the following calibration date.


Some manufacturers offer a hand-held loaner when the probe is at the factory for calibration. Some units feature automatic calibration while the sensor is online.

Installation
The probe should be mounted upstream of the production area being monitored. A 1/4-in. stainless steel sampling line should tap into the top half of the air line to protect the probe and sampling system from liquid water. Some systems include a filter either outside or inside the assembly. Consider installing a flowmeter to control flow to the probe.

The air sample should be delivered to the probe at full line pressure because dewpoint is a function of pressure. Install the sampling system with a pressure gauge to verify it's seeing full line pressure. Keep the sampling line as short as possible to prevent condensation in transit. Plastic and rubber sampling lines absorb moisture and yield false dewpoint readings. Slope the sampling line to drain condensation back into the header.

Technical tip: If dewpoint readings seem high, check each fitting in the sample line and measuring unit for leaks. Moisture can travel against the flow of air, which leads to wicking at leaky connections and false high readings.



Figure 2. Schematic of an aluminum oxide probe.

Direct insertion probes
Some probes can be installed through a ball valve while the system is pressurized. The perceived benefit is simplicity and lower installation cost. This type of installation doesn't seem to offer improved accuracy. There are, however, several issues of concern. The laminar effect may lead to inaccurate readings. Rust, scale, debris and other contaminants can affect the probe. You need to remove the probe for maintenance while the system is still pressurized. Many feel that a probe installed in an offline sample system, where it's easily accessible, will be easier to maintain.

The three probe types thin film polymer, aluminum oxide and ceramic oxide offer high accuracy and fast recovery from saturation. All three are reasonable selections for either desiccant or refrigerated systems. However, the thin film polymer is somewhat less accurate at the lower pressure dewpoints found in desiccant systems. 

Don van Ormer is senior auditor with Air Power USA Inc., Pickerington, Ohio. Contact him at donaldvanormer@aol.com and (740) 862-4112.

Figures: Air Power USA Inc.

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