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Quotes - Engineering - SalesDew Point Monitors for Compressed Air Dryers
A portable instrument that tells you what the dryer is actually delivering
A refrigerated dryer has no gauge that proves it is working. The panel display on a current unit shows the dew point the controller calculates from its own sensor, and an older unit shows a green light or nothing; neither tells the plant what the air in the header measures after the afterfilter, on a hot afternoon, at full flow. A dew point monitor does, and it settles the argument between the dryer and the wet air at the far end of the shop in twenty minutes.
The cataloged instrument is a self-contained sample-and-measure unit in a NEMA 4X case: a ceramic hygrometer probe reading -148 to +68°F pressure dew point, or 0 to 3,000 ppmv, on a four-digit display, fed by a sample line through a built-in 0.3 micron filter and flow meter. It carries between dryers in its case for audits and troubleshooting, or screws to a wall as a permanent monitor with a 4-20 mA output and two programmable alarm relays wired to the dryer alarm or the plant PLC.
The range reaches desiccant-dryer dew points as well as refrigerated ones, so one instrument checks a 38°F refrigerated dryer, a -40°F desiccant dryer and the class 2 air on a paint line. It is sold as a single model with 6 mm quick-connect sample fittings, a 145 psig sample pressure limit and an 85 to 265 V supply.
- -148 to +68°F pressure dew point, or 0 to 3,000 ppmv, accuracy plus or minus 3.6°F
- Built-in 0.3 micron sample filter, flow meter and 6 mm quick-connect fittings
- 4-20 mA output and two alarm relays for a dryer alarm or PLC
- NEMA 4X case, carrying case included, wall-mountable for permanent monitoring
The dew point monitor
One instrument covers the whole site. The series page carries its range, accuracy, connections and supply details.
| SERIES | TYPE | FLOW RANGE | PART NUMBERS | NOTES |
| PDM Dew Point Monitor | Dew Point Monitor | See series | 1 listed | 122°F max |
Dew point monitor series
How to check a refrigerated dryer
Tap the sample after the afterfilter, at line pressure, through the supplied fittings or downstream of a regulator set below 145 psig, and let the probe settle; a reading that is still falling after twenty minutes is the sample line drying out, not the dryer. Note the flow, the inlet temperature and the room at the same time, because the 38°F rating applies at 100 psig, 100°F inlet and 100°F ambient and nowhere else. A dryer reading 42 to 48°F at 115°F inlet on a 105°F afternoon is doing what its correction factors predict; the same reading in a cool room at half flow means something is wrong.
Reading the number
A steady dew point above 50°F at rated conditions points at an overloaded dryer, a fouled condenser, a low refrigerant charge or a hot-gas bypass valve that has drifted. A dew point that is fine most of the time and then jumps wet points at a failed drain or a flooded separator carrying water over, or at a bypass valve passing wet air around the dryer. A reading that never gets below 45°F while the panel says 38°F is a dryer sensor that has drifted, and the monitor is the only way to know. Measure at the header, not at the dryer outlet, when the complaint is water at a machine; the two can differ by 20°F where a long uninsulated run cools the air below the dryer's dew point.
Permanent monitoring
Wall-mounted, the instrument watches one dryer continuously and its two relays give the plant a dew point alarm that the dryer's own controller may not, set at whatever the process tolerates: 45°F for plant air, 40°F for a paint line. The 4-20 mA output logs to the plant system and turns a dryer complaint into a trend that shows when the dew point started climbing, which is usually weeks before anyone noticed water. Food, pharmaceutical and electronics plants that certify air quality to ISO 8573-1 keep one on each dryer for the record.
Sample line and care
Keep the sample line short and in PTFE, because ordinary tubing absorbs and releases water and slows the reading by hours; the instrument ships with PTFE tubing, and the built-in flow meter sets the 2 to 10 standard cubic feet per hour the probe wants. Never sample from the wet side of the dryer at full line pressure without the regulator, and keep the instrument within its 23 to 122°F ambient range. The ceramic probe holds calibration for years in clean air; a plant that wants traceable readings sends it for recalibration on a schedule. The dew point and ISO classes page explains what the numbers mean, and the maintenance page covers what to do when the reading is wrong.
Measuring what the dryer really delivers
Leave room above the rating. A dryer rated 38°F will read in the 40s on a hot afternoon with a warm inlet and still be healthy, so an alarm at 40°F on plant air trips every summer day and gets ignored by August. Something near 45°F catches a fouled condenser or a failed drain without crying wolf. A paint line is tighter and justifies the lower setting and the nuisance alarms that come with it.
Nylon and polyurethane walls take up water and give it back slowly, so the probe ends up measuring the tubing rather than the air, and a dry sample can read high for hours. PTFE barely absorbs anything. Keep the run short, set the flow on the built-in meter, and do not swap in a spare length of plant tubing, because a nylon line that once carried wet air holds that moisture for days.
When the sample pressure changes. Pressure dew point falls as the air is let down through a regulator, while ppmv is a concentration and stays the same, so ppmv lets you compare a reading taken at a regulated 90 psig with one taken at full line pressure without a conversion chart. Dew point remains the figure the ISO 8573-1 classes and the dryer rating are written in, so record both.
As often as the plant's quality system says. In clean, dry air the ceramic sensor drifts very little over years, so a plant using it for troubleshooting rarely needs more than an occasional check against a known dryer. A site certifying air to an ISO class will want a traceable certificate on a fixed schedule. Any probe that has been flooded with liquid water or oil should be checked before its readings are trusted again.
Very little, and it risks the probe. Air leaving an aftercooler or receiver is saturated, so its dew point is simply its temperature, which a thermometer tells you for a fraction of the effort. The inlet stream also carries liquid that the 0.3 micron sample filter is not built to separate, and a flooded probe takes a long time to recover. Read the outlet, after the afterfilter, and log the inlet temperature beside it.
Within about 3.6°F either way, which is plenty for the decisions a refrigerated dryer forces. The faults worth finding show up as a reading that climbs 10°F over a few weeks, or a 20°F gap between the dryer outlet and the far header, both well outside the instrument's tolerance. Where two readings differ by only a degree or two, look at the trend and the operating conditions rather than arguing over the last digit.
