Dew Point Meter
Dew point meter is a tool that measures the temperature and relative humidity in the air. Dew point allows you to determine whether condensation will form on a surface. measures dew point by utilizing microprocessor circuitry to convert sensor signal by the use of psychometric equations. Assembled in a rugged handheld IP65 protected plastic enclosure and equipped with quick-disconnect tubing couplings and an internal sensor manifold. Dew point meter used for compressed air and nitrogen gasses
Excess moisture vapor can often cause severe equipment damage, shorten the service life of process systems and equipment, or interfere with chemical mixtures.
What is the dew point?
Definition: Dew point is the temperature to which air must be cooled down, for the water vapor in it to condense into dew or frost.
- At the dew point temperature, the air is 100% saturated with water.
- The hotter the air, the more water vapor can be stored.
rH[%] = [Absolute Humidity / Maximum Humidity ] * 100
What is the effect of pressure on dew point?
The ambient air can be imagined like a wet sponge. In a relaxed condition it can absorb a certain quantity of water. If the sponge is squeezed, part of the water drops out.
Working Principle of a Dew Point Analyzer
Dew point analyzers rely on an impedance-based humidity sensor. These sensors rely on an assembly constructed from a ceramic substrate that supports a thin hygroscopic layer sandwiched by two conductive layers.
The top conductive layer is porous, allowing moisture to seep through into the hygroscopic layer. Since this hygroscopic layer is so thin-typically less than 0.1µ-it detects even miniscule changes in moisture.
The changes in electrical impedance of the hygroscopic layer are measured and converted into usable humidity readings.
Why is moisture problematic?
Water in compressed air lines could lead to:
- A reduced life span of topped users, and pneumatic components.
- Corrosion can occur in functional elements, which can lead to leaks.
- Lubrication films getting washed away.
- Ice formation in compressed air lines leads to a diameter reduction.
- Increased failure probability of machines.
- Finished goods could get damaged, for example powdery goods.
- An undefined quality of the produced goods.
- An interruption of my production process.
Quality problems in the food, beverage and pharmaceutical industries because of wet compressed air
- Excessive moisture leads to sticking of goods transported by compressed air, e.g., sugar, cement, granules, etc.
- Risk of bacteria formation in the breathing air, in pharmaceutical products and in PET bottle production
Quality problems: Painting process with moist compressed air
- Moisture in spray gun of painting robots leads to bubble formation in paint surface
- Consequence > considerable additional costs:
- Varnish must be removed
- Car body must be repainted
- How do we measure dew points?
- Capacitive polymer sensor to measure the humidity content
- Water vapor can enter the polymer layer by diffusion.
- This leads to a change of the capacity of the sensor.
- The porous cover electrode protects the sensor from dust or oil.
- Sensitivity depends on the polymer (IP of producer).
- Sensor can be cleaned easily.
How is dew point reliably measured?
1. Select an instrument with the correct measuring range
2. Understand the pressure and temperature characteristics of dew point
3. Install the sensor correctly
Other dew point applications
• Natural gas
• Ambient air
Operation of Dew Point Meter
- The mirror is constantly cooled by a cooling medium. The cooling medium is maintained at a constant temperature.
- To this mirror is attached a thermocouple whose leads are connected to a millivoltmeter.
- Constantly a light is made to fall in an angle on the mirror and the amount of reflected light is sensed by a photocell.
- Now an air jet is made to fall in an angle on the mirror and the water- vapor (moisture) contained in the air starts condensing on the mirror and they appear as small drops (dew) on the mirror.
- This moisture (dews) formed on the mirror reduces the amount of light reflected from the mirror and it is detected by photocell. When for the first time, there is a change in the amount of transmitted light; it becomes an indication of dew formation.
- At this instance (that is, when the due formation is detected first), the temperature indicated by the thermocouple attached to the mirror becomes the dew point temperature.
- Thus this arrangement is used to determine the time at which the dew appears for the first time and dew point temperature.
Application of dew point meter
- This instrument is used on ships to protect cargoes from condensation damage by maintaining the dew point of air in holds lower than the cargo temperature.
- Used in industries for determining dew point.
- Limitations of dew point meter
- There are limitations in cooling fluids and light measurement.
- The temperature at which air is cooled to become
saturated and condensation begins
- More water vapor in the air
- Less the air has to cool
- Higher the Dew Point
- Comparison of the specific humidity and the
maximum amount of water vapor that the air can
hold at a given temperature and pressure
- UNIT -
- Relative humidity specific humidity X 100
- maximum capacity
- 100 Rel. Hum. saturated air
- 0 Rel. Hum. dry air
- Instrument to measure humidity
- Dry bulbs measure air temp.
- Wet bulb has a water soak wick around the bulb
water evaporates cools the air
Why is dew point important in compressed air?
- The dew point is the temperature to which air must be cooled to become saturated with water vapor
- When cooled further, the airborne water vapor will condense to form liquid water
- When air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface
What is the pressure dew point?
- The term “pressure dew point” is encountered when measuring the dew point temperature of gases at pressures higher than atmospheric pressure
- It refers to the dew point temperature of a gas under pressure. This is important because changing the pressure of a gas changes the dew point temperature of the gas - Vaisala
Negative effects of moisture in compressed air
- Washing away required oils
- Rust and scale formation within pipelines and vessels
- Increased wear and maintenance of pneumatic devices
- Sluggish and inconsistent operation of air valves and cylinders
- Malfunction and high maintenance of control instruments and air logic devices
- Product spoilage
- Rusting of parts after sandblasting
- Freezing in exposed pipelines during cold weather
- Further condensation and possible freezing of moisture in mufflers whenever air devices are rapidly exhausted
Dew point issues
- Instruments on refrigerated dryers do not measure actual dew point, output could be different
- Dew point controls on desiccant dryers:
- Incorrect readings may cause air quality issues
- Incorrect readings may cause air dryer inefficiency
- Example 1,000 cfm air dryer in fixed cycle 15% = 150 cfm
- At light load of 50% purge = 30% of actual
- Properly working dew point control could save up to 75 cfm
Example dew point problems – Refrigerated Dryer
- Food plant with refrigerated areas
- Audit showed dew point spikes during day production hours
- No alarms on dryer
- Constant problems with moisture in air, testing revealed contamination
- Overheating due to poor ventilation and minimal size of dryer
- Solution fix room temperature
- Install dew point probe and alarm
Example dew point problems – Heated Dryer
- Dryer was constantly cycling, would never reach -40 switching point
- Reading checked against portable dew point probe and found in error
- Malfunction in dryer cooling was damaging probe
- Replaced probe and operation returned to normal
Example dew point problems – Heated Blower Dryer
- Poor dew point in plant
- Dryer constantly running
- Compressor coolers plugged
- Dryer over temperature
- Excessive moisture in air
- Dew point probe flooded and out of calibration
- Cooling repair and probe recalibration
Example dew point problems – Dual dryers
- Poor dew point in plant
- Refrigerated areas in plant
- Dryer 1 frozen on right side dry
- Dryer 2 over temperature
- Excessive moisture in air
- Excessive purge flow during light loads
- Dryer repair and probe calibration
- Dew point not always what is seems
- Refrigerated dryers need probes
- Secondary probes on desiccant
- Proper calibration schedule
- Hook up alarms
- Make sure personnel are trained to recognize bad readings
- Handheld or data logger connected probe useful for CA auditing