PC-06Process measurement · MT
Humidity & Dew-Point Measurement
Moisture decides product quality, drying efficiency and corrosion risk. Relative humidity suits ambient and HVAC conditions; dew point states the absolute moisture content of compressed air and process gases.
01Scope
What's included
Relative humidity and dew-point measurement for air and process gases.
- Humidity and temperature transmittersDuct, wall and probe versions for HVAC, cleanrooms, warehouses and process air.
- Dew-point transmittersCompressed air, dryer outlets and low-dew-point industrial gases.
- Process moisture analyzersMoisture in natural gas and hydrocarbon streams, where contact sensors struggle.
- Sampling cells and accessoriesSampling blocks, flow control and filters that give the sensor a representative sample.
- Portable and reference hygrometersSpot checks and on-site verification of installed transmitters.
02Compare
Sensing principles
The expected dew point and the gas decide the principle: polymer sensors for air, oxide sensors for dry gases, optical methods for reference and hydrocarbons.
| Principle | How it works | Suited for | Considerations |
|---|---|---|---|
| Capacitive polymer (RH) | How it worksA thin polymer film absorbs water vapour, changing the sensor's capacitance with relative humidity. | Suited forHVAC, cleanrooms, dryers and process air — usually combined with temperature to derive dew point or absolute humidity. | ConsiderationsAccuracy falls near saturation and with condensation or contamination; periodic calibration checks are expected. |
| Metal-oxide dew point | How it worksWater vapour adsorbed in a porous oxide layer changes its capacitance with the partial pressure of water. | Suited forLow dew points in compressed air, dryer outlets and dry process gases. | ConsiderationsDrift requires periodic recalibration; oil and some contaminants affect the layer. |
| Chilled mirror | How it worksA mirror is cooled until condensation forms on it; the mirror temperature at that moment is the dew point. | Suited forReference and laboratory measurement, and calibration of other hygrometers. | ConsiderationsHigher cost, slower response and a mirror that must be kept clean. |
| Laser absorption (TDLAS) | How it worksA tunable laser measures absorption at a water-vapour line to give moisture concentration. | Suited forMoisture in natural gas and hydrocarbon streams where contaminants would foul contact sensors. | ConsiderationsAnalyzer-class cost; the background gas composition must be known for the calibration. |
General engineering guidance. Limits and performance depend on the specific instrument and variant — confirm them against the manufacturer's datasheet for the selected model.
03Specify
What to specify
The parameters that decide the selection. State each with its unit; where a value is not known yet, say so — it becomes part of the engineering review.
Measured quantity
Relative humidity, dew point, absolute humidity or ppmV — each suits a different principle and unit.
Range
The expected dew point or RH range, including upset conditions such as a dryer failure.
Operating pressure
Dew point depends on pressure — state whether pressure dew point at line pressure or atmospheric dew point is needed.
Gas and contaminants
Oil aerosol, corrosive components or hydrocarbons can damage or bias sensors.
Process temperature
Sets the probe construction and how close the gas is to condensing.
In-situ or sampled
Direct insertion or a sampling cell with controlled flow — sampling eases maintenance and pressure handling.
Accuracy and response time
Low dew points and dryer control need fast, stable response; HVAC monitoring is more forgiving.
Output and communication
4–20 mA or Modbus to the control system; relay outputs for dryer and process alarms.
Calibration
Interval, traceability and whether exchangeable sensors avoid gaps in monitoring.
Area classification
Process gases may require certified sensors and transmitters.
04Integrate
Outputs & communication
Interfaces typically offered in this category, and what each means for the control system. Availability depends on the specific model.
- 4–20 mAAnalog
- Current loop with a live zero: 4 mA is the bottom of the range, so a broken wire (0 mA) is distinguishable from a zero reading. Two-wire (loop-powered) on most transmitters; four-wire with an active output on line-powered instruments such as analyzers — the I/O channel must match. Accepted by practically every PLC and DCS analog input.
- HARTDigital over analog
- A digital signal (Bell 202 FSK, 1200/2200 Hz) superimposed on the 4–20 mA loop — configuration, diagnostics and secondary variables over the existing two wires.
- Modbus RTUSerial
- Serial client/server (formerly master/slave) protocol, typically over RS-485: several devices share one multi-drop cable and the client polls each for its values — common on analyzers, multivariable meters and packaged equipment.
- Relay / discreteDiscrete
- Switched contact or transistor output for alarms, interlocks and pump or valve control — it carries a state, not a value.
05Install
Installation practice
- Place dew-point sensors where the gas is representative and flowing — in a sampling cell or main line, not in dead legs.
- Use stainless-steel sample tubing (electropolished for very low dew points); hygroscopic materials absorb and release moisture and slow the response, and plastic tubing lets moisture permeate through its wall.
- Protect RH sensors from condensation, direct spray and liquid contamination.
- Plan exchangeable sensors or on-site verification so calibration does not interrupt monitoring.
06Products
Humidity & Dew Point products
Status · no models listed
Specific models are not listed on the site.
Our engineers can recommend instruments against your process data. The selection checklist on this page is the information that decides it — send what you have.
How a review runs
MT01Share the process data
Medium, range, conditions, connection and the system it reports to.
02Engineering review
The measuring principle and the installation are checked against the application.
03Instrument options
Suitable devices are proposed with the data needed to confirm the selection.
Next steps
Where to go next
- 01SolutionInstrumentationProcess measurement and final control elements — selected for the process conditions, installed, calibrated and loop-checked.
- 02Resource4–20 mA and HART: How Analog Instrument Signals WorkLive zero, loop power, loop resistance and the digital layer HART adds.
- 03Related categoryGas AnalysisOnline gas analyzers with sample extraction and conditioning.
Engineering enquiry
Measuring moisture in air or process gas?
Share the medium, the range, the process conditions and the system it has to connect to. Our engineers can help match the principle and the instrument to the application.