Dew point temperature and relative humidity are both measures of the moisture content of air or gas, but they serve different purposes — dew point gives an absolute measure of moisture content that is independent of temperature, while relative humidity expresses moisture as a percentage of the maximum the air could hold at that temperature. Understanding which parameter to measure, and choosing the right instrument for the task, is essential for applications ranging from compressed air quality verification to pharmaceutical cleanroom monitoring, cold store management and industrial gas purity control.
Unitest Instruments supplies humidity and dew point measurement instruments from Rotronic and Michell, and provides SAC-SINGLAS accredited calibration for humidity instruments through our ISO/IEC 17025 accredited laboratory (LA-2023-0845-C).
Relative Humidity vs Dew Point: Key Differences
Relative humidity (RH) expresses the actual water vapour content of air as a percentage of the maximum water vapour the air could hold at the same temperature. Warm air can hold more water vapour than cold air, so the same absolute moisture content gives a different RH reading at different temperatures. For example, air with a dew point of +10 °C has a relative humidity of approximately 100% at +10 °C, but only about 45% RH at +25 °C.
Dew point temperature is the temperature at which the air becomes saturated — the temperature to which the air must be cooled (at constant pressure) for condensation to begin. It is an absolute measure of moisture content that does not change with temperature. Dew point is the preferred parameter for applications where moisture must be controlled to an absolute level, regardless of temperature — such as compressed air quality, gas pipelines and drying processes. Relative humidity is preferred for applications where the human or process experience of humidity matters — HVAC, agricultural greenhouses, museum conservation.
Humidity Measurement Methods
Capacitive Thin-Film Sensors
The most widely used humidity measurement technology. A thin polymer or metal oxide film changes its dielectric constant as it absorbs water vapour, changing the capacitance of a capacitor with the film as its dielectric. The change in capacitance is proportional to relative humidity. Capacitive sensors offer:
- Good accuracy (typically ±1.5–3% RH over the 10–90% RH range)
- Fast response (seconds to a few minutes depending on housing)
- Wide humidity range (0–100% RH)
- Suitability for most HVAC, pharmaceutical, cleanroom and food monitoring applications
- Relatively low cost and compact size
Capacitive sensors are prone to contamination by certain chemicals and can drift if exposed to repeated condensation. They require periodic calibration to maintain accuracy. Rotronic capacitive humidity sensors, available from Unitest Instruments, incorporate ROTRONIC's proprietary HC2 capacitive element with on-board temperature correction and are designed for long-term stability in demanding environments.
Chilled Mirror Hygrometers
The most accurate method of dew point measurement. A mirror surface is cooled (by a thermoelectric element) until condensation just begins to form, detected by an optical sensor that monitors the reflectivity of the mirror surface. The temperature of the mirror at the condensation point is the dew point temperature of the gas. Chilled mirror hygrometers offer:
- Very high accuracy (±0.1 °C dew point or better for laboratory-grade instruments)
- Direct traceability to the ITS-90 international temperature scale
- Wide measurement range (from very low dew points in dry gas applications to near-saturation)
- Use as reference instruments and transfer standards for calibrating other humidity sensors
Chilled mirror hygrometers are more expensive than capacitive instruments and require more maintenance (the mirror must be kept clean). They are used as reference standards in calibration laboratories, and in applications where the highest accuracy is required — natural gas moisture measurement, pharmaceutical process gases, semiconductor process gases.
Psychrometers (Wet and Dry Bulb)
A psychrometer measures humidity from the difference in reading between a dry-bulb thermometer and a wet-bulb thermometer (whose sensing element is covered with a water-saturated wick). As water evaporates from the wick, it cools the wet-bulb thermometer. The greater the difference between dry-bulb and wet-bulb readings, the lower the humidity. Psychrometers are simple, low-cost and do not require electrical power, but they are less accurate than modern electronic sensors and are labour-intensive to use properly.
Aluminium Oxide Dew Point Sensors
An aluminium oxide sensor uses an aluminium substrate anodised to form a porous oxide layer, which absorbs water vapour in proportion to the dew point of the surrounding gas. The water changes the impedance of the sensor, which is measured electronically. These sensors are compact, suitable for in-line process installation and can measure very low dew points (-100 °C and below) — making them ideal for compressed air quality, inert gas systems and specialty gas applications. Michell Instruments, one of Unitest Instruments' key supplier brands, specialises in aluminium oxide and chilled mirror dew point instruments for demanding industrial applications.
Applications of Humidity and Dew Point Measurement
Compressed Air Quality
Compressed air systems in manufacturing plants, pharmaceutical facilities, hospitals and food processing plants must meet air quality standards — most commonly ISO 8573-1. Dew point is one of the three main compressed air quality parameters (alongside oil content and particle count). ISO 8573-1 defines air quality classes by dew point; Class 1 (the driest) requires a pressure dew point of -70 °C or below, while Class 4 allows up to +3 °C pressure dew point. Our article on compressed air energy savings covers related topics in the context of air system efficiency.
Dew point measurement in compressed air systems is performed at the dryer outlet to verify dryer performance, and at critical points in the distribution system to detect moisture infiltration. Aluminium oxide or chilled mirror instruments are used for this application.
Pharmaceutical Manufacturing and Storage
Humidity control is critical in pharmaceutical manufacturing and storage. Moisture accelerates the degradation of many active ingredients and excipients, promotes microbial growth and causes physical changes in tablets and capsules (swelling, hardening, capping). GMP guidelines require that relative humidity in production and storage areas is controlled and monitored. Calibrated capacitive humidity sensors and data loggers are the standard monitoring tools.
For hygroscopic products and those with very low moisture tolerance, dew point measurement of process gases or the manufacturing environment may be required in addition to relative humidity monitoring. See our detailed guide on pharmaceutical temperature monitoring for GMP context.
HVAC and Building Environmental Monitoring
Building HVAC systems control both temperature and humidity to maintain occupant comfort and building health. In Singapore's tropical climate, relative humidity control is particularly important — without active dehumidification, indoor RH can reach 80–90%, promoting mould growth, causing discomfort and damaging moisture-sensitive building materials and equipment. Combined temperature and humidity sensors are standard in HVAC control systems and building monitoring programmes.
Cold Store and Cold Chain
In cold stores, humidity control is important for food quality. Too dry and food loses moisture and weight (economic loss); too humid and condensation forms on packaging and surfaces, promoting mould and bacterial growth. In refrigerated pharmaceutical cold rooms, humidity monitoring supports GDP compliance. Monitoring both temperature and humidity in cold chain environments provides a complete picture of storage conditions.
Cleanrooms
As described in our article on cleanroom environmental monitoring, relative humidity in cleanrooms must be controlled to prevent ESD (below 40% RH) and microbial growth (below 60% RH). Calibrated capacitive humidity sensors provide the monitoring required for ISO 14644 compliance.
Instrument Selection Criteria
Choosing the right humidity or dew point instrument requires matching the technology to the application:
- For HVAC, pharmaceutical and food monitoring: Capacitive relative humidity sensors with data logging — Rotronic instruments from Unitest Instruments cover these applications.
- For compressed air and process gas: Aluminium oxide or chilled mirror dew point instruments — Michell instruments from Unitest Instruments are designed for this purpose.
- For calibration laboratory reference: Chilled mirror hygrometer with the highest available accuracy and direct traceability.
- For field measurement: Handheld combined temperature and humidity meters with probe capability — Rotronic handheld instruments provide good accuracy in a portable format.
Calibration of Humidity and Dew Point Instruments
Humidity sensors are particularly prone to drift from contamination, exposure to high humidity cycles and ageing of the sensing element. Regular calibration is essential to maintain measurement integrity. For pharmaceutical and food applications in Singapore, annual calibration by a SAC-SINGLAS accredited laboratory is the minimum standard.
Unitest Instruments' SAC-SINGLAS accredited laboratory (LA-2023-0845-C) provides calibration for relative humidity sensors and dew point instruments across the ranges relevant to Singapore's industrial, pharmaceutical and environmental monitoring sectors. Our certificates include measurement uncertainty and full traceability to national standards, meeting the requirements of HSA, SFA, NEA and international auditors. Standard in-lab turnaround is 3–5 working days. View our calibration services or contact us to arrange calibration.