Absolute vs Relative Humidity

• Humidity is a measure of water vapor in the air.
• Meteorologists use different ways to describe water vapor in the atmosphere.

Absolute Humidity

• Describes the actual amount of water vapor in the air.
• It is measured in grams per cubic meter (g/m3).
• Measurement is regardless of the temperature.

Relative Humidity (RH)

• Indicates the actual water content of air.
• Air content is expressed as a percentage of the maximum amount it could possibly hold.

Relative Humidity vs Absolute Humidity

• Absolute humidity commonly refers to the actual amount of water vapor present in the air.
• Relative humidity is a percentage of the amount of moisture the air could possibly hold.
• Absolute humidity is expressed in moisture per cubic meter of air (g/m³).
• Relative humidity is expressed in percentage as the ratio of vapor pressure to saturated vapor pressure.
• Absolute humidity is independent of the temperature.
• When temperature goes up, relative humidity goes down and vice-versa.

Why Humidity Is Important

• Crucial for human health and comfort
• Important for animal health
• Plays a role in weather and climate
• Critical for industrial and manufacturing processes
• Significant for agriculture and plant growth

Qualities of Water Vapour

• Water vapour is invisible and behaves like a gas, condenses into water or gas.
• It reacts with surfaces and penetrates materials.
• A gas's ability to hold water vapour increases with temperature.

How to Quantify Humidity

• Relative Humidity
• Dew Point
• Fraction or Ratio
• Concentration

Other Important Humidity Concepts

• Saturation
• Partial Pressure
• Saturation Vapour Pressure (SVP)
• Supercooled Water

Hygrometer

• It is an instrument used for measuring the moisture content in the atmosphere.

Hygrometer Types

• Mechanical Hygrometer
• Electrical Hygrometer
• Dew-Point Hygrometer
• Psychrometer

Humidity Measurement

• It is an essential aspect of various industrial, scientific, and environmental applications.
• Selecting an appropriate instrument and ensuring proper measurement techniques are crucial for obtaining accurate and reliable results.
• Considerations for their use, sampling methods, and data recording procedures are essential.

Selection of Instruments

• The instrument has to have the correct measurement type, operating conditions, instrument performance.
• You have to configure the instrument correctly, and then consider power-supply and data output.
• Also, consider alarm and control features, environmental factors, safety and robustness, cost and maintenance, and user considerations.

Additional Instrument Considerations

• Operating Procedures
• Maintenance
• Calibration
• Interim Checks

Measurement Set-Up and Sampling

• Consider configuration, avoiding sample contamination, preventing condensation and measuring dry gases.
• Ensure there is a pump placement, temperature consideration and repeated readings.

Recording Humidity Measurement Results

• Data Logging
• Manual Recording
• Comprehensive Documentation

Humidity Measurement Examples

• Humidity measurement plays a crucial role in various industrial and environmental applications.
• Different settings require specific instruments and methods to ensure accurate and reliable monitoring.

Room Humidity Monitoring

• Aim for a long-term record of temperature and humidity in a moderate range.
• Consider unsupervised operation, whether data at intervals are enough e.g. hourly, real-time information needed, or record to view later.
• A possible solution is Automated "data logger".
• It can download reading easily to a computer.

Industrial Oven

• Humidity is measured at high temperatures to show progress of heat treatment such as drying or baking.
• Consider whether there will be dust, or other vapours apart from water.
• One solution is using relative humidity probes specified for high temperatures.
• Can extract a sample flow of gas to a cooler temperature (avoiding condensation).
• Check sensitivity to any chemicals given off during heating.

Compressed Air Supply Line

• Monitor air supply inside a pipeline to confirm level of dryness is a requirement.
• Consider any level of dust or oil in the gas stream.
• Consider if there is unsupervised use, remote reading and alarms needed.
• One solution is use a dew-point capacitive probe because the sensor usually has a sintered filter.
• Upstream filtering of the gas flow helps protect the sensor.
• Dew-point probes vary in their speed of response and long-term performance.

Climatic Test Chamber

• Has a range of temperatures and humidities.
• Rapid changes may need tracked.
• Condensation might occur at times (depending on the test performed).
• Consider response time, the robustness of instrument at hot and wet extremes and the actual planned range of test conditions.
• Possible solution: relative humidity sensors (probes) are designed to cope with a wide humidity and temperature range.
• Wet- and dry-bulb hygrometers (psychrometers) can be suitable, although they need careful setting up and maintenance.

Calibration

• Calibration is essential for accurate humidity measurement.
• Hygrometers (humidity measuring devices) do not always give exact readings.
• This is because of long-term drift due to sensor aging.
• Calibration tells you how much error is present.
• Calibration does not necessarily fix the instrument.

How is Calibration Done?

• A hygrometer is compared to a reference instrument in a controlled environment.
• The reference instrument needs to be calibrated and traceable to a standard.
• Done in laboratories or using field methods like salt solutions.

Uncertainty of Measurement

• All measurements have some level of uncertainty, meaning the actual value might be slightly different.
• Common causes of uncertainty consist of instrument issues, and environmental factors.
• Instrument issues are sensor drift, noise, and limited resolution.
• Environmental factors are temperature changes and unstable conditions.
• The level of confidence associated with the uncertainties is 95%.
• Normal uncertainty in ideal conditions: ±2% RH to ±3% RH
• Normal uncertainty in difficult conditions: ±5% RH to ±10% RH or worse.
• The best dew-point hygrometers in labs can achieve uncertainty as low as ±0.05 to ±0.1.

Humidity Measurement Dos and Don'ts

• Make use of the manual, and of manufacturer's advice
• Have the instrument calibrated, and take account of any calibration corrections required
• Let the instrument reach the temperature of the location
• Allow time for the humidity reading to stabilise (especially for dry gases)
• Record measurement results with suitable care
• Be clear when expressing humidity differences
• Check any hygrometer that has been exposed to extreme conditions or contaminants.
• Don't handle humidity sensors roughly, and don't abuse well-mounted sensors
• Don't use sensors outside the temperature or humidity range specified by the manufacturer – this could cause a shift in calibration.
• Don't expose sensors to condensation – unless they can definitely tolerate this
• Don't mix and match humidity probes with different electronics units.
• Unless they are definitely designed for this, instrument suppliers can advise if a probe is a “self-contained" interchangeable unit with its own signal processing inside

Watch Out For:

• Droplets, or stray water in any form. If present, humidity measurements may be misleading.
• Dust – impacts most instrument types.
• Watch pressure differences (dew point) or temperature differences (relative humidity).

Troubleshooting Incorrect Hygrometer Readings:

• Make sure all equipment is switched on and all cables are connected
• Consider whether the hygrometer and the conditions have had long enough to stabilise
• Check that the temperature is as expected (particularly for relative humidity measurement).
• Check the most recent calibration certificate and comparing against another hygrometer can help confirm a correct reading.
• Beware of human error mistakes in recording, mis-reading, wrongly numbering sets of sensors.

Special Cases of Humidity Measurement

• Consider the temperature change if measuring gas at one temperature and using it at another because measured values may need converted or interpreted
• Also consider the pressure change if measuring at gas at one pressure and using it at another because measured values may need to be converted or interpreted.
• In-vacuum Humidity measurements can be made below atmospheric pressure, requiring careful setup and interpretation.
• In non-air gases, select a humidity is measured like fuel ("natural gas") and various process gases by choosing a measurement method suitable for the particular gas.
• In harsh environments dust, vibration, condensation, airborne droplets, chemicals all pose challenges to most hygrometers.
• In older hygrometers modern electronic humidity sensors have increasingly long lifespans, if treated well.

Humidity Calculation

• Humidity is the amount of water vapor present in the air.
• Absolute humidity is the mass of water vapor in a given volume of air, typically measured in grams per cubic meter (g/m³).
• Relative humidity is the percentage of water vapor in the air compared to the maximum it can hold at a specific temperature.
• Specific humidity is the ratio of the mass of water vapor to the total mass of air, often expressed in grams of water vapor per kilogram of air (g/kg).

Why Calculate Humidity?

• Human Comfort and Health
• Weather Prediction and Climate Control
• Industrial and Agricultural Applications
• Preventing Mold and Damage to Materials
• HVAC and Energy Efficiency