Humidity / Temperature

Fundamental terms of humidity measurement


Water vapor density (absolute humidity)

This is the amount of water vapor (kg) contained per unit volume (m3) of the gas mixture. In a gas mixture the water vapor generates a certain partial pressure that is part of the total barometric gas pressure. The vapor pressure can only rise to its saturation limit, which is determined by the temperature. Thereafter water is given off in liquid form (dew). The maximum pressure is called saturation pressure and is temperature dependent. The temperature dependency is, however, not included in the term of absolute humidity.

Relative humidity

Relative humidity is the relationship between the actual water vapor pressure and the maximum possible water vapor pressure.

%RH = 100xp/ps

%RH: Relative humidity percentage
p: Water vapor pressure in the gas mixture at ambient temperature
ps: Water vapor saturation pressure at ambient temperature

100% RH corresponds to the maximum amount of water vapor a gas mixture can contain at constant pressure and constant temperature. At constant water vapor partial pressure and changing ambient temperature, the water vapor saturation pressure changes and consequently the relative humidity also changes (see water vapor saturation pressure).

To obtain useful measurements of relative humidity, it is extremely important that the measurement probe and measured material have the same temperature.

Equilibrium relative humidity (ERH)

A hygroscopic material always tries to reach humidity equilibrium with the surrounding air. Equilibrium relative humidity is the free water content in a hygroscopic material after equilibrium is reached in an environment with constant relative humidity and temperature.

Humidity equilibrium then prevails when the amount of water absorbed and given off is equal.

Response time of Rotronic sensors 

Rotronic defines the response time of its sensors as the time taken to complete 63% of a step change in humidity levels. The response time becomes greater at low temperatures and low air movement. It also increases when a filter is used as the water vapor is transported through the filter more slowly due to the reduced air flow and the water exchange takes place with slower diffusion of the water molecules.

Dew Point / Frost Point (DP / FP)

The dew point is the temperature at which the air over water is saturated with water vapor at a constant air pressure. The water vapor pressure that then prevails is the same as the water vapor saturation pressure.

Wet bulb temperature (TW)

This is the lowest temperature that can be reached by evaporative cooling. The water given off by a wet surface is then in equilibrium with the water absorption capacity of the surrounding atmosphere.

Enthalpy (H)

To obtain useful measurements of relative humidity, it is extremely important that the measurement probe and measured material have the same temperature. The specific enthalpy of moist air is an energetic property. It is composed of the specific enthalpies of the components in the mixture (dry air, water vapor) and is related to the mass fraction of the dry air. It is given in J/kg.

Specific humidity (Q) in g/kg

This is the ratio of the mass of the water vapor to the mass of the complete gas mixture containing the water vapor.

Vapor concentration (DV) in g/m3

This is the ratio of the mass of the water vapor to the volume of the complete gas mixture containing the water vapor.

Mixing ratio (R) in g/kg

This is the ratio of the mass of the water vapor to the mass of the dry gas mixture containing the water vapor.

Water vapor partial pressure (E) in hPa

This is the fraction of the total pressure of a gaseous mixture due to water vapor.

Water vapor saturation pressure (EW) in hPa

This is the maximum pressure that water vapor can reach over a water surface at a given temperature.

Muan kinetic temperature (MKT)

The mean kinetic temperature is the total influence of temperature on an object or product over a certain period of time.

As a world-leading manufacturer of humidity measurement instruments, Rotronic is fully aware of its responsibility to offer instruments that can withstand the harshest operating conditions, while remaining user-friendly and requiring minimal maintenance. At the same time we urge our users to ensure excellent performance of the measurement instruments at the expense of little effort. The following checklist is provided as a guide.

  1. Analyze the environment in which the humidity probe is used. What suspended substances and/or chemicals exist and in what concentration?
  2. Install the probe at a place representative of the measured climate with good airflow across the sensor.
  3. Choose the right filter. Measurement is fastest without a filter. For wind velocities higher than 3 m/s, however, a filter must be used. The filter protects the sensor up to airflow velocities of 40 m/s. Suitable filters must also be used in the case of contaminants/pollutants and in harsh environmental conditions.
  4. Install the probe correctly to suit the application.
  5. For calibration, use one of our calibration services or the SCS-certified humidity standards. This will ensure your calibration is traceable to national standards.
  6. Inspect and replace the filter more frequently in harsh operating conditions. Filters can be cleaned in an ultrasonic bath. However, always keep a new filter set in stock.
  7. Check that the measurement probe is working correctly by performing a calibration at least every 6 to 12 months.

A Pt100 sensor changes its electrical resistance with every change in temperature in its environment. Its resistance value is 100 Ohms at 0 °C. This characteristic is used in a bridge circuit to generate a signal suitable for further processing.

There are five quality classes with the following tolerances at 0 °C.

Class B: ±0,3 K
Class A: ±0,15 K
Class B 1/3: ±0,1 K
Class B 1/5: ±0,06 K
Class B 1/10: ±0,03 K

The table below illustrates the tolerances for each Pt100 sensor class at different temperatures.

Pt100 temperature sensors

New standard

The manufacturing tolerances were formerly sub-divided into the accuracy Classes A and B (see above). The new standard contains the additional classes AA and C. Within the validity range of every class for wire-wound resistors and film resistors, the limit deviations (tl) are given in dependence on the temperature (t) in Celsius:

Class AA: tg = 0,1 K + 0,0017 · t
Class A: tg = 0,15 K + 0,002 · t
Class B: tg = 0,30 K + 0,005 · t
Class C: tg = 0,6 K + 0,01 · t

Example for Class B: At 200 °C deviations in the measured value of up to ± 1.3 K are allowed.

Accuracy specification for humidity and temperature

With its accuracy specification, Rotronic states the maximum permissible deviation of the HygroClip probe from the Rotronic SCS reference. The accuracy specification applies at the adjusted humidity and temperature values. A validated and permanently monitored process guarantees that all HygroClip probes undergoing production match the Rotronic references used. In addition to this, samples are taken from every production batch and checked for accuracy against SCS references.

Accuracy of humidity measurement over measuring range

HygroClip2 industrial probes
HC2-IC / HC2-IM/ HC2-IE / XD-Industrial

HygroClip2 probes
HC2-S(3) / HC2-SM / XD
HC2-HK / HC2-C / HC2-P / HC2-HP / HC2-HS

Continuous load

Rotronic HC2A industrial probes are designed for continuous loads of up to 190 °C. Rotronic standard probes up to 100 °C.

Accuracy of HC2 probes - humidity

1 Peak load:

The peak load at 200 °C is 100 h. See the sensor data sheet for detailed information on the pollution loads for the sensor.

Accuracy of temperature measurements over the measuring range

HygroClip2 industrial probes
HC2-IC / HC2-IM/ HC2-IE / XD-Industrial

HygroClip2 probes
HC2-S(3) / HC2-SM / XD
HC2-HK / HC2-C / HC2-P / HC2-HP / HC2-HS

Accuracy of HC2 probes - temperature

Some gases and contaminants/pollutants can damage Rotronic humidity sensors. The contaminants/pollutants can be divided into two categories: gases without influence and gases with an influence on the humidity sensors.

For contaminants/pollutants with an influence on the sensors and therefore with an influence on the measurement result, the maximum constant concentration must be known (see table below).

Contaminants/Pollutants with an influence

Contaminants/Pollutants with an influence

Contaminants/Pollutants without influence

Contaminants/Pollutants without influence

Note that the common sealing material silicone damages the sensor! When probes are installed, silicone must not be used!