Humidity and Temperature Measurement in Different Pressure Environments
In industrial and scientific applications, accurate measurement of temperature and humidity is essential for maintaining the integrity of processes and products. However, varying pressure conditions – such as those found in cleanrooms, high-altitude environments, and vacuum or pressurized chambers – can make it challenging to determine the appropriate sensors for each scenario. Therefore, this blog will focus on the importance of selecting the right probes to ensure accurate results and a safe measurement setup.
Before delving into specific conditions, it is important to revisit the basics. A clear understanding of the fundamental concepts of humidity is crucial for comprehending how these parameters interact across different environments. Humidity represents the quantity of water vapor present in air or other gases and can be expressed in various forms: relative humidity, absolute humidity, dew point, or mixing ratio.
The relative humidity corresponds to the ratio of the current amount of water vapor in the air (e) to the maximum amount the air can hold at a given temperature (es) , expressed as a percentage
The saturation vapor pressure es depends on temperature and can be calculated using several empirical formulae. In atmospheric pressure applications, humidity is mainly dependent on temperature, because relative humidity (RH) is determined by the ratio of the current water vapor content to the maximum possible amount of water vapor in the air that can be maintained at a given temperature. As the temperature changes, so too does the maximum amount of water vapor that the air can hold, and so does the humidity.
Outside the atmospheric pressure, overpressure (e.g pressure chambers) or negative pressure (Vacuum), the pressure also has an influence on the humidity measurement. In a vacuum or under positive pressure, the saturation vapor level changes as the physical conditions of the air change. Therefore, the pressure acts on the maximum amount of water vapor that can be maintained at a given temperature. In a vacuum, where the pressure is greatly reduced, the air can hold less water vapor, which affects the measurement.
The same scenario is given for some psychometric calculated values like wet-bulb temperature, enthalpy, specific humidity, etc., where the pressure must be considered. For a clear understanding of how pressure affects these values, try out the Humidity and Dew Point Calculator on our website. This is a helpful, easy-to-use tool that provides accurate insights.
Pressure Scenarios – What are the Differences?
In general, there are three key scenarios where temperature and humidity measurements are required under varying pressure conditions: vacuum, atmospheric pressure, and overpressure environments. The following sections provide a detailed explanation, offering comprehensive insights and clarifications on the key aspects.Vacuum…
is a condition where the pressure is significantly lower than atmospheric pressure, with very few or no air molecules present. Vacuums are generally classified by the amount of pressure below atmospheric pressure, which corresponds to 1.013 bar or 1013 hPa.
Vacuum Chambers – Humidity and Temperature Measurement
Vacuum chambers are used in various industrial and scientific applications to create environments that are free of air and other gases. This enables experiments and processes that would not be possible under normal pressure conditions.
Typically, a vacuum pump system initiates the process by removing air from the chamber, reducing the pressure to approximately 10⁻³ mbar. In certain applications, a high-vacuum pump may be employed to further decrease the pressure, achieving high vacuum levels or even ultra-high vacuum conditions of up to 10⁻⁹ mbar. Once the desired vacuum is established and the necessary processes are completed, air is reintroduced into the chamber in a controlled manner using a needle valve. This step is carried out gradually to prevent sudden pressure changes that could cause damage. The controlled intake of air restores ambient humidity, enabling sensors to accurately measure relative humidity.
For those applications requiring precise humidity and temperature measurements under low pressure conditions, we offer the Rotronic HC2A-IE probe. This probe features a specialized screw-in connection, ensuring a secure installation that prevents leaks, thereby maintaining measurement accuracy and preventing pressure loss. The HC2A-IE is engineered to withstand pressures ranging from 0 to 100 bar and is available with two mounting thread options: G 1/2" or NPT 1/2".
The response time of our humidity sensors Rotronic HYGROMER HT-1 is defined under standard conditions of 23 °C, 1013 mbar, and 1 m/s airflow. The humidity sensor detects fluctuations in this partial pressure, with its performance shaped by its response time under the prevailing ambient conditions.
Atmospheric pressure (also known as barometric pressure) refers to the force exerted by the weight of the air in the Earth’s atmosphere on a given area. It is the "normal" pressure experience at sea level and varies slightly depending on altitude and weather conditions. At sea level, atmospheric pressure is slightly higher than 1bar.
Pharmaceutical Drug Production - Humidity and Temperature Measurement
Maintaining precise temperature and humidity levels in the pharmaceutical industry is essential to ensure product quality, efficacy, and compliance with Good Manufacturing Practices. Deviations from optimal conditions can cause chemical degradation, physical changes, microbial contamination, and loss of stability, compromising the safety and effectiveness of drugs. These issues can also disrupt manufacturing processes by altering powder flow properties, causing uneven dosing or difficulties in tablet compression, and leading to inconsistent product characteristics such as density, hardness, or dissolution rates. The consequences are severe, often resulting in batch rejections and financial losses. At the same time, poor environmental control may lead to negative findings during inspections by regulatory bodies like the FDA or EMA. Effective monitoring and management of these conditions are crucial to protecting product integrity and maintaining regulatory compliance.
For industrial environments under atmospheric conditions, we recommend two robust industrial probes: the Rotronic HC2A-IC, made of PPS, and the Rotronic HC2A-IM, made of stainless steel. These probes are designed to measure a wide temperature range of up to 200 °C and can be seamlessly integrated with transmitters for controlling alarms and HVAC systems.
Overpressure: refers to any pressure that is higher than atmospheric pressure. It occurs when a system is pressurized above the normal atmospheric pressure.
Compressor Monitoring – Humidity and Temperature Measurement
An air compressor is a mechanical device designed to increase atmospheric air pressure by reducing its volume. It operates by drawing in ambient air, compressing it within an enclosed chamber, and storing it in a pressure vessel. The compressed air can then be released in a controlled manner to power a wide range of tools and equipment, from basic inflators to sophisticated machinery.
The air flows into the compressor tank through the compressor input line (marked in red). To ensure that the mounted probe accurately measures the humidity and temperature within the tank, a pressure chamber is installed on top of the compressor line. This setup provides a controlled airflow directed through the sensing element, allowing precise monitoring of the humidity and temperature levels in the application. The mounted measuring chamber, equipped with an overpressure valve, ensures that the actual pressure within the tank remains unchanged, even with the measuring chamber in place.
The Rotronic HC2A-IE probe, when integrated with the Rotronic HygroFlex Advanced HF5A Transmitter, offers precise monitoring of sudden changes in humidity and temperature. This setup enables the configuration of various alarms, providing timely notifications to address potential issues before they impact the compressor's connected systems and equipment. Such proactive monitoring ensures continuous and reliable operation.
Most air compressors designed for general industrial and commercial applications operate within a pressure range of up to 12 bar (approximately 174 psi). This pressure level is adequate for powering a wide variety of pneumatic tools, applications, and production machinery, making it suitable for diverse operational needs.
The HC2A-IE industrial probe is well suited for various overpressure applications due to its robust design and compatibility with specific measurement chambers. When directly screwed into a line, the HC2A-IE can withstand pressures of up to 100 bar. Alternatively, when used with measurement chambers equipped with fixed and quick-action compressed-air couplings – such as the LDP-FCPB1 (made of POM) and the LDP-FCSB1 (made of stainless steel) - it can handle pressures up to 16 bar.
Important: The measurement chamber must not be connected directly to the compressed air supply without a properly secured probe screwed in place. Additionally, the probe should never be removed from the measurement chamber while the chamber is still under pressure. Failure to follow these precautions could lead to safety hazards and potential damage to the equipment.
To ensure optimal performance, it is recommended to position the probe as close as possible to the application being monitored. Long pipelines between the measurement chamber and the main line can complicate measurements, reducing accuracy and increasing response times. For reliable readings, any potential drops in environmental humidity and pressure must be avoided by eliminating leaks in the system, except for the outlet valve of the measurement chamber, which is specifically designed for controlled airflow.
Related Content
PST Rotronic Humidity Probes OverviewContact Us
Reach out to PST for more information on our products and how we can assist with your measurement needs.
Related Products
Industrial Humidity Probes - Rotronic HC2A-IE
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Industrial Humidity Probe - Rotronic HC2A-IC
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