blog How to choose the right instrument for measuring humidity and dew point Share Industrial Manufacturing and Processes Industrial Measurements Humidity measurement and control is called for in a wide variety of industrial applications. Each application has a different set of requirements for humidity instruments, such as required measurement range, tolerance to extreme temperature and pressure conditions, ability to recover from condensation, ability to operate in hazardous environments, and options for installation and calibration. There is no single device that is suitable for all needs. In fact, the range of available equipment is quite large, varying both in cost and quality. The right instrument for high humidity Environments with >90%RH are defined here as high humidity environments. At 90%RH a difference of 2°C can cause water to condense on the sensor, which in an unventilated space may take hours to dry. Vaisala humidity sensors will recover from condensation. However, if the condensed water is contaminated, the instrument accuracy can be affected due to deposits on the sensor, especially salt deposits. Even the life of the sensor may be shortened. In applications with high humidity where condensation can occur, a warmed sensor head probe such as the Indigo compatible Vaisala HUMICAP® Humidity and Temperature Probe HMP7 or Vaisala HUMICAP® Humidity and Temperature Transmitter HMT337 should be used. The right instrument for low humidity Environments with <10%RH are defined here as low humidity environments. At low humidities, the calibration accuracy of instruments measuring relative humidity may not be adequate. Instead, measuring dew point will provide a good indication of humidity. For example, Vaisala DRYCAP® products are designed for measuring dew point. If a dryer fails in a compressed air system, water condensation may appear and the instrument will need to recover. Many dew point sensors are damaged or destroyed in such situations, but Vaisala DRYCAP® dew point sensors withstand high humidity – and even water spikes. The right instrument for extreme temperature and pressure conditions Continuous exposure to extreme temperatures may affect sensor and probe materials over time. It is therefore very important to select a suitable product for demanding environments. In temperatures above 60°C the transmitter electronics should be mounted outside the process and only a suitable high temperature probe should be inserted into the high temperature environment. Moreover, built-in temperature compensation is required to minimize the errors caused by large temperature swings or operation at temperature extremes. When measuring humidity in processes operating at around ambient pressure, a small leak may be tolerable and can be reduced by sealing around the probe or cable. However, when the process needs to be isolated, or when there is a large pressure difference between the process and the external environment, a sealed probe head with appropriate mounting must be used. Pressure leaks at the point of entry will alter the local humidity and result in false readings. In many applications it is advisable to isolate the probe from the process with a ball valve to enable the removal of the probe for maintenance without shutting down the process. See the Indigo compatible Dew Point and Temperature Probe DMP8 with adjustable installation depth for pressurized pipelines or Dew Point Temperature Transmitter DMT348 also for pressurized pipelines. When is a sampling system needed for dew point measurement? Wherever possible, the probe should be mounted in the actual process to achieve the most accurate measurements and a rapid response time. However, direct installations are not always feasible. In such situations, sample cells installed in-line provide an entry point for a suitable measurement probe. Note that external sampling systems should not be used to measure relative humidity because the change in temperature will affect the measurement. Sampling systems can instead be used with dew point probes. When measuring dew point, sampling systems are typically used to lower the temperature of the process gas, to protect the probe against particulate contamination, or to enable easy connection and disconnection of the instrument without ramping down the process. The simplest dew point sampling setup consists of a dew point transmitter connected to a sampling cell. Vaisala has several models suitable for the most common applications and sampling needs. For example, the easy to install DSC74 sampling cell is designed for the flow and pressure conditions in compressed air applications. In demanding process conditions, sampling systems must be designed carefully. As dew point is pressure dependent, a flow meter, pressure gauge, special non-porous tubing, filters, and pump may be needed. In a pressurized system a sample pump isn’t needed as the process pressure induces a large enough flow to the sampling cell. When measuring dew point with a sampling system, trace heating should be used when the ambient temperature around the cooling coil or connecting tube is within 10°C of the dew point temperature. This prevents condensation in the tubing that connects the dew point instrument to the process. Hazardous environments Only products with appropriate certification can be used in potentially explosive areas. For example, in Europe products must comply with the ATEX directive, which has been mandatory since 2003. Intrinsically safe products are designed in such a manner that even in the event of failure they do not generate enough energy to ignite certain classes of gas. The wiring from the intrinsically safe product into the safe area must be isolated via a safety barrier. For example, the Vaisala HMT360 series of intrinsically safe humidity transmitters are specially designed for use in hazardous environments. Shock and vibration When the probe will be subject to excessive shock or vibration, the choice of probe, mounting method, and installation location needs careful consideration. Download a Humidity Guide for Smart Industries Measuring humidity in industrial processes and facilities usually brings you energy savings and improves end-product quality. Also indoor air quality is better when the humidity levels are optimal. Get access to Vaisala Humidity Calculator and Humidity Conversion Formulas, measurement glossary and knowledge about intrinsic safety, theoretical and practical humidity webinars and videos, a drying simulator and use cases and typical applications & instruments. Download here
Relative Humidity and Temperature Probe HMP7 for high humidities Vaisala HUMICAP® Humidity and Temperature Probe HMP7 is designed for applications which involve constant high humidity or rapid... Read more
Dew Point and Temperature Probe DMP8 with adjustable installation depth for pressurized pipelines <40 bars Vaisala DRYCAP® Dew Point and Temperature Probe DMP8 is designed for... Read more
Humidity and Temperature Meter Series HMT330 for demanding humidity measurement Measure humidity in harsh environments with the instrument you can depend on. The Vaisala HUMICAP® Humidity and... Read more
Dew Point and Temperature Meter Series DMT340 for demanding low humidity conditions Condensation-resistant DMT340 is designed for industrial low-humidity applications such as industrial drying... Read more
Intrinsically safe Humidity and Temperature Transmitter Series HMT360 for hazardous environments Note that the HMT360 will be discontinued during 2021. View the new HMT370EX, also designed for use in hazardous and... Read more
Dew Point and Temperature Probe DMP5 for in-line measurement in high temperature applications <180 °C Vaisala DRYCAP® Dew Point and Temperature Probe DMP5 is designed for in-line... Read more
Dew Point Probe DMP6 for in-line measurement in very high temperature applications <350 °C Vaisala DRYCAP® Dew Point and Temperature Probe DMP6 is designed for in-line... Read more
Relative Humidity and Temperature Probe HMP4 for high pressures Vaisala HUMICAP® Humidity and Temperature Probe HMP4 is designed for high-pressure applications such as compressed air systems in... Read more
Relative Humidity and Temperature Probe HMP5 for high temperatures Vaisala HUMICAP® Humidity and Temperature Probe HMP5 is designed for high temperature applications such as baking ovens, pasta... Read more
Relative Humidity and Temperature Probe HMP8 for pressurized processes Vaisala HUMICAP® Humidity and Temperature Probe HMP8 is designed for pressurized applications in compressed air systems... Read more
Compact Humidity and Temperature Probe HMP9 for rapidly changing environments The Vaisala HUMICAP® Humidity and Temperature Probe HMP9 is designed for easy installation in rapidly changing... Read more
Miniature Dew Point Transmitters DMT143 & DMT143L (Long) for OEM applications Miniature size dew point transmitters DMT143 and DMT143L are ideal choices when you want to measure dew point accurately in small... Read more
Dew Point Transmitter DMT152 Dew point measurement in very dry environments The Vaisala DRYCAP® Dewpoint Transmitter DMT152 is designed for measuring low dew point in OEM... Read more
Dew Point and Pressure Transmitter DPT146 for compressed air measurement DPT146 for compressed air measurement makes monitoring compressed air simple and convenient. The DPT146 measures both... Read more
Dew Point Meter DMT132 for refrigerant dryers Vaisala HUMICAP® Dewpoint Transmitter DMT132 is an affordable dew point measurement instrument designed to verify the... Read more
Humidity and Temperature Module HMM170 for environmental chambers Vaisala HUMICAP® Humidity and Temperature Module HMM170 is a robust open frame transmitter for integration into... Read more