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. Sampling systems can instead be used with dew point probes. Note that external sampling systems should not be used to measure relative humidity because the change in temperature will affect the measurement. In such situations, sample cells installed in-line provide an entry point for a suitable measurement probe. However, direct installations are not always feasible. Wherever possible, the probe should be mounted in the actual process to achieve the most accurate measurements and a rapid response time. When is a sampling system needed for dew point measurement?

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. 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. Pressure leaks at the point of entry will alter the local humidity and result in false readings. 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.

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. Moreover, built-in temperature compensation is required to minimize the errors caused by large temperature swings or operation at temperature extremes. In temperatures above 60☌ the transmitter electronics should be mounted outside the process and only a suitable high temperature probe should be inserted into the high temperature environment. It is therefore very important to select a suitable product for demanding environments. The right instrument for extreme temperature and pressure conditionsĬontinuous exposure to extreme temperatures may affect sensor and probe materials over time. Many dew point sensors are damaged or destroyed in such situations, but Vaisala DRYCAP® dew point sensors withstand high humidity – and even water spikes. If a dryer fails in a compressed air system, water condensation may appear and the instrument will need to recover. For example, Vaisala DRYCAP® products are designed for measuring dew point. Instead, measuring dew point will provide a good indication of humidity. At low humidities, the calibration accuracy of instruments measuring relative humidity may not be adequate. 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.Įnvironments with <10%RH are defined here as low humidity environments. Even the life of the sensor may be shortened. However, if the condensed water is contaminated, the instrument accuracy can be affected due to deposits on the sensor, especially salt deposits. Vaisala humidity sensors will recover from condensation. At 90%RH a difference of 2☌ can cause water to condense on the sensor, which in an unventilated space may take hours to dry. In fact, the range of available equipment is quite large, varying both in cost and quality.Įnvironments with >90%RH are defined here as high humidity environments. There is no single device that is suitable for all needs. 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. Humidity measurement and control is called for in a wide variety of industrial applications.

Dew point measure