Industry instrumentation : Units and conversion tables for process control - Instrumentación industrial : Unidades y tablas de conversión para el control de procesos


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The Foxboro Dew Point Measuring System measures dew Point temperatures between - 50 F and + 142 F within the limits of ambient temperature shown (from Foxboro® technical information ).

The Foxboro Dew Point Measuring System is a device used to measure the dew point of a gas or a compressed air system. The dew point is the temperature at which water vapor in the air begins to condense into liquid water. Measuring the dew point is important in many industrial processes, as it can help to prevent corrosion and freezing, and ensure the quality of the final product.

The Foxboro Dew Point Measuring System uses a chilled mirror dew point sensor, which measures the temperature at which the moisture in a gas or air sample condenses on a mirror surface. The system then calculates the dew point based on the measured temperature and the pressure of the sample.

The system is capable of measuring dew points in the range of -100°C to +20°C (-148°F to +68°F) and can operate at pressures up to 400 bar (5800 psi). It is designed to provide accurate and reliable measurements in a variety of industrial applications, including natural gas processing, petrochemicals, and power generation.

Left chart , X axis :Ambient Temperature ºF (from -60 to +260 )  , Y axis : Dew Point temperature ºF (from -60 to +160 )

Right chart , X axis :Ambient Temperature ºF (from -60 to +220 ) , Y axis , Relative Humidity %

The operating range is shown in the left chart in terms of dew point temperature. For example, at an ambient temperature of 60 ºF dew point temperatures from 60 ºF down to 10 ºF may be measured.

For convenience, the same operating range is shown in the right chart in terms of relative humidity. At the same ambient temperature of 60 ºF used in the example above, it can be seen that the corresponding relative humidity range is 100 percent down to 13 percent.

Another example illustrates the use of the chart in determining a working range of ambient temperatures. To measure a dew point temperature of 60 ºF it will be seen that the range of ambient temperatures must be between 60 ºF and 133 ºF

Dew points at higher operating temperatures are measured by cooling a sample to a temperature within the operating range shown.

For example, with a furnace atmosphere at 1600 ºF temperature and 2 ºF dew point, the sample is cooled in a refrigeration unit to 35 ºF Its dew point is then readily measured by the DEWCEL element.

Blast furnace air at 300 ºF temperature and 70 ºF dew point, may be measured by passing it through an uninsulated pipe leading to a sampling chamber. Here, it is cooled to 120 - 130 ºF enabling accurate dew point measurement.

Most dew point meters are capable of measuring dew points within a range of -80°C to +80°C (-112°F to +176°F) or even wider ranges. Some devices can measure dew points as low as -100°C (-148°F) or as high as +120°C (+248°F). It's worth noting that the dew point range can also be affected by the specific gas or liquid being measured and the environmental conditions of the measurement.


Absolute Humidity-The most accurate method of determining the moisture content of air is to measure its dew point. Dew point values are readily converted to absolute humidity units or vapor pressure units by reference to standard tables. Instruments reading absolute humidity directly are also available.

Relative Humidity-A two-pen Recorder can be furnished if a relative humidity measurement is desired. One pen records dew point; the second pen records dry-bulb temperature. A table or chart is used to convert these readings to relative humidity.

Foxboro Wet- and Dry-Bulb instruments, and direct-reading Relative Humidity instruments of the hair type are also available.


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