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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 ).

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.


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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