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
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.
READINGS ARE EASILY CONVERTED TO:
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
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
Foxboro Wet- and Dry-Bulb instruments, and
direct-reading Relative Humidity
instruments of the hair type are also available.