The 373 Dew Point Mirror is a precision hygrometer exploiting
advances in digital technology to satisfy the highest requirements in the
measurement of humidity. As a true laboratory reference instrument, the 373
relies on proven optically detected chilled mirror techniques. This ensures a
direct, accurate, and stable measurement of the actual humidity over a wide
range of temperatures. The measured humidity may be displayed in a variety of
parameters including Dew Point, Frost Point, %RH, PPMv, PPMw, Absolute and
Specific Humidity. Choose your parameters. Choose your units. Choose your
language. Its all user selectable from the front panel.
Head heating, and built in control for external
heated hoses, allow for accurate measurement of above ambient dew points. And
with the internal gas pump and electronic flowmeter, gas sampling requires the
connection of only one external tube. The system is supplied as either a bench
style instrument, or can be fitted with optional mounting hardware for a
standard 19" rack.
General Information
Graphic Display
- Full Color Hi-Resolution LCD Touch Screen -
- User Selectable Display Parameters -
- Parameters Selectable as Numeric or Graphic -
- User Selectable Units -
The system uses an active matrix 640x480-pixel full
color liquid crystal display with an integral touch panel. It has a high
contrast ration and a wide viewing angle for easy readability. Data is displayed
with large, easy to read characters. Using the on screen menus, you can
configure the display for for a variety of humidity, temperature, and pressure
parameters. The parameters can be viewed in either numeric or graphic (strip
chart style) format. System units are user selectable and may be set to any
combination of SI and non-SI units.
Digital Control
Digital Measurement and Control of Mirror Temperature
- The reflected light from the mirror is continuously measured by a high
resolution A/D converter to detect the dew/frost layer thickness. This digital
signal is then used in the control algorithm to properly drive the peltier
element, establishing and maintaining the dew thickness at the equilibrium
point. The mirror temperature is measured by a 100-ohm platinum resistance
thermometer which is connected directly to a high accuracy, high precision A/D
converter. The resistance of the thermometer is measured by the A/D converter
and used, along with calibration coefficients, to computer mirror
temperature.
Dew/Frost Determination
Automatically Discriminates Between Dew and Frost -
For mirror temperatures above 0ºC, water vapor condenses on the mirror as liquid
water (dew). A condensation layer resulting from a mirror temperature above 0ºC
is considered a dew point.
For mirror temperatures far below 0ºC, water vapor
condenses on the mirror as solid ice (frost). A condensation layer resulting
from a mirror temperature far below 0ºC is considered a frost point.
However, for mirror temperatures between 0 and
approximately -20ºC, the state - water or ice - of condensed layer is
indeterminate. In this temperature range, it is difficult to know, without
visual observation, whether the mirror is controlling at the dew point or at the
frost point. Since these two states occur at different temperatures for gas of
the same water vapor content, it is important to determine which it is. The
errors resulting from this problem can be in excess of ± 2ºC
To correct the situation, this system can be
commanded to automatically force all sub-zero condensation to a known state of
frost. This is accomplished by rapidly cooling the mirror to below -40ºC, then
quickly returning it to the previously predicted frostpoint temperature. It is
then allowed to stabilize while ensuring the mirror temperature remains below
0ºC. Once forced to frost in this manner, the condensation will remain in frost
for all subsequent mirror temperatures that continue to remain below
0ºC.
Once the mirror temperature has risen above 0ºC, any
further attempts to stabilize in the indeterminate range between 0 and -20ºC
cause the system to once again cycle through forced frost formation. A forced
frost cycle may also occur at the completion of any automatic or manual mirror
check.
By ensuring that sub-zero mirror temperatures are
always forced to frost, the mirror temperature can be taken as the frost point
temperature. Since the dew point vapor pressure and the frost point vapor
pressure are equal, dew point temperature can be mathematically
computed.
ORIS
Optical Response Injection System - ORIS allows for
faster measurements at low frost points, generally below about -60ºC. Typically,
at these low frost point conditions, a chilled mirror hygrometer must cool the
mirror to a value well below the actual frost point temperature in order to
start the condensation process on the mirror. But due to the low water vapor
content of the gas, it can take a very long time to establish a suitable frost
layer on the mirror, then stabilize it at the proper equilibrium temperature.
This can often take in excess of several hours. And the lower the frost point,
the longer it takes.
ORIS solves this problem by momentarily injecting a
small amount of water vapor into the gas stream to assist the initial formation
of frost on the mirror, significantly reducing the amount of time required for a
stable measurement. Measurements that once took several hours or more, can now
be performed in a matter of minutes thanks to ORIS.
Programmable Outputs
- 2 Configurable 12-Bit Analog Outputs -
- User Selectable Parameter and Range -
Communication Interface
- Bi-directional RS-232 -
- Obtain Readings -
- Read/Store System Configuration Parameters -
- Verify/Edit Calibration Coefficients -
Abbreviated
Specifications
Dew/Frost Point Accuracy: |
0.1°C |
Dew/Frost Point Ranges Available |
-60°C to +20°C |
(option-L) |
-80 to 20°C |
(option-LX) |
-95 to 20°C |
(option-H) |
-30 to +70°C |
(option-HX) |
-20 to +100°C |
Readout |
High Contrast Color Graphic LCD,
Numeric Digits 0.5" (1
cm) High |
Power Requirements: (specify when ordering)
|
115VAC 60Hz, or 230VAC 50Hz |
|