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.

Automatic Dew/Frost Determination

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.

Optimum Response Injection System (ORIS)

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 (optional)
• User Selectable Parameter and Range

Communication Interface

• Bi-directional RS-232
• Obtain Readings
• Read/Store System
• Configuration Parameters
• Verify/Edit Calibration Coefficients