The MBW 373 dew point mirror has been the standard in humidity measurement for over 20 years. Incremental development based on operational experience and customer feedback means that the 373 continues to meet the expectations of hundreds of users in some of the most challenging applications.
Despite its high performance and specification, the MBW 373 is easy to use and simple to maintain. Even early instruments remain serviceable thanks to the use of industry standard component architecture and careful control of software build records.
Recent 373 developments include improved materials of construction to further reduce sorption effects, higher pressure capability based on new mechanical designs, data acquisition software with external generator control, and multi-channel thermometer data integration.
Measurement and Control of Mirror Temperature
The intensity of reflected light from the mirror is continuously measured with a photo diode and a high resolution A/D converter to detect the dew/frost layer thickness. This measurement is used in the control system to drive the peltier element, establishing and maintaining the layer 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. A second mirror PRT with a 4 wire connection can be connected to an external bridge so that two measurements of mirror temperature are possible.
Optical 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 vapour content of the gas, it can take a very long time to establish a suitable frost layer on the mirror, then stabilise 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 vapour 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.