E-M-HG2-S-V1_04
Rotronic AG
Bassersdorf, Switzerland
Document code
Unit
HygroGen2 humidity and temperature
generator: Instruction Manual
Instruction Manual
Document Type
Page
32
of 46
Document title
© 2009-2010; Rotronic AG
32
E-M-HG2-S-V1_04
6.4 Chilled Mirror Humidity Reference
For the lowest possible calibration uncertainty, the ROTRONIC recommendation is to use a chilled mirror
instrument to provide the best possible reference humidity (contact ROTRONIC for details of tested
chilled mirror systems). This type of instrument is generally accepted as a secondary humidity standard.
Advantages:
•
Chilled mirrors are capable of highly precise measurement with low drift.
•
A chilled mirror can be used over a broad range of humidity conditions.
Disadvantages:
•
Depending on the instrument used, certain operational effects can lead to increased uncertainty
(e.g. instrument resolution, mirror cleanliness).
•
Regular mirror cleaning is required to maintain best possible performance.
•
The need to have a second instrument may limit practicality when the HygroGen is used for on-
site measurements
•
Chilled mirror probes fitted through the HygroGen door can induce temperature gradients or
condensation within the chamber
•
Sampling type units require that the sample lines must be heated to avoid condensation when
passing through ambient conditions lower than the dew point generated within the HygroGen
chamber
•
Chilled mirrors may be more expensive to calibrate
Unquestionably a chilled mirror reference can resolve the best possible calibration uncertainty, but they
do require a good level of user expertise to ensure that best possible results are obtained. The user
should always consult with the manufacturer to ensure that the specific operational requirements of the
instrument used are considered to ensure best performance is attained.
6.5 Practical Guidance - Chilled Mirror Reference
Whilst the HygroGen is generally capable of reaching stable conditions within 5 to 15 minutes, the same
cannot be said of some chilled mirrors. Depending on the conditions being measured, some chilled
mirrors may require as long as 60…90 minutes to stabilise.
Attention should be paid to the following: (a) some types of chilled mirror sensors have the tendency to
over or under shoot, to “hunt” and can be off-the-mark for quite a long period of time, (b) at values
below freezing, the indications of a chilled mirror instruments without a force frost feature can be
ambiguous as readings may correspond to either the dew point or the frost point (c) whenever
measuring conditions result in a dew point that is higher than the laboratory room temperature, heat
the entire external sample loop, including the sensor head. Heating is required to avoid condensation.
