USB DrDAQ User's Guide
19
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usbdrdaq.en r4
If you are going to compare the values measured with the DD101 and a calibrated
reference, your container will have to be large enough for the reference sensor too.
Make sure that the material that your container is made of is compatible with the
chemicals you are going to use. If the chemicals react in any way with the container,
the humidity reference will not be correct.
Standard Solutions
The standard solutions may be harmful to your skin, eyes or when
swallowed. Take all necessary precautions to avoid contact when
preparing and using the standard solutions.
There are several chemicals that have been measured under laboratory conditions to
find out what humidity they provide. Whichever chemicals you choose, make sure that
the humidity they give is within the operating range of the DD101 sensor (that is,
greater than 20% and less than 90% relative humidity). You should use at least two
standard solutions to give two fixed points for your calibration. Standard reference
books such as Kaye & Laby "Tables of Physical and Chemical Constants" (Longman)
give tables with the humidity of standard solutions. For example:
Compound
Humidity at 20 °C
Potassium chloride
85%
Magnesium nitrate
54%
Magnesium chloride
33%
Note:
The humidity produced by the standard solution depends on the temperature. If
the temperature changes from 20 to 21 Celsius this could give a change in relative
humidity of as much as 3% for a nominal 50% value. This change would be worse for
higher values of relative humidity.
Insulated box
Because the humidity produced by the standard solution depends on temperature, you
should use your test container inside an insulated box. A domestic cool box (without
the cold blocks) is suitable.
Preparation
The standard solutions may be harmful to your skin, eyes or when
swallowed. Take all necessary precautions to avoid contact when
preparing and using the standard solutions.
Prepare your standard solutions before you start the calibration procedure. Follow the
instructions given by the manufacturers of the solutions you are using. A slurry of
undissolved chemical in your solution should improve the stability. Make sure that all
the containers you use for preparation are thoroughly clean before use as
contamination of the solution will alter the humidity. Do not use tap water to make up
the solution as this is insufficiently pure: use distilled and de-ionized water. Allow time
for the solution to reach room temperature before use. Always use fresh solutions to
ensure that the chemicals have not become contaminated or degraded.
Allow the standard solutions and other equipment to reach the same temperature
(ideally 20 C) before you start.
