User manual

Table Of Contents

Product information22

Test container

Your test container should be only just big enough to hold the DD101 and DD100

sensors, the stand, and a small quantity of standard solution (the solution should fill

approximately 5% of the volume of your container). It will also have to be water-tight

with a seal that allows you to pass the leads for the sensors into the container.

Note: The smaller your container, the more quickly the humidity will stabilise. You

can reduce the time taken for the system to stabilise by forcing the air in the

container to circulate. This will require a fan that does not introduce heat into the

container (it must be driven by a shaft or magnetic coupling so that the motor is

outside the container).

[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.