DT722 User’s Manual
Michell Instruments
15
APPENDIX D
Relative humidity in general
•
Due care must be taken of temperature. The effect of temperature on humidity is highly
significant. Failure to take this into account can sometimes lead to errors so large that the
measurement is meaningless. In many situations, the largest single source of uncertainty
in a humidity measurement is the effect of temperature differences from place to place
in the process, room or chamber. The importance of considering the temperature effects
carefully cannot be overstated when relative humidity is the parameter of interest.
•
Care must be taken when expressing uncertainties, changes or fractional differences in
relative humidity. For example, the difference between 50% RH and 52% RH is 2% RH.
This can also be expressed as a difference of 4% of value. It is important to distinguish
clearly between these two kinds of statement.
Recommendations specifi c to ranges of measurements
• Ambient
humidity
-
Avoid using hygrometers near the body, which is a source of heat and
moisture. Do not breathe close to the measurement.
•
High humidity, above the ambient range -
Sample lines should be maintained above the
dew point of the gas being measured, to avoid condensation. Electrical trace heating is
often the most practical method.
•
Low humidity, and very dry gases -
If possible, prepare for measurements by flushing
sample lines and hygrometers with dry gas, or by evacuating to low pressure. Drive
off stray residual water by baking assemblies if possible (but not instruments – unless
designed for this!). The lower the moisture content to be measured, the more dramatically
the required drying time multiplies.
•
Avoid hygroscopic materials. At low humidity (anything much below a dew point of 0°C)
the amounts of water given off by organic and porous materials can dramatically affect
the value of humidity. The lower the level of moisture, the more significant the effects.
•
Choose impermeable materials, to avoid inward diffusion of moisture through sampling
tubes and enclosures. Steel and other metals are practically impermeable. PTFE (‘Teflon’)
is only slightly permeable and will usually be satisfactory for dew points above -20°C, and
sometimes below this level. Materials such as PVC and rubber are relatively permeable
and so totally unsuitable at low humidity, and not really satisfactory in any humidity range.
•
Surface finish of pipework is important for very dry gases. Even the tiny quantities of
water adsorbed on the surfaces of non-hygroscopic materials can have significant effect.
Polished or electropolished steel is recommended for the best results.
•
Clean environments are always best for humidity measurements, but this is especially
critical at very low humidity. Even fingerprints harbour water. High purity cleaning agents
are recommended: Analytical Reagent (AR) quality solvents for oil-based contaminants,
and purified water (distilled or de-ionised) for salts. Cleaning should be followed by
thorough drying by a clean method.
•
Sample tubing should be as short in length as possible. The surface area should be
minimised by using the narrowest tubing that the flow conditions will permit.
•
Avoid leaks. Minimising the number of connections (elbows, tees, valves, etc.) helps with
this.
•
Adequate flow of the gas sample should be ensured, to minimise the influence of sources
of stray water in the flow path.
•
‘Dead ends’ should be avoided, as they cannot easily be flushed.
•
Back-diffusion of moisture should be minimised, e.g. by fast flow rates of gas, long exhaust
tubes after the sensor, or by valves which isolate the low-humidity region from ambient air.
