Michell Instruments PC33, Руководство пользователя

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PC33 & 52 User’s Manual
Michell Instruments 19
appendix d
dew point in general
• The measuring environment and all parts of the sampling pathway must be kept
above the dew point if condensation is to be avoided. Electrical trace heating or
other heating methods should be used if necessary. An excess temperature of
10°C above the dew point is usually a safe margin.
• For measurements in the region below 0°C it must be clear whether the
condensate is dew or frost. Failure to distinguish between these can result in
errors of about 1°C for every 10°C below zero.
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 specific 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 - sa mple 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.