In most cases, the errors caused by temperature arise as a result of thermal
expansion (in solids, liquids, or gasses), or from the change in electrical
resistance caused by changes in temperature. Sometimes temperature errors
are self-compensating. For example, when measuring a piece of steel with a
steel micrometer, if both items are at the same temperature, regardless of
what it is (within reason, of course), there will be no temperature error
since both items will expand or shrink by the same amount. This situation
does not exist, however, where different metals are involved, as they will
certainly have different coefficients of thermal expansion.
When
calibrating carboloy gage blocks against steel gage blocks, they must be
normalized to some particular temperature.
Many instruments contain
temperature compensating mechanisms to reduce or eliminate the temperature
error. Some of the ways in which temperature affects the measuring process
are:
1. Thermal expansion.
2. Electrical resistance.
3. Chemical action.
4. Thermoelectric effects.
(f) It is necessary to know how heat affects the properties of
materials in order to plan for its compensation. Most solids, liquids, and
gases expand when heated. This expansion is not always linear. There may
be more expansion at some temperatures than at others.
There are some
exceptions.
In a certain temperature rangy, water contracts instead of
expanding as it gets hotter.
(g) Some substances conduct heat better than others. The specific
heat capacity of a substance is the quantity of heat required to increase
the temperature of a unit mass of the substance 1.
We have to put more
calories of heat into a gram of water to raise it 1 C than we would into
one gram of aluminum to get the same temperature change.
(h) When humidity changes, the dielectric constant of air changes.
This causes air-spaced capacitors to change in value and air-filled
waveguides to change their characteristic impedance and propagational
velocity.
These changes will cause errors in the readings unless the
humidity is controlled or allowed for. Natural air humidities as low as 5%
or as high as 95% are not uncommon. The leakage currents of many components
vary with the humidity. Some high voltage equipment breaks down completely
under high humidity conditions. Humidity affects the dimensions of certain
organic materials; for example, soft wood, human hair, and manila rope.
Human hair is often used in relative humidity indicating instruments.
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