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5.2 Temperature Measurement
5.1.4 Calibration of Pressure Meters
Laboratory calibration of meters of pressure above atmospheric pressure is generally performed with a device in which calibrated weights are positioned onto a piston in an oil-containing cylinder (so-called dead weight tester). Such a device is not
suitable for small positive values and it cannot generate pressure below atmospheric
pressure. U-shaped liquid columns are used for low positive or for negative pressures. Pressure above or below atmospheric pressure is generated in a container by
means of a compressor. The pressure is derived from the height difference between
the two levels in the liquid column. There are devices allowing a very precise reading of the height difference. Mostly water is used as a liquid, but also lower-density
liquids. For negative or positive pressure ranges until 1 bar, a mercury column is
used, as in a barometer. In a laboratory, liquid columns are used directly as well
for pressure measurement in air. In an industrial environment, calibration is mostly
performed by comparing with calibration manometers and with a device for the
generation of positive or negative pressure. These calibration manometers are regularly checked in a calibration laboratory.
5.2 Temperature Measurement
5.2.1 The Glass Thermometer
The principle is the expansion of a liquid, mostly mercury or an alcohol, within
a spherical reservoir, by the influence of temperature. Glass thermometers do not
require periodical calibration, which is an advantage. Once the scale is determined,
it is definitive. The use of glass thermometers is awkward because of their vulnerability. There are types with metal protective tubes.
5.2.2 The Temperature Transducer
A temperature probe is mostly based on the temperature-dependence of an electric
resistance or by generating a voltage by means of a thermocouple. The variable
resistance is part of a Wheatstone bridge, together with three fixed resistances. A
thermocouple detects a temperature difference, typically with respect to the atmospheric temperature. This last one is then determined by a resistance measuring
device. The electric signal is read into a computer system. The difference between
both types is the more limited range of a resistance temperature transducer compared to that of thermocouple temperature transducer, but with a higher accuracy.
5.2 Temperature Measurement
5.1.4 Calibration of Pressure Meters
Laboratory calibration of meters of pressure above atmospheric pressure is generally performed with a device in which calibrated weights are positioned onto a piston in an oil-containing cylinder (so-called dead weight tester). Such a device is not
suitable for small positive values and it cannot generate pressure below atmospheric
pressure. U-shaped liquid columns are used for low positive or for negative pressures. Pressure above or below atmospheric pressure is generated in a container by
means of a compressor. The pressure is derived from the height difference between
the two levels in the liquid column. There are devices allowing a very precise reading of the height difference. Mostly water is used as a liquid, but also lower-density
liquids. For negative or positive pressure ranges until 1 bar, a mercury column is
used, as in a barometer. In a laboratory, liquid columns are used directly as well
for pressure measurement in air. In an industrial environment, calibration is mostly
performed by comparing with calibration manometers and with a device for the
generation of positive or negative pressure. These calibration manometers are regularly checked in a calibration laboratory.
5.2 Temperature Measurement
5.2.1 The Glass Thermometer
The principle is the expansion of a liquid, mostly mercury or an alcohol, within
a spherical reservoir, by the influence of temperature. Glass thermometers do not
require periodical calibration, which is an advantage. Once the scale is determined,
it is definitive. The use of glass thermometers is awkward because of their vulnerability. There are types with metal protective tubes.
5.2.2 The Temperature Transducer
A temperature probe is mostly based on the temperature-dependence of an electric
resistance or by generating a voltage by means of a thermocouple. The variable
resistance is part of a Wheatstone bridge, together with three fixed resistances. A
thermocouple detects a temperature difference, typically with respect to the atmospheric temperature. This last one is then determined by a resistance measuring
device. The electric signal is read into a computer system. The difference between
both types is the more limited range of a resistance temperature transducer compared to that of thermocouple temperature transducer, but with a higher accuracy.