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Absolute pressure |
In rare cases it can prove necessary to measure the - physical - pressure in absolute terms (e.g. in closed systems). The sensors used in such cases must be specially encapsulated, however.
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Accuracy |
The actual permissible extent of deviation from the ideal characteristic curve is specified in terms of accuracy.
"FS" = Full Scale = final value (e. g. 20 mA)
"FSO" = Full Scale Output = span (e. g. 16 mA)
The accuracy specifications are always given as a percentage of the range of measurement (FSO).
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Characteristic curve |
The relationship between measured value and the output value is described by the characteristic curve. This is ideally a straight line.
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Characteristic curve |
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Conversion factor |
1 mbar = 100 Pa
1 bar = 14,5 PSI
1 PSI = 68,95 mbar
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Deviation of
characteristic curve |
Hysteresis
The hysteresis is the difference of the output value for a fixed measured value under increasing or decreasing pressure.
Deviation from the linear
The deviation from the linear is the amount of deviation from the characteristic curve of a reference line.
Reproducibility
The reproducibility defines the largest deviation of the output value for repeated runs at the measured value.
Temperature error
The temperature error shows the maximum deviation of the characteristic curve from the ideal case - for measurements taken at different temperatures - within the compensated sector. The temperature error is normally given as a value applicable to the entire compensated sector and not per 10K or 1K.
Deviation of the characteristic curve in accordance with IEC 60770
This value defines the maximum overall error in accordance with IEC 60770, composed of non-linearity, hysteresis and reproducibility in keeping with the boundary setting, i.e. the deviation of the initial and final values from the straight line.
As compared with the initial value setting or BFSL (best fit straight line), this definition gives the largest characteristic-curve deviation, but it is the easiest method for the user to understand. When accuracy details are given in data sheets, therefore, the error definition must also be specified.
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Deviation of characteristic curve |
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Final value
(full scale) |
The highest measured value to which the measuring device has been collimated. e.g.: 20 mA
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Hydrostatic level measurement |
The pressure exerted at a particular depth of a given fluid is directly proportional to the height of the column of this fluid above the position in question. For water, which has a density of 1 g/cm³, the pressure increases by around 100 mbar per metre of water depth. By calibrating on the basis of the density applicable for the fluid in question, the output signal of a hydrostatic level probe can be adjusted to the depth of fluid to be measured.
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Initial value (offset) |
The lowest measured value to which the measuring device has been collimated. e.g.: 4 mA
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Range / Full scale output (FSO) |
The algebraic difference between final value and initial value e.g.:
20mA - 4mA = 16mA
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Gauge pressure |
The pressure measurement is normally made relative to the existing air pressure, i.e. the air pressure corresponds to 0 bar. For pressures greater than the air pressure the value obtained is a positive one (overpressure).For pressures less than the air pressure it is negative (negative pressure).
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Gauge
pressure |
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Response time |
The response time describes (act. to DIN EN 61298 and DIN EN 60770) the period between the start of the step response time and the time for the output to reach and remain within 1% of output span of its steady value. The step corresponding to 80 % of output signal, giving an output change from 10 % to 90 %, then another from 90 % to 10 %. Synonymical (in the standards DIN EN 61298 and DIN EN 60770) it can be used the definitions response time and settling period.
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Response time |
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Types of sensors |
Piezoresistive silicon sensors
non-encapsulated for gasses and dilute, non-aggressive fluids with stainless steel membrane for standard industrial applications
Thick-layered ceramic sensors
for aggressive environments
Capacitive ceramic sensors
for aggressive environments and very small ranges of measurement
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