Electricity + Control February 2018

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SENSORS, SWITCHES + TRANSDUCERS

Effects ofTemperature-related Density Changes on Hydrostatic Level Measurement Oleg Greber, WIKA W ith hydrostatic level measurement, temperature fluctuations in the medium have an effect on the accuracy of the meas-

urement result. But why is that, and how can you minimise the influence of temperature on hydrostatic level measurement? In the blog post ‘Hydrostatic level measurement in open geom- etries and vessels’ the calculation of the filling height is explained in greater detail. Hydrostatic level measurement is not dependent upon the shape of the vessel, and can be calculated with the formula:

h = p / ( ρ * g)

a precise hydrostatic level measurement with fluctuating medium temperatures is needed, then a compensation for the tempera- ture-related density change is absolutely required. With the knowl- edge of the current medium temperature, the actual density can be used to calculate the filling height. A temperature-related meas- uring error is thus prevented. Enquiries: Wika Instruments South Africa. Tel. +27 (0) 11 621 0000 or email greg.rusznyak@wika.com or visit www.wika.co.za

h: p:

Filling height

Hydrostatic pressure

ρ ρ:

Medium density

g:

Gravity

m:

Mass

V:

Volume

However, the medium density ( ρ = m / V) is subject to the influ- ence of temperature. The basis for this is the physical law that a volume expands when the temperature increases under constant pressure. This means that with rising temperature the density of the medium reduces and vice versa. Since the hydrostatic pres- sure p in open vessels remains constant, the influence of the tem- perature has a negative effect on the measurement result. There- fore, the measured hydrostatic pressure of a liquid should always be correlated to the medium temperature.

Oleg Greber, B.Eng, is in Product Management at WIKA Alexander Wiegand SE & Co. KG.

Case example: Deviations in accuracy without temperature compensation The density of water at room temperature (20°C) is 998,20 kg/m³. If the same density value is now assumed for the calculation with warm water at 80 °C, then there is a measuring error of 2,7%, since the density of warm water at 80°C is only 971,79 kg/m³ (see main picture). For oils and fuels, the temperature-related density change is even greater and thus a measuring deviation of approx. 4,5% is to be expected for the same example using motor oil. If you only want to monitor the fill level, while the accuracy plays a subordinate role, a temperature compensation by correcting for the density is not necessary. This is not required if either no or only small temperature fluctuations prevail in the process. If, however,

20 Electricity + Control

FEBRUARY 2018