Hello.
I wonder if it is fine to assume stream water as pure water when I calculate water density.
Pure water density is 1.0 kg / L at 4'C, and it decreases as the temperature increases. So, the density stream water at 20'C would be less than 1.0kg/L.
I think the water density must be affected by many ions and other substances exist in the stream water.
Salinity and ionic concentrations do greatly influence water density. So, it all depends on why you're calculating density. Small differences in salinity can have drastic effects on organisms, for example.
Dissolved minerals affect water density. You would need to measure the density with a hydrometer.
It depends on what are you looking for and where you are going to use the density of water. If you are working on water quality then no one will suggest to to use that assumption. For example, if streamflow water has high turbidity then it's density will be very different than pure water.
I'm working on figuring out the relationship between the water pressure and the barometric pressure in order to deduce the stream water level. The formula is h=(P-Pr)/(rho * g)
(h: water level, P: the water pressure, Pr is the barometric pressure, rho: water density, g: gravitational acceleration).
P and Pr are measured by sensors and g is about 9.8m/s^2, but I would like to know how much I input rho(water density).
Since you do not know what is the exact water density for your study stream, it is always normal to use standard value which is 1000 kg/m3 at 4 deg C and 998.2 kg/m3 at 20 deg C.
Dear Beomjeong Kim,
as you pointed out, several factors (chemical composition, temperature) influence the density of the water. Moreover, if you're working on rivers, also the presence of suspended sediments might increase it and it might be variable with time. You should measure it. Anyway this is seldom possible. In my experience, I used a similar instrument to measure the discharge from a spring and in that case I assumed constant density.
You could check your the results from the pressure sensor data with results from another instrument (e.g. level gauge) in order to verify if the assumption of constant rho is verified. Moreover you could perform a sensitivity analysis in order to check the maximum variations corrseponding to different values of rho (selected in a reliable range for your river) and then decide how to operate based on the uncertainty you're accepting in your problem.
Best regards
Luca Milanesi
In the case of sediment transport calculations, in freshwater environments (thus low salinity) and low suspended sediment concentrations, it is common use to use standard water density values (numbers are given in a previous answer) for calculations. The uncertainties caused by other parameters (sediment density, sediment size, Choice of the transport formula, hydraulic roughness issues) are orders of magnitude larger and govern the uncertainty of the result.
Very accurate densities of pure water have been widely tabulated as function of temperature. This you can find in many handbooks of chemistry and chemical engineering. By the way precise value of the maximum density of water at 4oC is not equal to 1.0 g/mL. It is very close to unity that you can find in many tables.
I don't understand the meaning of the equation, but for a field study you should use the density of the water in the field. Get a good scale and carefully weigh a gallon of water.
Beomjeong Kim,
you could not do that. Every water has special quality.
The density of water may depend on the constituents and environmental temperature.
Kim you are very correct, the water density decreases with increasing of temperature. Steam water has less density with comparison to normal temperature water. This, we have practically observed during our geothermal water/steam/ geysers water study.
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