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Jan 08, 2020
04:57 AM

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Jan 08, 2020
04:57 AM

help unit problem

Hello why can I not solve this I want to solve Q to me in [m^3/hr]

5 REPLIES 5

Jan 08, 2020
07:23 AM

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Jan 08, 2020
07:23 AM

Because you cannot add apples to pears; or subtract them.

Under the root you've got 2801.0 (unitless) and

800 *DeltaPpump/rho*g

which calculates to something like 2056 m (so this one has units!)

Either 800 must have a unit of 1/m, or 2801 must be metres; or you will not be able to subtract one from the other.

Going back to your quadratic: If Q has a unit of m^3/hr, then 0.2 must have that same unit, and 3.5 must have a unit of m^6/hr^2. Then, in order to get the equation right, DeltaPpump/rho*g must also have a unit of m^6/hr^2, which it does not have (it has a unit of m, as I stated above).

All this is under the assumption that the formulae you are working with are physically based, that is, they are derived from a sound physical understanding and modelling of the problem. If thta's not the case, and this formula is experimentally determined, such that it 'happens to work', then you are better off stripping the units.

Success!

Luc

Jan 08, 2020
08:49 AM

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Jan 08, 2020
08:49 AM

You expect Q in m^3/hr?

If the equation is physically based and unit balanced, then the coefficients must have the units you see in the picture below.

Chances are that its an empiric formula and then you should strip off the units by dividing the input values by the units, the empiric formula expects them to be in.

Jan 08, 2020
06:02 PM

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Jan 08, 2020
06:02 PM

This is a beautiful illustration of why we need "UnitsOf". 😉

Jan 08, 2020
06:46 PM

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Jan 08, 2020
06:46 PM

@Fred_Kohlhepp wrote:

This is a beautiful illustration of why we need "UnitsOf". 😉

"UnitsOf" does not help if the emiric formula expects a variable in a different than the default unit.

Personally I'd prefer to use "SIUnitsOf" or rather chosing the units myself for better control as shown above. Otherwise a change of the unit system of the sheet has the effect of different results of the very same sheet.

Jan 10, 2020
05:35 AM

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Jan 10, 2020
05:35 AM

Thank you guys, I think i will use unit off in this simple situation, that to complicated 😄