Hydraulics: head loss through instrument that discharges to waste

[thearete]

Setup: I have a 350mm main to which a y-shaped fitting of small diam piping is attached via a hydrant. A pressure logger is attached to one fork of the fitting and a water quality instrument containing ~2m of ~10mm ID hosing and a flow cell is attached to the other. Water discharges to waste after passing through the cell. Water is driven through the instrument solely by the pressure in the large diam main.

My problem: flow through the instrument influences the logged pressure. I'd like to be able to determine the pressure in the trunk as the flow is adjusted using a (distant) flow control valve without having to repeatedly disconnect the instrument. Can anyone think of a simple means to compensate for the presence of the instrument when intepretting the logged pressure?

Knowns:
Pressure in fitting ('point A'; logged)
Pressure at downstream side of instrument ('point B'; atmospheric)
Difference in elevations between A&B (most likely negligible given the pressures in the system (~140m))

Can calculate:
Equivalent Colebrook White roughness height (or Hazen Williams coefficient) of the instrument away from the field site.

Unknown:
Flow through the main at the location of the hydrant
Flow through the instrument to waste.

Has anyone got any tips on where to start with this one? I suspect that the orifice equation and/or the Bernoulli eq will be required but keep tieing myself in knots when trying to determine how they should be applied.

Cheers,

Will

 

[coloeng]

Not sure exactly what pressure you are trying to get. If you want the mainline pressure, connect your pressure logger to the other outlet on the hydrant. Because your flow for the water quality instrument is small, it won't effect the pressure in the hydrant compared to the mainline pressure.

 

[thearete]

Flow and losses through the instrument do seem to be significant: logged pressures jumped from 134m to up to 139m on detaching the device. I want to know whether there's a way of back-calculating the pressure in the main whilst the instrument and pressure logger attached to the same fitting.

 

[bimr]

There is no method of back calculating the water main pressure. The simple idea as suggested by the coloeng is to use a separate sample line.