I spoke with Siemens today about their Magmeters they want us to use on our equipment.
He stated flat out that they care not at all about flow runs or pipe and fittings before or after their flow meter.
The 2" model I looked at was flanged on either side, about 10" long.

This seems about right to me because this is a mass meter, but is that agreed upon by peers or the engineering community?

I have used magnetic flowmeters of Krohne Marshall and Yokogawa makes. As I remember, they recommend a straight pipe run of about 5D upstream and 3D downstream of the meter - with pipe size the same as the meter size.
(Often, for a given flow, you will find that it is more economical or even necessary to use a flowmeter one size smaller than the process pipe size, depending on the flow velocity through the meter).

Within this 5D / 3D distance, no bends, fittings, etc. are allowed.

Further, the angle for reducers / expanders in the vicinity of the flowmeter has to be maximum 4 degree.

I think, it would be prudent for you to follow the above guideline - since your line size is only 2", this might not pose any problems.

Actually, volumetric flowmeters that measure fluids which have no significant effect in density with respect to changes in temperature and pressure can be considered as massflow meters. If the density gets effected then you have to have density compensation.

But there are true massflow meters(for ex. corriolis, thermal etc.) that don't require density compensation. Straight length requirement is insignificant in this case.

There is no problem with conductivity if the water is potable(you will have problems with purified water and better quality waters apart from other pure liquids). As a thumb rule you can calculate conductivity as TDS x 0.6.

I once again caution you to check for full pipe flow as the volumetric flow calculations consider full pipe diameters. BMSG's suggestion about the expander and reducers should not be overlooked.

I had an application of using 2 mag meters, I was limited in space so the configuation was, 10" mag meter, 12" diaphram operated control globe valve, 12" tee (run-run), 24" of 12" pipe, second 10" mag meter,  On each side of each mag meter was a 12" x 10" reducer,  we were not looking for high accuracy in the second mag meter, just to find out about how much water was flow in or out of the tee,  The first mag meter was within the accurancy of a turbine meter 2 miles up stream, so it was OK,  the second mag meter read up to 10% low or high (depending upon the position of the control valve) when it was known there was no change in flow due to the tee.  When the valve was closed and flow from the tee (bull) to the second mag meter accuracy was better.
Results: 5D is vital when a globe valve is upstream, anything downstream 1d maybe

As for conductivity, a means of grounding the meter to the water is required, you can use ground rings installed in the flanges or a corporation on the downstream side,  lack of a good ground will give fluctating readings at steady flow.

A Mag meter works by inducing a magnetic field and measuring the deformation of the field by the fluid moving through the pipe, so small eddies should cancel out hence the reducers had no effect except to increase velocity for higher accurancies. 

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