Which is More Accurate DPT or Flow Meter

This sounds good...

http://www.cidra.com/products/pw_vf100.html

1. OMEGA ENGINEERING - Selecting A Flowmeter
2. FLOWMETER SELECTION GUIDE - INSTRUMENTATION GUIDE.
   
3. Flowmeter Selection Guide - Cole-Parmer
   
4. Flowmeters Selection Guide
   
5. Flow Meter Selection: Choosing the Right Flow Meters by icenta ...
   
6. [PDF] DIFFERENTIAL FLOW METER SELECTION GUIDE

If the water is not high purity water, (de-ionized) like boiler water, then a magnetic flow meter has accuracy close to a Vortex meter, without the low flow cutoff range of a vortex flow meter at a slightly lower cost.

As a rule-of-thumb, the lower 10% of span of a vortex meter is regarded as zero flow, because of the inability to generate sufficient number of vortices at that low flow rate. So flow goes from zero to 10% (roughly) in a step function. The assumption with vortex is that the flow is not in the less-than-10%-flow-region because there is no accuracy below 10%, it's always zero.

Both mag and vortex need upstream/downstream meter runs, vortex needs longer significantly longer meter runs than mag.

The line pressure requires a tube that will hold that pressure, but all the major manufacturers cover that pressure range.

Clamp-on flow meters don't care about pressure and work well on uncorroded pipe.

The accuracy of an orfice plate drops off dramatically at lower flows. The flow is not automatically zeroed, but accuracy goes off the map. See Spitzer's article here for a summary of orifice plate accuracy (Spitzer wrote the book on flow)
http://www.flowcontrolnetwork.com/measurement/flowmeters/article/accuracy-of-orifice-plate-differential-pressure-flowmeters

It seems that someone "in the know" has spoken.

Orifice plates and DP transmitters are used because of thier relatively low cost when compared with other flow measurement instruments and because a DP TX can be transferred to a level or pressure scenario, it generally cuts down on the amount of spares that need to be warehoused. It's all an exercise in cost! If you want greater accuracy at those kind of flow rates with a medium like water, a low cost solution is a turbo impellar type flow meter. Very basic, very reliable, and easy to maintain. Hope this helps!

How about a volume positive displacement meter, similar to those at fuel stations?

As it has already been said if accuracy is your goal a magnetic flow meter is an inexpensive and effective means of measuring flow of a liquid that is conductive... Accuracy can be as good as 0.25% of flow rate. Process dependant.

For example:

http://tinyurl.com/cyuvb9a - Siemens

http://tinyurl.com/cken5eg - E+H

If You need better accuracy or if you are flowing a liquid that is non-conductive and the line size is less then 10-8 inches the best way to go is a Coriolis flow Sensor.....

The accuracy can be as good as 0.10% of flow rate.

For example:

http://tinyurl.com/7nmfpcw - Siemens

http://tinyurl.com/75zlmbk - E+H

Accuracy is just a matter of money how much do you need is up to you.

Cheers,

What is the line material?

What is the purpose of this water, i.e. what is it going to do when it arrives at its destination?

Flowmeter!