As energy usage has become a lightning rod for conversation and studies regarding efficiency, the ability to measure and quantify energy consumption has become forefront in the battle to get a handle on what is being used and how efficient or inefficient that use may be.
However, before we can truly measure consumption, we really need to determine what constitutes a valid measurement and what does not. In any hot or chilled water measurement, what is the single most limiting factor in accuracy? If you look at the following measuring components; you have a matched pair of temperature sensors, a flow meter, and a flow computer.
Given matched pair temperature sensing technology accurate to +/- 0.01% of span (typically 200ºF) or 0.02ºF of reading, combined (+/- 0.04ºF) with a differential reading of typically 10ºF or greater, this equates to a worst-case accuracy of +/- 0.4% of differential temperature reading. This accuracy in a worst-case scenario is better than the best-case accuracy of most flow meter technologies being implemented currently as you will see below.
At optimal flow velocities, the most accurate volumetric flow technologies available may be accurate to +/- 0.2% of rate or reading. But, this does not take into account what velocity range the meter will be required to operate over in the application, or the flow profiling that is available or required in order to meet this accuracy. Typical volumetric flow meter technologies (Turbine, paddle wheel, Vortex, orifice plates, Variable area devices, pitot tubes, target meters, and magmeters) require a fully developed flow profile to make any accuracy statement. This typically means 10 pipe diameters upstream and 5 pipe diameters downstream of piping straight run.
In addition, all of these flow meter technologies have a turndown associated with the accuracy statement. This defines the minimum velocity at which the meter may operate and still meet its accuracy specification. Below this minimum velocity the meter typically has another accuracy specification, which is a fixed velocity error that is independent of the actual flow velocity. This means that below the minimum velocity defined by the turndown, the meter becomes logarithmically more inaccurate as the velocity drops.
In the case of mechanical meters such as turbines (at best +/- 0.5% of reading at 20:1 turndown) and paddle wheels (at best +/- 0.75% of reading at 10:1 turndown) there is a point where the meter just quits operating. Over time, this point of inoperability will climb higher and higher as the flow meter gradually fouls and wears.
In the case of a vortex meter (at best a +/- 0.8% of reading at 25:1 turndown), once the velocity drops to a point where vortices are no longer created, the meter ceases to function even though 15-20% of the flow span is still passing through the meter.
In the case of differential producers (at best +/- 1.0% of reading at 10:1 turndown), such as orifice plates, variable area, target, pitot tubes or bars, volumetric flow is measured on the premise of square root extraction. This fundamentally makes these technologies the most inaccurate at low velocities since any error is squared in the process of making the flow calculation.
In the case of typical Magnetic flow meters, which are the best of all (at best +/- 0.2% of reading at 100:1 turndown), a lack of moving parts prevents any pressure loss or heat (excluding insertion magmeters). This measurement technology can operate with less than ideal flow profile conditions (3 upstream and 2 downstream) and still maintain its accuracy statement.
Flow computers today have the ability to make calculations, provide linearization adjustments, and even further reduce temperature error by being able to zero out error in the operating differential span of the application. With OIML R75 compliance, flow computers can effectively provide reading accuracy to +/-0.1%, by far providing the most accurate component of the entire system.
So, in conclusion, the most limiting factor in measuring energy in a hot or chilled water application is by far the flow meter technology component. However, this still does not take into consideration the most influential factor of all, which we will save for our next post.
Read the next post: Not All Flow Meters are Created Equal, Part 2