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Interpreting Product Accuracy Specification with Ohio Semitronics Products

May 5, 2016

PC5 Watt Transformer with Current Transformer

Industrial design engineers and project managers contact OSI with a list of requirements in mind to fulfill an electrical measurement in their application. Requirements may include a product with the smallest size, or the least expensive cost to do the job. Agency listings, Shock ratings and harsh environments can be factors in the consideration with providing existing products or developing a new design to function economically and reliably. Government and Agency standards are requiring stricter efficiencies in certifying appliances and equipment testing facilities that in turn require monitoring equipment to be more and more precise in the measurements. Accuracy is often discussed in the requested project outline. It is important to understand the various products and the differences between their accuracy statements, what they mean and how they are calculated to determine the product’s tolerance over a linear span.

Accuracy is a reference to the transducer's input range and/or output range. In a calibration facility setting, the input Current, Volts or Power supplied to the transducer are fixed values and accurate to N.I.S.T. or higher standards. Due to the precise input equipment, Accuracy is proven or measured with the product analog or display output. When discussing applications or when a transducer is installed into its end use, the inputs are not fixed and the only reference is the output. In this case accuracy and tolerance limit is referenced to the transducer input scale.

For the most part Voltage, Current & Power Transducers have an accuracy listing that is ± % based on F.S. (Full Scale). Accuracy may include linearity (tolerance over the input or output span from 0 to 100%). A more accurate linearity specification may be listed separately. Repeat-ability is often included in the accuracy statement and relates to the transducers ability to measure consistently over and over.

Specified accuracy as “Percent of Reading” referenced with highly accurate transducers is calculated based on any selected point over the input or output span resulting in a sliding tolerance that gets smaller as the input decreases from full scale. These Percent of Reading based models will also have a second listing for accuracy in percent that is added to the first number. This two-step spec insures that the tolerance at very low inputs is achievable.

Temperature change is also a factor in calculating a transducers tolerance limit and is additive to the listed accuracy tolerance. Basic models will specify an operating temperature range listed in Celsius and followed by the “Effect”. The effect is expressed in a ± percent based on Full Scale. A more accurate transducer will have the temperature effect listed as a percentage per degree Celsius. This temperature related tolerance limit will increase proportionally with the amount that the temperature rises or falls from room/ambient.

“System Accuracy” is a calculation based on the use of several products used together that are not calibrated as a system. Some examples of this type of system is a watt transducer that will be installed into an end-user site that already has existing current transformers and in some cases there will be potential transformers as well.

Using the Square Root of the Sum of the Squares will give a reasonable tolerance limit as opposed to simply adding all the system item accuracies together. An example of a system using a watt transducer that is rated ±0.5% F.S. and using with a single current transformers that is rated ±1.5% F.S. the system accuracy is as follows:

  • Watt Transducer accuracy: ±0.5%
  • Current Transformer accuracy: ±1.5%
  • Square the ratings: .5 x .5 = .25 along with 1.5 x 1.5 = 2.25
  • Sum the two values squared = 2.5
  • Square Root of the squared value = ±1.58% applied to the system accuracy of the two components being used together. Specifically, electrical transducers measuring AC power systems can include up to 3 Current Transformers (CT’s) and 3 Potential (Voltage) Transformers (PT’s), so the above formula could start with seven items squared in the start.