Indicated Value vs. Full Value
Issues to consider when selecting an electronic torque tester:
1 Accuracy: Generally there are two ways of stating accuracy:

A. % of full-scale deflection or FSD
B. % of indicated value or reading

The following example will show the difference between the two methods:

Case 1 - Assume you have a 100 ft. lb. tester (maximum), and that the stated accuracy is +/- 0.5% of full scale.

At 100 ft. lb. +/- 0.5% full scale error = .5 ft, lb. This represents the “best case” error of the system. However, when a lower range is utilized, this .5 ft-lb becomes more significant. That is, on the same 100 ft. lb. tester;

      at 50 ft. lb. - .5 ft. lb. error = 1% accuracy
      at 10 ft, lb. - .5 ft. lb. error = 5% accuracy
      at 1 ft. lb. - .5 ft. lb. error = 50% accuracy

Therefore, what looks to be a good accuracy reading at full-scale actually translates into substantial error at the low range of the tester.

Case 2 - Assume you have a 100 ft. lb. tester (maximum), and that the stated accuracy is +/- 0.5% of indicated value.

      at 100 ft. lb. - .5% error .5 ft. lb.
      at 50 ft. lb. - .5 % error .25 ft. lb.
      at 10 ft. lb. - .5% error .05 ft. lb.

As can be seen by the above examples, error as related to full-scale value increases significantly as you go lower in the range, while error as related to indicated value stays constant throughout the useful range of the tester.
2 Range: Generally when manufacturers advertise % error of full-scale, their useful ranges will be advertised from zero to full-scale. That is, +/- 0.5% accurate (full-scale) from 0-100 ft. lb. This is interesting because at zero ft. lb., the system is only accurate to within +/- 0.5 ft. lb. Basically, error goes to infinity at zero.

Furthermore, transducers which are used to convert the mechanical torque into an electrical signal may become inconsistent below 10% of full-scale deflection.

It is for the above stated reason that systems which have accuracy as related to indicated value should state the useful range to be 10% to 100% of the tester range.

Therefore, if a tester has 100 ft. lb. maximum range, it should not be used at less than 10 ft. lbs. if the desired accuracy is needed.

It is CDI’s belief that in order to be completely honest to the customer, accuracy should always be stated as a percent of indicated value and the useful range should correspond to that stated accuracy. This will prevent the user from having to calculate what the
real error is at any given range.

3 Circuitry: There are two basic ways of measuring the output of a torque transducer.
  1. Analog (non-microprocessor based pure analog)
  2. Digital (microprocessor based plus analog input)

Without in-depth explanations of these two systems, the following advantages of having digital circuitry are well known throughout the electronics industry.

  1. Digital systems are economical, flexible and compact.
  2. Digital systems improve reliability in the face of hardware imperfections.
  3. Digital systems allow the ability to make logical decisions, carry out digital computations (unlimited unit conversion) and store the results in memory.

Basically, full digital systems are computer controlled. It is important that the terms “digital display” or “digital memory” do not necessarily mean that the system has full digital circuitry.