FAQs
Force - FAQs
The answers to some frequently asked questions about the measurement of force are grouped below.
Force, Mass, Weight, Load & Gravity
- The variation in the value of g across the earth's surface is about 0.5 % due to latitude, plus a change of approximately 0.003 % per 100 m altitude. Local topography and tidal forces also can have small effects.
- The term g-force, or Gs is not well defined but is sometimes used to convey something like a value of force, expressed as a proportion of the nominal gravitational force experienced when standing on the earth's surface or a value of acceleration, expressed as a proportion of the nominal gravitational acceleration experienced when in free-fall just above the earth's surface.
- Force is a measure of the interaction between bodies, mass is a measure of the amount of material in an object, weight is the gravitational force acting on a body (although for trading purposes it is taken to mean the same as mass) and load usually means the force exerted on a surface or body.
Units & the SI System
- Equivalent force values are given here.
- The Système International d'unités - the SI system - is the coherent system of units adopted and recommended by the General Conference on Weights and Measures (CGPM). It is based on seven base quantities: length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity.
- The International System of Units (SI) is widely used for trade, science, and engineering. The SI unit of force is the newton, symbol N.
Accuracy & Uncertainty
- No, the difference is not very meaningful at all.
- Probably not. The figures should be interpreted with caution, especially if an instrument has not been independently calibrated for some time.
- Yes there is a difference.
- The measurement uncertainties associated with specifications for force-measuring devices are often expressed as a percentage of full-scale reading. This is not always the case, however, and sometimes percentage of reading is used instead and the differences can be very significant, particularly when measuring forces that are quite small for a particular instrument.
Calibration, Specification, Adjustment, & Traceability
- No, definitely not.
- Instrument calibration is often thought to be expensive but the information contained in the resultant certificate is usually worth considerably more.
- Calibration is the process of comparing a measuring instrument with a measurement standard to establish the relationship between the values indicated by the instrument and those of the standard.
What 'uncertainty in the measurement' will be quoted on my certificate of calibration? (FAQ - Force)
When requesting a calibration it is reasonable to try and establish beforehand something about the measurement uncertainties that are likely to be provided on the ensuing certificate; unless they are going to be adequate there is not much point in asking for the calibration.- Multi-axis calibration is similar to single axis calibration but done once for each axis. There are very few sources of traceable dynamic force calibration – and certainly none in the force and frequency ranges measured by the majority of commercially available dynamic force transducers.
- Calibration is defined as a set of operations that establish, under specified conditions, the relationship between the values of quantities indicated by a measuring instrument or measuring system … and the corresponding values realised by standards.
- There are three main options available for calibrating a force measuring system and establishing its measurement uncertainties.
Instrumentation
- Yes there are - some are listed here.
- There are many types of force transducer and they are used with instrumentation of varying complexity. In designing or specifying a force measurement system for an application, it is useful to understand the basic operation of the transducer to be used and also their broad operating characteristics.
- Force measurement systems can involve a number of different physical principles but their performance can be described by a number of common characteristics and terms.
- The figures in the table below represent typical characteristics but force transducers and force measurement systems will often operate outside these ranges with increased measurement uncertainty.
- A force measurement system is made up of a transducer and associated instrumentation.
- A transducer is a device that provides an output quantity having a determined relationship to the parameter being measured - force in this case.
- Machines capable of undertaking force calibrations are known as force standard machines and they may be categorised as either primary or secondary.