Terminology
- 1. Accuracy
- 2. Backlight
- 3. Balance Beam
- 4. Calibration
- 5. Calibration Certificate
- 6. Calibration Error
- 7. Capacity
- 8. Cornerload Error
- 9. Digit
- 10. Divisions
- 11. Drift
- 12. External Calibration
- 13. Flexures (Flexible Bearings)
- 14. Force Motor
- 15. Hysteresis
- 16. Instability
- 17. Internal Resolution
- 18. Legal for Trade
- 19. Level Indicator
- 20. Linearity
- 21. Minimum Weight
- 22. Net Weight
- 23. NTEP
- 24. Overload
- 25. Precision
- 26. Readability
- 27. Repeatability
- 28. Resolution
- 29. Roberval Suspension
- 30. Seal
- 31. Stability Indicator
- 32. Strain Gauge
- 33. Tare
- 34. Temperature Range
- 35. Zero (Auto-Zero)
1: Accuracy
The ability of a scale to produce the correct absolute weight for a standard known weight. For example, if a properly zeroed and calibrated scale with a capacity of 100grams were used to measure a known standard weight of 50grams and it produced a reading of 50.002grams it would be said to be accurate to +0.002 grams at that weight. This scale might also be said to be off linearity by +0.002 grams at this point. It is very common to see scales which are specified in linearity rather than in absolute accuracy, and these specifications are usually given as +/- a number least significant digits (most often +/- 2).
2: Backlight
This refers to enhancing the contrast of LCD displays by illuminating the background behind the opaque characters being displayed.
3: Balance Beam
Refers to the beams which support the 2 pans in the older style equal arm balances. Sometimes refers to the beams which carry sliding weights in place of the second pan.
4: Calibration
The very, very important act of placing a specified known standard weight on the scale, after having carefully zeroed (tare) the scale, and causing the scale to indicate the correct exact weight of the standard weight. This adjusts the scale to the actual gravity in this location, and to the barometric conditions in this location. It also compensates for any drift due to temperature changes or other factors. It is very important to do this at regular intervals when doing high accuracy projects where drift can produce serious errors in the results.
5: Calibration Certificate
A variety of certificates are available for weights to satisfy a number of different weight standards. Some of these are multi-page and require considerable effort to accomplish. It would be wise to be guided by an established firm that performs these services routinely.
6: Calibration Error
The error measured when the scale is allowed to weigh a known standard weight after being properly zeroed.
7: Capacity
The amount of weight specified as full scale when the pan is empty. This capacity includes any weights which may be zeroed out by tare when in actual use. For instance, a scale with 1000gram capacity which has a 200gram container on it that has been zeroed out through the use of tare, can only weigh unknowns up to 800grams.
8: Cornerload Error
This is the deviation in readings as the load is moved from corner to corner on a rectangular weigh pan or from compass point to compass point on a circular pan. Almost all high accuracy scales use a Roberval type suspension to minimize these errors. These are sometimes referred to as eccentric errors.
9: Digit
Usually refers to the resolution of a digital scale. For instance a scale with a capacity of 1000grams and a resolution of 0.001gram would be said to have a digit value of 1 milligram.
10: Divisions
Classically it refers to the total number digits which can be read. A 1000gram capacity scale with a resolution of 1mg would be said to have 1,000grams / 0.001gram = 1,000,000 divisions.
11: Drift
The amount the reading on the scale changes without any changes in the load on the pan. Some of the causes of drift are temperature changes (which can be internal and caused by the load on the scale), variation in the voltage feeding the scale, the introduction of static electricity (someone touches the scale), the relief of stresses in the mechanism with time, and other factors. The way to combat these problems is good weighing procedures and frequent calibrations.
12: External Calibration
The act of Calibrating a scale using an external weight or may be used to refer to a scale that requires an external weight for calibration.
13: Flexures (Flexible Bearings)
Flexures are the hinges used in Roberval suspensions. The suspension is basically a parallelogram configuration with upper and lower trusses, a rigid end truss, and a floating end truss to which the pan is attached. It requires 4 flexures to attach the rigid end truss to the middle trusses, and 4 flexures to attach the floating end truss to the middle trusses. In TORBAL mechanical balances these flexures are replaced by taut bands.
14: Force Motor
The magnetic motor which generates the restoring force in high accuracy scales. A h3 permanent magnet is used to generate a high density magnetic field. A very lightweight coil on a non-magnetic bobbin has a current generated by a force restoration servo pass through it and produces the exact force required to hold up the unknown weight which has been added to the weigh pan. This increase in current is measured very precisely by an analog to digital converter and the result is passed to a computer which calculates the weight of the unknown. The invention of this type of motor is what led to the digital scales of today.
15: Hysteresis
In this glossary the term is used to refer to a situation where the result of weighing a particular weight is affected by whether the weighing was done by adding weight or removing weight. In mechanical balances the term “stick-tion” was used to describe the condition. It is somewhat disconcerting to have a scale produce two different results for the same weight. In the modern digital scale these effects are usually caused by the hysteresis characteristics of iron used in the magnetic circuit of the force motor.
16: Instability
This refers to a condition where the result of a weighing jumps around in a discontinuous fashion (as opposed to drift where the change is very gradual and continuous). This type of condition is indicative of a wide variety of problems and is frequently associated with pan problems.
17: Internal Resolution
This refers to the resolution of the Analog to Digital Converter as opposed to the resolution of the display. There is usually a 5 to 10 times higher resolution in the A to D output which allows the computer to smooth out some secondary problems.
18: Legal for Trade
A term used to describe a scale which has been approved for use in commerce by the National Type Evaluation Program and received a Certificate of Conformance which describes the approval and the balance. These approvals may be viewed at the NTEP website.
19: Level Indicator
It is very important to the accuracy of the scale that it be set up in a level condition. The acceleration due to gravity, which is what produces weight, is exactly perpendicular to the level condition. Small angular errors can produce large errors. Almost all high accuracy scales have built in bubble type level indicators which, in conjunction with adjustable legs, allow the user to level the scale accurately.
20: Linearity
Refers to the deviation of a scale from the perfect straight line which joins the Zero condition (0,0) to the full scale or calibration condition (max weight, full scale). We force the Zero condition with the Zero button, and we force the full scale condition when we calibrate the scale. We, thus, have both ends of the line perfect. Now we put on another known weight of high accuracy, usually mid scale (for a 100.000gram capacity scale we would use 50.000 grams). This mid range weight will usually produce the maximum non-linearity result (or very close to it). If we get a reading of 50.002grams (instead of the correct 50.000grams) we would record a non-linearity of +0.002grams. Most often the accuracy of a scale is specified as its linearity (which indeed is its non-linearity).
23: NTEP (National Type Evaluation Program)
This organization, in conjunction with the NCWM (Nation Conference on Weights and Measures) sets the standards and does the type evaluations that determine whether or not a scale is approved for use in commerce. If a scale is approved it is granted a Certificate of Conformance (COC). These certificates may be viewed on the NTEP website.
24: Overload
Describes the condition where the Tare Weight plus the Unknown Weight add up to more than the capacity of the scale.
25: Precision
Refers to the ability of a scale to repeat a measurement (frequently called Repeatability). It does not refer to the accuracy of the scale, which is a measure of its ability to produce the correct result. A scale can be very precise and yet be very inaccurate. For instance if a scale measures 50.000 grams ten times and the results are five readings of 50.020grams, and five reads of 50.021grams, it would be said to have a precision of .001gram, but an accuracy of 0.021 grams.
26: Readability
Refers to the resolution of the display, or the value of the least significant digit.
28: Resolution
The smallest amount the scale is able to resolve, usually the LSD (least significant digit).
29: Roberval Suspension
An ingenious suspension system for eliminating the effects of eccentric loads and allowing the unknown weight to be placed on a pan on the top of the suspension system. The invention of a French Nobleman in the 19th century.
30: Seal
A way of adding an indicator that prevents tampering with the balance and its calibration. Very important in Pharmacy applications and all legal for trader scales.
31: Divisions
Something in the scale display which tells the operator that a stable reading has been reached.
32: Strain Gauge
An electrical resistor that varies its resistance in accordance with the forces being applied to it. Used in conjunction with a metal strain gauge bridge and wired into a group of four to make a wheatstone bridge. The ideal strain gauge bridge produces a linear error signal in proportion to the load placed upon the bridge. These bridges have not achieved the accuracy of the force motor system, but are the next most accurate devices and are heavily used in industrial types of scales.
33: Tare
The process of zeroing the weight on a pan, usually used to hide the weight of the weighing container.