Procon Engineering. Technical Document PELR TERMS and DEFINITIONS

Transcription

1 Procon Engineering Technical Document PELR 1002 TERMS and DEFINITIONS The following terms are widely used in the weighing industry. Informal comment on terms is in italics and is not part of the formal definition. ACCURACY: The closeness of agreement between the results of a measured load and the true value of the load. Unfortunately the use of the term often causes misunderstandings and may sometimes be confused with terms such as repeatability, resolution, combined error or non-linearity. APPLIED LOAD: the force or weight on a load receiving element of a weighing system beyond that required to maintain the zero-load display. Sometimes called the LIVE LOAD or NET LOAD. AUTOMATIC TARE ADJUSTMENT: the process or means for automatically compensating for tare weight in a weighing system so that the instrument will read in net weight. CALIBRATION: the set of operations which establish under specified conditions the relationship between the values of load applied and the corresponding value of the weighing system output. In the weighing industry it is generally accepted that calibration includes adjusting to specification as necessary. CALIBRATION CURVE: the presentation of calibration results in graphical format CAPACITY: with respect to a weighing system, the maximum specified load that can be applied to the load receiving element. Sometimes also called RATED CAPACITY. RATED LOAD or FULL SCALE LOAD For outdoor applications, the effects of wind loading must be taken into account CORNER TEST: see ECCENTRICITY TEST 1

2 CREEP: the change in weighing system output occurring with time, while under constant load, with all environmental and other influence quantities remaining constant. Creep results from mechanical or load cell shortcomings and its affect often reduces with time following the application or removal of load. DEAD WEIGHT or DEAD LOAD: the fixed weight of the weighing vessel and its supporting structure, the value of which is to be permanently balanced or cancelled out in the weight measuring system. The process by which a weighing system is caused to record or memorise the dead load is usually set up during system calibration. Dead weight is a different concept to tare weight (See TARE WEIGHT section below). DEFLECTION: the displacement of the weighing structure caused by a change in applied load. DRIFT: the slow variation with time of the output of the weighing system with all other influence quantities remaining constant. EARTH (GROUND EARTH): a tie point in an electrical system that is electrically common with a rod driven down in the earth below the water table. More commonly, EARTH is taken to mean a point electrically common with the earth system installed by the Electricity Supply Company, and which is accessible at every 3-point electricity mains socket in the building. Earth should not be confused with the common connection point of the low voltage circuits of a system such as a Weight Display Unit. This point is usually referred to as the zero volt connection. It may or may not be connected to earth. ECCENTRICITY TEST: a test of a weighing structure in which the load is distributed over the different areas of the load receptor, or placed in different parts of a weighing container or vessel. EXCITATION VOLTAGE: the recommended voltage applied to the input terminals of an electrical load cell. (The load cell power supply) The excitation voltage may be up to 18V, but is lower if the load cell is in a hazardous area and driven through Zener Safety Barriers. FEEDBACK: the coupling of a signal from the output of a system back to the input of the same system for use in correcting or modifying the output. The sensing system associated with the excitation voltage of some Procon weighing systems is an example of feedback. 2

3 HYSTERESIS: The maximum difference between measurements of the weighing system output for the same applied load; one reading is obtained by increasing the load from zero, the other by decreasing the load from maximum rated load. IMPACT: Forces arising from the dynamic effect of live load. Impact is the effect of applying a load suddenly rather than gently to a weighing system. For example, jumping rather than stepping on to the bathroom scales, and thus imparting additional unwanted energy into the weighing transducers. In an industrial weighing system, impact most often occurs when material to be weighed is in large lumps and is dropped into the weigh vessel from too great a height. In such cases, the overload level of the load cells may be exceeded momentarily, damaging or destroying them. INCREASING LOAD TEST (LINEARITY TEST): The normal basic performance test for a weighing system, in which weight display readings are recorded as increments of test-weight load are successively added to the vessel. INCREMENT: the value of the smallest unit that can be indicated by a weigh system display in normal operation. The increment depends on the display device. For example The Procon EP11 has a 4½ digit LED display. This means that it can display five digit numbers starting with a 1 but only four digits if the leading digit is 2 to 9. If the rated capacity of a weighing system using the EP11 is 10000kg, the increment can be 1kg. If the rated capacity is 20000kg, only four digits can be displayed (as 0 to 2000), and the increment will be 10kg. With a 3½ digit display, the increment will be even larger, 10kg for a 10000kg capacity and 100kg for a 20000kg capacity. The increment is closely related to the resolution of a system. The increment size is often selected during commissioning primarily based on the amount of load cell signal change available between zero and maximum capacity. INCREMENT d (approved): the smallest weight increment that can be legally displayed. Typical approvals are for d, which means the weighing range can be divided up into this number of parts, thereby determining the displayed increment. 3

4 IN-FLIGHT MATERIAL: additional material being supplied or taken from a weighing system after an action is taken to stop the flow. (Often a head of material between a feed valve and weigh vessel). INFLUENCE FACTOR: environmental element that may alter or interrupt the output of the weighing system such as temperature, humidity, radio frequency interference, barometric pressure etc. LINEARIZATION PROCEDURE: the process by which the electronics are programmed to automatically compensate for non-linearities. Significant non-linearities in weighing systems usually arise from mechanical factors (eg force shunts). Note that linearity compensation can only work with systems where such characteristics are repeatable. LOAD CELL: a device which produces an output signal proportional to the applied load. Procon Engineering Ltd load cells use the principle of sensing the strain set up in a metal block or other shape when a load is applied to it. The strain is sensed as a change in the electrical resistance of one or more strain gauges bonded to surfaces of the metal block. Procon specialize in designing customised load cells to fit into locations in existing equipment and installations. MAXIMUM CAPACITY: the maximum weighing capacity, not taking account of the dead weight. Similar to the LIVE LOAD. The maximum capacity will be less than the rated capacity of the load cells, which have to carry the dead weight (i.e. the empty vessel) at all times, as well as the load being weighed. NON-LINEARITY (best straight line): the deviation of weighing system output, obtained for increasing applied loads from a best-fit straight line passing through zero applied load, computed using the method of least squares. (See Fig 1 in appendix) NON-LINEARITY (terminal): the deviation of weighing system output, obtained for increasing applied loads from the line drawn between zero and maximum applied load. (See Fig 2 in appendix) OUTPUT: the signal voltage produced by a load cell. The signal is usually expressed in terms of millivolts per volt of excitation. Most Procon load cells have outputs of 1 or 2 mv/v of excitation voltage. OUTPUT, RATED: the difference between the outputs of the weighing system at no-load and at rated load. 4

5 OVERLOAD RATING, SAFE: the maximum load in percentage of rated capacity which can be applied without producing a permanent shift in performance characteristics beyond those specified. RATED CAPACITY or RATED LOAD (load cell): the maximum axial load that the load cell is designed to measure within its specifications. This property is often described as the Range in load cell specifications. REPEATABILITY (Precision in measurement): the degree of reproducibility among several measurements of the same test load under specified conditions. This property is usually expressed as a percentage of full scale output. RESOLUTION: also known as INCREMENT, the minimum change in the measured variable which produces an effective response of the instrument. Note that the resolution for an industrial weighing system may be considerably smaller than the stated accuracy increment. It is a common mistake to choose a display resolution, where the increment size is much smaller than the expected accuracy of the system. This often results in the least significant display digit being misleading and very unstable. SAFETY FACTOR See FACTOR OF SAFETY SENSITIVITY or DISCRIMINATION: in a weighing system, this denotes the degree of responsiveness to a change of load. It defines the minimum amount of gently applied load which is required to produce a visual change in reading or a change in electrical output. Sensitivity or Discrimination are also used to define the output from a load cell at rated load. It is measured in mv/v of excitation voltage. SIDE LOAD: any load acting at 90º to the vertical axis of the weighing structure. SPAN (see also OUTPUT, RATED): the difference between the maximum operating capacity and zero live load of a weighing system. SPAN CALIBRATION: the operation to set and optimise the instrument output between zero and full scale. (Adjustment of the slope) STRAIN GAUGE: A metallic or semiconductor material of small cross sectional area that is bonded to a surface and elongates or contracts with that surface. This deformation results in a change of its electrical resistance, providing means for subsequent measurement. On load cells, strain gauges are usually employed in sets of four, and wired to form a Wheatstone bridge from which output signals are derived. 5

6 TARE: the weight of an empty weighing container or vessel, or the allowance or deduction made from gross weight when calculating the weight of the contents. Taring is the action of adjusting out or cancelling the weight of the container or vessel in order to indicate net weight. In some instruments, this is done by a simple front panel adjustment; in others the tare weight is memorised by the electronic weight display unit for use whenever the weight display is updated. A new tare weight can be input at any time, usually by a single button on the unit. It may also be acquired automatically immediately after discharge of the weighing vessel. 1. A semi-automatic or automatic tare is a command to instantaneously memorise the current gross weight and subsequently subtract this from all live loads (the net weight). 2. A preset tare is where a numerical tare weight is entered by an operator into the instrument (perhaps representing a known container weight) TEMPERATURE EFFECT- ZERO: the change of zero live load output for a specified change of temperature at steady state conditions. Usually expressed as a percentage of full scale output per degree C. TEMPERATURE EFFECT- SPAN (or sensitivity): the change of weighing system span for a specified change of temperature at steady state conditions. Usually expressed as a percentage of maximum applied load per degree C. TEMPERATURE RANGE COMPENSATED (load cell): the temperature range over which the load cell is compensated to maintain rated output and zero balance within specific limits. Procon load cells typically have a compensated range of -10 to +40ºC. For special applications, high temperature versions are available considerably extending the range. TERMINAL RESISTANCE- INPUT (load cell): the resistance of the load cell measured at the excitation terminals at standard temperature with no load applied and with the output terminals open circuit. TERMINAL RESISTANCE OUTPUT (load cell): the resistance of the load cell measured at the output signal terminals at standard temperature with no load applied and with the excitation terminals open circuit. TEST LOAD: a load of known weight value applied to a weighing vessel for testing purposes. As part of their Calibration Service, Procon Engineering Ltd has a series of traceable Test Loads covering ranges from a few milligrams to tens of tonnes, which can be taken to site to conduct in situ calibrations. 6

7 TIPPING WEIGHTS: special incremental weights used to determine the weight display change points. TOLERANCE: a value fixing the limit of allowable error or departure from the true performance or value, as established from experience in the field of application. Tolerances are usually expressed as percentages. Smaller tolerances imply more accurate performance. TRACEABILITY: an accounting for, or statement of, the relationship of a device s calibration to the appropriate primary standard. Traceability is usually a step-by-step process, a master standard within the organization first being taken to the primary reference standard location (e.g. NPL or accredited Test House) and calibrated. This standard then acts as a transfer standard against which instruments within the organization are calibrated. They are then used for day-to-day calibration of load cells or other devices WEIGHING RANGE see MAXIMUM CAPACITY. WEIGHT: 1. The force with which a mass is attracted towards the centre of the earth. 2. An object usually of metal and usually having a definite mass, designed for testing purposes, such as a Test Weight. Technically, load cell weighing systems measure the force exerted by an object and the result should therefore be expressed in newtons. However in common parlance the term weight is accepted as the force and is expressed in kg. To eliminate air buoyancy effects the first definition should strictly be realised in a vacuum, and the result will be slightly less in air. It also explains why the weight of an object varies with height above sea level. There is also a variation with latitude, due both to the fact that the world spins on its axis and because it is not a perfect sphere, but flattened at the poles. Weighing systems are usually calibrated at their point of operation, using known traceable loads, and therefore variances in gravity are eliminated. ZERO-LOAD OUTPUT (load cell): the output signal from a load cell with no load applied, usually expressed as a percentage of rated output. ZERO TRACKING (Automatic): a means for automatically maintaining the weighing system output at zero within specified limits. ZERO TRACKING WINDOW: the limits (+ and -) over which the zero tracking device operates, typically ± 2% of span. 7

8 Appendix Fig 1: Representation of combined error based on best straight line through zero 8

9 Fig 2: Representation of combined error based on terminal straight line 9