Scales Measuring Instruments

SCALES

The balance is an instrument that measures the mass of a body or substance, used as a means of comparing the force of gravity acting on the body. The word comes from Latin words meaning two and bis linx dish. Keep in mind that the weight is the force exerted on the gravitational field uncuerpo mass, with such force the product of mass times the local acceleration of gravity.

[F = m x g]. The local term is included to emphasize that the acceleration depends on factors such as latitude, height above sea level and density of the earth, where the measurement is made. The force is measured in Newton. The balance has other names, among them scales and weighed.

PURPOSE OF THE BALANCE

The balance is used to measure the mass of a body or substance or the weight thereof, as between mass and weight there is a well-defined. In the laboratory scale is used to perform quality control activities, with devices such as pipettes, to prepare proporcionespredefinidas components in mixtures and to determine densities or specific gravities.

OPERATING PRINCIPLES

The scales are differentiated by design, the principles used and metrology criteria they use. Today could be considered that there are two main groups: mechanical balances and electronic scales.

Balance mechanical

Some of the most common are:

1. Spring balance. His funcionamientoestá based on a mechanical property of the springs, which is that the force of a spring is proportional to the elasticity of the spring constant [k] multiplied by the elongation of the same [x] [F =-kx]. This implies that the larger the mass [m] that is placed on the side of the scale, the greater the elongation, the same being proportional to the mass and spring constant. The calibration of a spring balance depends on the force of gravity acting on the object, so it must be calibrated in the workplace. Used if high accuracy is not required.

2. Balance of sliding weights. There are two known masses that can be moved on a scale with an alcohol-macro and the other with a micro-graduation, by placing a substance of unknown mass on the tray, sliding weight is determined by the masses on the scale mentioned until obtain the equilibrium position. At that time the reading is taken by adding the amounts indicated by the position of the mass on the scales above.

3. Analytical balance. It works by comparing the mass of known weight of a mass withthe weight substance desconocido.Está built based on a symmetric bar or lever is supported by a blade type support at a central point called the fulcrum. In their extremes there stirrups or caps that are also supported by blades that allow them to swing smoothly. From there two pans are suspended. Placed in a mass or weight certified and the other those that must be analyzed. The whole system has a freezing or locking lever allows stably main stand when not in use or when you need to modify the counterweights. It has an external box that protects the balance of political interference, such as drafts, which might arise where it is installed. Today, it is considered that an analytical balance is one that can weigh ten-thousandths of a gram (0.0001 g) or one hundred thousandth of a gram (0.00001 g) have a capacity generally reaches up to 200 gramos.Para use required to have a certified set of weights, which provides mass parts with different magnitudes.

The game is usually composed of the following parts:

4. Balance of the top plate. This type of balance has a loading plate placed on the top, which is supported by a column that is held in position by two pairs of vertical guides with flexible couplings. The effect of force, mass produced, is transmitted from one point in the vertical column either directly or through some mechanism to the load cell. The demand for this type of mechanism is to maintain the parallelism of the guide with an accuracy of ± 1 micron. Deviations from parallelism cause an error known as lateral loading occurs when the mass being weighed show differences, if the reading is taken at the center of the dish or one of its ends. The scheme is included below explains the operating principle, which some manufacturers have introduced electronic scales.

5. Balance of substitution. It is a single-pan balance. Is placed on the weighing plate an unknown mass is balanced by removing the side counterweight masses of known magnitude, using a mechanical cam system until it reaches an equilibrium position. The fulcrum is usually offset with respect to the length of the load beam and placed near the front of the balanza.Cuando mass is placed on the weighing plate and released the balance of the locking mechanism, the movement of the beam load is projected by an optical system to a screen located at the front of the instrument.

Verification of operation

The following describes the procedure to check the operation of a typical mechanical balance. The process description is based on the balance of sustitución.1. Verify that the balance is level. The leveling is accomplished by threading adjustment mechanisms, located at the base of the scale. The level is achieved by centering a bubble on a scale visible in the front of the base of the balanza.2. Check the zero point. Zeroed, and release controls balance. If the scale reading to zero is not maintained, it is necessary to adjust the zero adjustment mechanism which is a knurled screw located in a horizontal position near the fulcrum. For this it is necessary to block gently balance and adjust the mechanism. The process continues until the zero adjustment in the scale of correctly lectura.3. Check and adjust the sensitivity. This is reset whenever an adjustment is made internally. Is performed with a known standard weight and proceed by following these steps: a) Block balanza.b) Place a weight on the pan pattern, equivalent to óptica.c scale range) Place the graduation of the decade lower weight one (1). d) Release the balanza.e) Adjust the point cero.f) Replace the decade of graduation from underweight to zero (0).

The balanzadeberá dial 100. If the scale scores less or more than 100, you must adjust the sensitivity control. This means the balance lock, lift the top cover and turn the screw of sensitivity: if the scale over 100 brand, turn the screw clockwise to clockwise, ie down. If the scale scores less than 100, you must first unscrew the screw. Then the process is repeated until the required balance (set to zero and sensitivity) .4. Check the brake plate. This is mounted on a threaded shaft, when the balance is locked, tap the plate to prevent wobble. In case of mismatch should gently rotate the shaft until the distance between the brake and the plate is zero when the balance is locked.

Maintenance of mechanical balance

The maintenance of mechanical scales is limited to the following routines:

Frequency: Diaria1. Check nivel.2. Check cero.3 graduation. Check sensibilidad.4 adjustment. Clean the weighing plate.

Frequency: Anual1. Calibrate the balance and document process.2. Disassemble and clean internos.Se components must follow the process specified by the manufacturer, or sign especializadapara firm effect.

Electronic scales

Electronic scales involve three basic elements: 1. The object to be weighed is placed on the weighing plate exerts a pressure that is distributed randomly on the surface of the dish. From there, through a transfer mechanism, levers, supports, guides,, concentrates the weight load on a simple force [F] that can be measured. [F = ∫ P ∂ a] The integral of the pressure on the area to calculate the force.

2. A measuring transducer, known as load cell, produces an output signal proportional to the load force in the form of changes in voltage or frecuencia.3. A digital analog electronic circuit that finally presents the result of weighing in digital form.

Laboratory scales operate according to the principle of electromagnetic force compensation, applicable to displacement or torque, which combines the accuracy of the mechanical components with automatic reading.

Operation. Moving parts (weighing plate, support column [a], coil loading position indicator and [G]-weighing process object-) are maintained in equilibrium-a float-off force [F] that is equal to the weight. The compensation force is generated by the flow of electrical current through a coil located in the space of air in an electro-magneto-cylindrical. The force F is calculated by the equation [F = I XlX B], where: I = current, L = total length of wire from the coil and B = the magnetic flux in the air space of any changes electroimán.Con -peso/masa- the load, the system responds mobile-mechanical, moving vertically away a fraction, detected by a photosensor [e], as a result sends an electrical signal to the servo amplifier [f] that changes the current flow electricity passing through the magnet coil [c], so that the mobile system returns to the equilibrium position to adjust the magnetic flux in the electromagnet. Consequently, the weight of the mass G can be measured indirectly from the flow of electric current passing through the circuit by measuring the voltage [V], through a precision resistor [R].

[V = I x R]. To date, many systems have been developed that use electronics to make very accurate mass and weight. The scheme presented below explains how it works electronic balance.

System signal processing

The system signal processing circuit comprises the power transforms laseñal issued by the transducer as numerical data that can be on a screen. The signal processing comprises the following functions:

1. Tara. It is used to zero the value placed on reading, at any load within the capacity range of the scale. Controlled by a button usually located in the front of the balanza.2. Control for adjusting the integration time. The weight values ​​are averaged over a predefined period of time. This function is very useful when you perform weighing operations in unstable conditions. For example, the presence of air currents or vibraciones.3. Rounding the result. In general, electronic scales internally process data of higher resolution than those presented on the screen. In this way achieves the exact center at ground zero balance when the balance is tared. The net domestic value is rounded to the display.4. Detector stability. It is used in sequential weighing operations and allows to compare the results with each other. When the result holds, is released and put on display, something that is detected at power-unit symbol seleccionada.5 weight. The electronic processing of signals can have other functions such as parts counting, percentage value, target value, among others. These calculations are performed by the microprocessor, according to the instructions the operator inputs via the keyboard of the balance.

REQUIRED SERVICES

To successfully install and use a scale, the following is required: 1. Provide an environment that does not present drafts, sudden temperature changes and is free of polvo.2. Having a perfectly nivelado.Es inn ideal a platform of high inertia, isolated structures located in the vicinity, to reduce the effect of vibrations emitted by certain equipment such as centrifuges and refrigerators. It must have a sufficient area to install the balance and that auxiliary equipment with which it interacts in the process of weighing. Similarly, it should provide the space required for the interconnecting cables, power, connection to the information system, printer, etc.3. Avoid in the vicinity are installed equipment that produces strong magnetic fields or vibration as centrifuges, electric motors, compresoresy generadores.4. Avoid being under the direct influence of air conditioning systems, air currents and light solar.5. Have an electrical outlet in good condition, equipped with ground pole switches provided that complies with electrical regulations in force in the country or the laboratory.

Operation of the electronic balance

The operation of modern electronic balance is clearly defined in the operation manual that is usually supplied fabricantes.En must meet the following procedure: 1. Allow the balance to balance its environmental conditions where instalada.2 meeting. Allow the balance to warm up before activities begin. Normally it is enough that is connected to the electric power system. Some manufacturers suggest that you leave an interval of time of at least 20 minutes from the time it is powered up when you start using it. The analytical balances Class I require at least 2 hours before starting utilización.3. Verify that the balance is calibrada.Las electronic scales have usually made at the factory calibration, stored in memory, which can be used if there is no mass calibración.Si perform calibration is required, you must have calibrated mass to perform the procedure indicated by the calibrated masses fabricante.Las used must meet or exceed ASTM tolerances. General information is included below a table showing the mass tolerances for ASTM Class 15.

4. Follow the instructions given by the manufacturer in the operating manual.

Calibration of balances

The balance calibration process must be performed by specifically trained in this activity. As a fundamental aspect is stressed that the calibration should be performed based on the guidelines of the OIML or other equivalent entity such as the American Society for Testing and Materials (ASTM), institutions have developed methodologies to classify pattern weights or masses , used in these processes. The following table includes the reference weights classification used by the OIML.

Classification Table OIML6 reference weights

Any calibration should be performed using a standard weight, and the results will be analyzed to determine if they are within acceptable tolerances. Weights should be selected pattern, depending on the capacity of the scale. The table below complements the previous one and is a guide to help determine the type of weight pattern to be used in the process of calibration of a balance in terms of their ability.

ROUTINE MAINTENANCE

The balance is characterized by a high precision instrument. For this reason, routine maintenance by the operator are minimal and are limited to the following:

Daily activities

1. Clean the weighing plate, so that it is free of dust or dirt. Lalimpieza is with a clean cloth that can be moistened with distilled water. If it is necessary to remove any stain, you can apply a mild detergent. You can also use a soft hair brush to remove particles or dust that had been deposited on the weighing plate.

2. Externally and internally clean the weighing chamber. Check that the windows are free of polvo.3. Verify that the adjustment mechanisms of the front door pesajefuncionen chamber properly.

Very important: Never lubricate a balance unless the manufacturer tells you expresamente.Cualquier substance that interferes with the mechanisms of balance retard or alter definitely response measure.

Note: Generally, the manufacturer or its representative in specialized facilities performing maintenance of the balance, following procedures that vary depending on the type and model of balance.

Any calibration should be performed using a standard weight, and the results will be analyzed to determine if they are within acceptable tolerances. Weights should be selected pattern, depending on the capacity of the scale. The table below complements the previous one and is a guide to help determine the type of weight pattern to be used in the process of calibration of a balance in terms of their ability.

ROUTINE MAINTENANCE

The balance is characterized by a high precision instrument. For this reason, routine maintenance by the operator are minimal and are limited to the following:

Daily activities

1. Clean the weighing plate, so that it is free of dust or dirt. Lalimpieza is with a clean cloth that can be moistened with distilled water. If it is necessary to remove any stain, you can apply a mild detergent. You can also use a soft hair brush to remove particles or dust that had been deposited on the plate pesaje.2. Externally and internally clean the weighing chamber. Check that the windows are free of polvo.3. Verify that the adjustment mechanisms of the front door of the weighing chamber to function properly.

Very important: Never lubricate a balance unless specifically directed by the manufacturer. Any substance that interferes with the mechanisms of balance retard or alter definitely response measure.

Note: Generally, the manufacturer or its representative in specialized facilities performing maintenance of the balance, following procedures that vary depending on the type and model of balance.

DEFINITIONS B? SICAS ASTM.

American Society for Testing and Materials (American Society for Testing andMaterials). Calibration. Determining the correct value of the reading of an instrument for measuring or comparing it against a standard or pattern. A scale is calibrated using weights patrón.Carga side. Ability to read a balance consistently the value of the masses, regardless of the position thereof on the weighing plate. In English it is called lateral load cornerload.Error. Deviation is presented in the results when weighing an object, placing it in different positions of the weighing plate. For example, between the center of the dish and then in one of his bordes.Error of linearity. Difference occurs when the balance is loaded in succession, increasing the amount of weight equal magnitude to achieve maximum capacity, and then download it by following a process similar to above. The differences that exist between the readings obtained and arithmetic values, corresponding to the weights used are interpreted as non-linearity.

Error sensitivity. Constant deviation over the range of weighing or balanza.Exactitud Deuna capacity. Sum of all errors in the balance. It is called tota of error.Histéresis band. Difference occurs in the results when the load increases or decreases in the balanza.Linealidad. Concept that applies to the capacity or ability of a scale to achieve accurate readings mass weight of less than full capacity. If you draw a graph of weight, compared to the weight display in a perfectly linear scale, the end result would be a straight line. To determine the linearity error of a balance, you should use certified masses. The procedure for calculating the differences in linearity is to make mass readings with the same mass-certified, with and without preload. The difference between the two readings to calculate the error linealidad.Masa. Property of matter that is manifested through phenomena such as the attraction of bodies, through the force of gravity or inertia-resistance to change the state of rest or motion under. The fundamental unit for expressing the concept of mass is the kilogram [kg]. Masses certified. Bodies that meet the tolerances defined by the certification bodies.

The ASTM Classes 1 to 4 are widely used and are required reference for calibración.OIML routines. International Bureau of Metrology Legal.Sensibilidad. Smaller mass can be detected by the scale. It is also understood as the mass scale smaller than the measured correctamente.Trazabilidad. Ability to relate a set of measures defined unestándar instrument.