Weight Measurement

Weight & Mass-:

In everyday usage, the mass of an object is often referred to as its weight though these are in fact different concepts and quantities .

In scientific contexts, mass refers loosely to the mass refers loosely to the amount of "matter" in an objectamount of "matter" in an object (though "matter" may be difficult to define), whereas weight refers to the weight refers to the force experienced by an object due to gravityforce experienced by an object due to gravity In other words, an object with a mass of 1.0 kilograms will weigh 9.8 newtons (Newton is the unit of force, while kilogram is the unit of mass) on Earth (its mass multiplied by the gravitational field strength).

Its weight will be less on Mars (where gravity is weaker), more on Saturn, and negligible in space when far from any significant source of gravity

weight is the vertical force exerted by a mass as a result of gravity. Simply put, it's the heaviness of an object dependent on gravity.

Mass is the property of a body that causes it to have weight in a gravitational field. Simply put, it's the amount of matter in an object.

Balance & Scale-:

A balance uses a lever with plates on each end atop a fulcrum, or a pivoting point. By using standard masses, like grams and kilograms, an unknown object's mass can be determined when it balances with an equal amount of known standard masses. This mass will always be constant

A scale measures a single object's weight using gravity. A scale reading will differ if the gravity is different. This means an object's weight measured atop a mountain will be less than at sea level, because there is less gravity at the peak of a mountain

A balance has two pans, you put measured weights in one and the item you are weighing in the other and when the balance is level you sum the known weights to get the unknown's weight .

A scale is an instrument with a readout (digital or analog) from which you read the weight of an object places on (in) the weighing surface/dish

Spring scale -:

In a spring scale, the spring stretches (as in a hanging scale in the produce department of a grocery store) or compresses (as in a simple bathroom scale) in proportion to how hard the Earth pulls down on the object. It is therefore affected by the local gravity.

By Hooke's law, every spring has a proportionality constant that relates how hard it is pulled to how far it

stretches ..

strain gauge scale which works by the spring principle.In electronic versions of spring scales, the deflection of a beam supporting the unknown weight is measured using a strain gauge, which is a length-sensitive electrical resistance. this technique is suitable for determining the weight of very heavy objects, such as trucks and rail cars, and is used in a modern weighbridge.

Strain gauge scale

It is also common in high-capacity applications such as crane scales to use hydraulic force to sense weight. The test force is applied to a piston or diaphragm and transmitted through hydraulic lines to a dial indicator based on a bourdon tube or electronic sensor

Hydraulic or pneumatic scale

Types of scales -:

Bench scales ,pocket scales,jewelry scales,industrial scales,laboratory scales ,bathroom scales ,and baby scales,Moisture analyzer,Platform scales,Pallet scales,Hang scale,Density balance (scale) andMedical scale.

Bench scales are ideally suited for parcel weighing, portion control, and parts counting. They are available in many models. These scales generally have the capacity range of 150 g to 300 kg.

Pocket scales are handheld scales that can be carried in pockets. In general, pocket scales are used to weigh fine items up to 500 grams. These are great for military, hunting, and building construction applications

Jewelry scales are specifically designed to meet the needs of jewelers for weighing pearl, silver, semi precious and precious stones, diamond, and gold. An extensive array of jewelry scales ranging from analytical to carat bench scales is available in the market. Many scale companies offer pocket versions also

Industrial scales are best suited for use in industrial environments including automotive and trucking, food service, processing, retail, hospitality, agriculture, railroad, engineering and construction

Laboratory scales are yet another type of weighing instruments used by medical and scientific communities. These are able to measure tiny objects such as dust and lint.

Bathroom scales are ideal for personal use. Lots of bathroom scales come with advanced features. By using bathroom scales, you can take measurement of body fat. People can monitor fitness levels in the comfort of their home. Bathroom scales are also used in fitness centers and health clubs.

Baby scales are made specifically for babies. They monitor not only the body weight of babies, but also the amount of milk consumption.

In addition to the above, retail scales, truck scales, floor scales, filling scales, and crane scales are among the other types of scales available in the market.

Micro Balances & Ultra Micro Balances

Wireless screen

Accuracy:

The extent to which a given measurement agrees with the standard value for that measurement The ability of a scale to provide a result that is as close as possible to the actual value.  Example, if a known calibration standard weight of 100.000 grams was placed on the Scientech SP250 and the display shows 100.002 grams we could say the accuracy of the balance is 0.002 grams or 2 milligrams.  Accuracy tells how close a balance gets to the real value.  The accuracy of the scale is very sensitive to the calibration process.  It is recommended to perform a calibration at the end user facility

To determine, check, or rectify the graduations of (any instrument giving quantitative measurements).

Calibration is the comparison between the output of a scale or balance against a standard value.  Calibration requires a standard weight and the balance to be set in the "calibration

mode  ".

Calibration technically means to determine the difference between the balance/scale readout and the actual weight on the weighing platform to determine accuracy .

Adjustment means to bring a balance/scale into the state of accuracy required for its use.  Therefore, 'calibration,"

actually means "adjustment ".

Calibration:

Some instruments contain internal calibration weights that are quite accurate, and can be used by the operator routinely, but we recommend external calibration.  The readability of the scale will determine which class calibration mass (Class 1, Class 2, ASTM 6, Class F, etc) will be appropriate for calibrating your balance.  Check your operator’s guide since most balances must be calibrated with a specific mass value.

Example: the Ohaus CS200 requires a 200 gram ASTM Class 6 mass.  You can not calibrate the CS200 if you have a 100 gram mass.  It must be a 200 gram mass.  The scale is programmed and the software within the scale is configured only to accept a 200 gram

There is no certificate of calibration included with any balance we sell since this can only occur at the place of installation.  A calibration certificate can only be attained at the place of installation for a balance.  One can't calibrate a balance here in Massachusetts and ship it to another destination with the calibration certificate that would be valid.

The reason for this included:• Any location in the world is positioned differently from "magnetic north", thus effecting the acceleration of gravity, which effect the force a cal mass exerts on the balance's weighing sensor. • Barometric Pressure makes a difference and if the balance is calibrated in the hills of the rocky mountains and you're at sea level say in the California Bay Area your elevation is different effecting the barometric pressure.

Calibration Certificate:

Cornerload refers to the ability of an instrument to deliver the same weight reading for a given object anywhere on the weighing pan Test this characteristic using the same test weight that was used to test repeatability.  Position the object at various locations on the weighing pan.  The reading

should be the same, within a few digits, at all positions  .

Cornerload

The amount of increments a scale offers.  The amount of divisions can be determined by taking the scale's capacity divided by the scales readability (the smallest number a scale can display. 

Example the Ohaus SC4010 features 4,000 divisions.  The capacity is 400 grams and the scale's readability or another way to say it is the numbers on the display increase in 0.1 gram intervals.  Therefore 400 / 0.1 = 4,000 divisions.  Another example would be the Ohaus SC2020 features 20,000 divisions.  The capacity for the SC2020 is 200 grams and the scale's readability is 0.01 gram.  Therefore 200 / 0.01 = 20,000 divisions. 

Divisions

It is the divisions which determines the cost of a scale - not the capacity or readability, but instead the combination of both the capacity and readability to determine the amount of divisions.  The more divisions the better the quality of the weighing sensor and larger the A/D converter needed to resolve the analog output from the weigh sensor to a binary number for the digital display

IP Ratings (Ingress Protection)

The IP rating system provides a means of classifying the degrees of protection from solid objects an\d liquids afforded by electrical equipment and enclosures.  The system is recognized in most European countries and is set out in a number of British and European standards. These include: Classification of Degrees of Protection Provided by Enclosures, BS (British Standards) 5490:1977; IEC (International Electrotechnical Commission) 529:1976.

First number Protection against solid objects 0 - no protection 1 - protected against solid objects up to 50 mm (e.g. accidental touch by hands) 5 - protected against dust-limited to ingress (no harmful deposits) 6 - totally protected against dust

Second number Protection against liquids 0 - no protection 1 - protected against vertically falling drops of water (e.g. condensation) 2 - protected against direct sprays of water up to 15° from the vertical 4 - protected against water sprayed from all directions—limited ingress permitted 6 - protected against strong jets of water—limited ingress permitted (e.g. for use on ship decks) 7 - protected against the effects of immersion between 15 cm and 1 m 8 - protected against the effects of extended periods of immersion under pressure

IP-43 Rating - The first digit designation "4" means protection against "solid objects equal or greater than 1mm (0.04") in diameter".  The second digit "3" means protection against "sprayed water" ; "Spray water falling perpendicularly at an angle of up to 60-deg may not have

any damaging effects." IP-54 Rating - The "5" means "Dust Protected.  Ingress of dust is not totally prevented, but dust does not enter in sufficient quantity to interfere with satisfactory operation of the equipment.  The "4" means "Protected against splashing water.  Water splashed against the enclosure from any direction shall have no harmful effect."

IP-66 Rating - A strong water jet directed at the scale from any direction must not have any harmful effects.  A jet nozzle with an inside diameter of 0.49 inches splashes a volume flow of approximately 26.4 gallons per minute, from a distance of approximately 8.2 - 9.8 Feet from all sides onto the scale.  The test time is 3 minutes.

IP-67 Rating - Temporary Submersion.  The device is placed in water at a depth of 3.28 feet for 30 minutes

Linearity

Linearity refers to the quality of delivering identical sensitivity throughout the weighing capacity of a balance or scale.  Test this characteristic by weighing two stable objects separately, each of approximately one half the weighing capacity.  The sum of the two readings should equal the reading obtained when both objects are weighed together.

Mass tolerances - weight classifications & applications:

ASTM Class 1: Can be used as a reference standard in calibrating other weights and is appropriate for calibrating high precision analytical balances with a readability as low as 0.1mg to 0.01mg.

ASTM Class 2: Appropriate for calibrating high-precision top loading balances with a readability as low as 0.01g to 0.001g.

ASTM Class 3: Appropriate for calibrating balances with moderate precision, with a readability as low as 0.1g to 0.01g .

ASTM Class 3: Appropriate for calibrating balances with moderate precision, with a readability as low as 0.1g to 0.01g .

NIST Class F

primarily used to test commercial weighing devices by state and local weights and measures officials, device installers and service technicians.  Class F weights may be used to test most accuracy class III scales, all scales of class III L or IIII, and scales not marked with a class designation. Calibrated according to NIST Handbook 105-1

OIML Class E1 - Used as primary reference standards for calibrating other reference standards and weights.

OIML Class E2 - Can be used as a reference standard in calibrating other weights and is appropriate for calibrating high precision analytical balances with a readability as low as 0.1mg to 0.01mg.

OIML Class F1 - Appropriate for calibrating high-precision toploading balances with a readability as low as 0.01g to 0.001g.

OIML Class F2 - For calibration of semi-analytical balances and for student use.

OIML Class M1, M2, M3 - Economical Weights for general laboratory, industrial, commercial, technical and educational use.  Typically fabricated from cast iron or brass. Class M2 commonly used for student brass weights .

Measuring Cell

(Also known as a is the part of the instrument which "senses" weight, and converts it to an electrical signal.  Similar to "mechanical system "

Precision The extent to which a given set of measurements of the same sample agree with their mean.

Amount of agreement between repeated measurements of the same quantity.  Also know as repeatability  .

A scale can be extremely precise, but not necessarily be accurate . Example, two balances were evaluated for precision.  Both

balances: Balance "A" and Balance "B" offers 200g x 0.001g.  A 100.000 gram ASTM Class 1 test mass was place on each balance 70 times .

Balance "A" displayed 103.005 grams 68 times and 103.004 grams 2 times .

Balance "B" displayed 100.000 grams 10 times, 100.001 grams 11 times, 100.002 grams 9 times, 100.003 grams 17 times, 100.004 grams 17 times, 99.999 grams 4 times, 99.998 grams 10 times and 99.996 grams 2 times.

Conclusion: Balance "A" is more precise even though the balance measured a 100.000 test mass as 103.005.  While Balance "A" is more precise Balance "B" is more accurate since it measured the 100.000 test mass more to the actual mass value.

Readability Smallest division at which the balance’s LCD increments

Examples:Ohaus CS2000 features a 2000 gram weighing capacity and increment in 1 gram increments (2000g x 1g.)  Therefore, the readability is 1 gram.  The LCD will increment 1 g, 2 g, 3 g, 4 g, .... 1999 g, 2000 g.  You will never see 0.1 g or 0.5 g with the CS2000.  The scale manufacturer also defines the CS2000 scale with an accuracy of +/- 3 grams, but a readability of 1 gram.  Therefore the scale increments in 1 gram intervals but it is on accurate to +/- 3 grams.

Tanita 1140 is a dual range scale.  This means when weighing item(s) on the weighing platform from 0 to 500 grams the LCD will increment from 0 to 1 g, 2 g, 3 g, ... 499 g, 500 g.  Again you will never see the LCD show 0.1g or 0.5 grams.  The display will ALWAYS BE IN WHOLE GRAM increments.  When the weight on the platform exceed 500 grams (501 - 1000g) the LCD will increment in 2 gram intervals automatically.  You have no control of this since the manufacturer has programmed the scale to operate in 2 gram intervals when anything is placed on the platform over 500 gram.  Therefore, the display will show 502 g, 504 g, 506, up to 996 g, 998 g, 1000 grams.

Reproducible

Refers to the ability of an instrument to return the same numeric result with repeated application of the same weight  .

Resolution

The smallest increment of weight which the numeric display can indicate.  Also referred to as 'display resolution."

Repeatability

Repeatability refers to an instrument’s ability to consistently deliver the same weight reading for a given object, and to return to a zero reading after each weighing cycle.  Test this by

repeatedly weighing the same object  .

Strain Gauge Transducer

 

Commonly called a loadcell.

Consists of an aluminum beam with 4 strain gauges bonded at the hinge areas.  The strain gauge is packaged as a film and when the strain gauge is bent the resistance value changes, similar to a potentiometer.  The 4

gauges are wired to form a wheatstone bridge  .When a load is placed on the beam, it bends at the flexures.  The bending changes the resistance value of the strain gauges (normally 350 ohms at rest) and the resulting output from the wheatstone bridge is proportional to the load.With the basic understanding of electronics including "Ohm’s Law" in which Voltage = Current x Resistance and the understanding that current is the same in a series circuit you will be able to understand the functionality of a load cell

The analog voltage is converted by the A/D into a digital signal which is processed by a microprocessor and the microprocessor outputs the appropriate control signals to illuminate the corresponding segments of the LCD to display the correct numerical number for a user to read from the LCD.  In between the steps there are some filters, but basically there really isn’t much to a scale.

The problem with scales incorporating a load cell is that an excessive load can permanently bend the load cell.  If the aluminum beam is permanently bent the scale will not work.  This is NOT covered under the scale manufacturer’s warranty.  In most situations the cost of replacing the load cell cost almost as much a replacing the scale, because of the ‘deep discount prices" we offer.  For example Ohaus charges approximately $90.00 to replace the load cell on the SP401 while we sell a brand new Ohaus SP401 for $142.00 .

Tare

Act of removing a known weight of an object, usually the weighing container, to zero a scale.  Taring allows you to display the weight of the material on the scale's LCD with the weight of the material only and not the material and container.  Most balances allow taring to 100% of the weighing capacity