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.

Two points of the wheatstone bridge are connected to an exciter voltage (from the battery or AC adapter) and an output analog voltage feed to a A/D Converter. The output voltage being fed in the A/D varies in proportion to the load applied to the platform of the scale. This occurs since the weighing platform of a scale is connected to the end of the load cell via a post. The applied force is transferred from the platform, through the post and onto the aluminum beam. Since the aluminum beam is milled out in the shape of a dog bone, the force applied results in a deformation of the beam. When the beam bends the strain gauges bend resulting in their resistance value to change. Since the current is the same in a series current and the resistance changes as the strain gauges bends, the voltage changes in proportion to the load applied to the platform.

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 low prices we offer. For example we charge approximately $70.00 to replace the load cell on the Touchscale while you can purchase a brand new Touchscale for about $90 online.

Therefore, great care needs to be taken when using a scale with a load cell. The person needs to have an idea how much an object weighs prior to placing it on the platform. Otherwise, you can destroy the scale.

The best way I describe this is if you go out and buy a brand new bicycle. Everything works great and you cruise down the street and hit a small pot hole. The bicycle’s rim doesn’t get bent yet, but you have stressed the metal of the rim. Now you decide to jump a few curbs and the rim is still okay. You feel stupid today, and decide to hit that pothole again. Now the rim is bent. You can’t go back to the bicycle dealer and say the bike is junk since the rim in bent. The rim got bent due to abuse and this is not covered under the manufacturer’s warranty. Well, the same works with scale that incorporates a loadcell. Many people don’t have an idea what something weighs prior to putting the object on the platform but you really need to get acquainted with what objects weigh prior to placing the object on the platform. We have taken precautions by installing down stop(s) which restrict the downward movement of the loadcell but prudence still must be followed.

Accuracy:

The extent to which a given measurement agrees with the standard value for that measurement ¹ 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 200.00 grams was placed on the iBalance 201 and the display shows 200.01 grams we could say the accuracy of the balance is 0.01 grams or 10 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. If calibration is well performed, we can usually say the accuracy of the scale should be within +/- one display resolution with most scales.

Calibration:

To determine, check, or rectify the graduations of (any instrument giving quantitative measurements). ¹ 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."

We always 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 i500 requires a 500 gram ASTM Class 6 mass. You can not calibrate the i500 if you have a 100 gram mass. It must be a 500 gram mass. The scale is programmed and the software within the scale is configured only to accept a 500 gram calibration value to store the calibration data within the EPROM on the scale’s motherboard.

Calibration error

The difference between what a weight of near the full capacity of the instrument reads on the digital display and its true mass.

Capacity

The actual or potential ability to perform, yield or withstand. ¹ The largest weight the balance is capable of weighing.

Capacitance Loadcell

The fundamental design of a capacitance loadcell is that of the electrical capacitor. The loadcell contains two closely spaced, parallel, electrically-isolated metallic surface, one of which is essentially a diaphragm capable of slight flexing when pressure is applied. When pressure is applied to the capacitance loadcell a minute change occurs in distance between the plates. The varying gap between the plates creates in effect a variable capacitor. The resulting capacitance is detected send to a linear comparator and amplifier which is then processed by a microprocessor and displayed on the LCD. Many of the older technology scales from the 1980's use a capacitance loadcell.

Cornerload

Cornerload refers to the ability of an instrument to deliver the same weight reading for a given object anywhere on the weighing pan. (Of course, an instrument that does not perform acceptably with regard to drift and repeatability cannot possibly deliver acceptable cornerload performance.) 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 error

Refers to variations in the displayed weight as the object being weighed is moved to various positions on the weighing pan.

Count

The smallest increment of weight which the digital display resolves. Also called "division.".

Digit

The smallest increment of weight that the digital display resolves.

Divisions

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 i500 features 5,000 divisions. The capacity is 500 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 500 / 0.1 = 5,000 divisions. Another example would be the iBalance 201 features 20,000 divisions. The capacity for the i201 is 200 grams and the scale's readability is 0.01 gram. Therefore 200 / 0.01 = 20,000 divisions. It is the divisons 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 analogic output from the weigh sensor to a binary number for the digital display.

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Drift

Drift is a progressive (continuously upward or continuously downward) change in the number displayed on the digital readout. The weight readings does not stabilize, or unstable readings with no weight applied. All analytical balances show some uncertainty. Some do so more than others.

Two environmental factors affect the instrument’s stability dramatically—temperature and static electricity. Temperature control is imperative. This includes both control of the room temperature and maintaining the internal temperature of the instrument. For best stability, maintain the room temperature within two degrees constantly (day and night). Leave the instrument plugged in and turned ON. Static discharge can also be accomplished by putting some ionizing devices around the weighing pan.

Drift may be related to RFI (radio frequency interference). There is not a lot you can do is RFI other than move the balance to a different area where the RFI is less. Leveling of the balance can also result in drift.

Flexible Bearings - Precision components in the measuring cell (force motor) which allow the force coil to move without friction.

Hysteresis - The lag in response exhibited by a body in reacting to changes in forces, esp. magnetic forces, affecting it. ¹

Hysteresis Error - Refers to the condition of repeatedly weighing the same object, but obtaining different readings on the numeric readout.

Instability - the tendency to behave in an unpredictable, changeable, or erratic manner. ¹ Refers to a displayed number which continues to vary randomly or sporadically, rather than progressively. See "drift."

Installation - Learn about selecting the best location of your balance for optimal performance.

Internal resolution - the smallest increment of the A/D converter. It is used by the hardware and software designers. For a scale using a strain gauge design, the ratio between internal and display resolution is about 4:1. It is possible to use 1:1 ratio (Many of the cheap-junk no name scales do this), but you will see a lot of unstable readings by changing of 1 increment. Having stable display is the main reason of the ratio. There are other concerns, such as measurement speed and temperature compensations.

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.

Linearity Calibration:

Linearity calibration utilizes three calibration points, one at zero, center span and full span. This method minimizes deviation between actual and displayed weights within the balance's weighing range.

Linearity Test:

Perhaps the most obvious test of a high precision scale would be to place a weight of accurately known value on the weighing pan, and observe the numerical result. But there is a better test, nearly as simple, that better reflects the measurement accuracy. This is called the linearity test.

The linearity test measures the ability of an instrument to have consistent sensitivity throughout the weighing range. The test requires several nominally equal weights, each a fraction of the weighing capacity. The group together should approximate the weighing range of the instrument. For example, a 150 gram capacity scale might be tested with three 50 gram weights.

Static electricity will cause erratic readings. Instruments should be operated on a static dissipating surface (antistatic mat). Operators should stand on antistatic floor covering. Avoid the use of plastic containers for items being weighed. Never replace broken glass doors on instruments with plastic ones. Maintain humidity at 65% or more. Eliminate sources of floor vibration and air currents. On analytical balances with glass doors, be sure the doors close fully.

Liquid Crystal Display (LCD) - A numeric readout device, often characterized by black numerals on a silver background.

Mass tolerances - weight classifications & applications:

ANSI/ASTM Class 1. Provides the greatest precision. Can be used as reference standard in calibrating other weights and appropriate for calibrating high-precision analytical balances (from 0.01mg to 0.1mg).

ANSI/ASTM Class 2. Appropriate for calibrating high-precision toploading balances with readabilities ranging from 0.001g to 0.01g.

ANSI/ASTM Class 3. Appropriate for calibrating balances with moderate precision, ranging from 0.01g to 0.1g.

ANSI/ASTM Class 4. Appropriate for student use and semi-analytical weighing.

Min Weight - typically used in the specifications of counting scales(i5000, Proscales, iBalance) The small piece weight required in a counting mode. At the beginning of any counting process, the scale's software needs to teach the scale what is the unit piece weight. The scale uses the information to count the unknown weight.

Precision - The extent to which a given set of measurements of the same sample agree with their mean. ¹ 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:The 3001 features a 3000 gram weighing capacity and increment in 1 gram increments (3000g 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 3001 scale with an accuracy of +/- 2 grams, but a readability of 1 gram. Therefore the scale increments in 1 gram intervals but it is on accurate to +/- 2 grams.

My Weigh Palmscale increments in 0.1 gram intervals. This means when weighing item(s) on the weighing platform from 0 to 200 grams the LCD will increment from 0.0 to 0.1 g, 0.2 g, 0.3 g, ... 99.9 g, 200 g. Again you will never see the LCD show 0.01g or 0.05 grams. The display will ALWAYS BE IN TENTH GRAM increments. Therefore, if you need 0.01 accuracy you need to look at iBal 201 (200g x 0.01g) since they don't make a scale that is 1000g x 0.1g..

The Ultraship is a dual range scale. This means when weighing item(s) on the weighing platform from 0 to 1000 grams the LCD will increment from 0 to 2 g, 4 g, 6 g, ... 498 g, 500 g. Again you will never see the LCD show 0.1g or 0.5 grams. The display will ALWAYS BE IN TWO GRAM increments. When the weight on the platform exceed 1000 grams (1000g-14000G) the LCD will increment in 5 gram intervals automatically. You have no control of this since our factory has programmed the scale to operate in 5 gram intervals when anything is placed on the platform over 1000 grams. Therefore, the display will show 2005 g, 2010 g, 2015, up to 14000 grams.

Reproducible - Refers to the ability of an instrument to return the same numeric result with repeated application of the same weight. See " hysteresis."

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. The best test object is a weight intended for that purpose. It should match the weighing capacity of the instrument. (Do not test a 200 gram capacity instrument with a test weight less than 100 grams) When a test weight is not available, an alternative object that is solid, non-porous, dirt free, non-magnetic, and non-static retaining can be used. Repeatability is sometimes referred to as "Standard Deviation" of a set of similar weight readings.

Span Calibration:

Span calibration utilizes two calibration points, one at zero and a choice of either half capacity or full capacity.

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.

Tare by subtraction means that you can keep on using the tare button providing the total mass on the platform does not exceed the weighing capacity of the scale. Therefore, if you had the My Weigh i5000 (5000g x 1g) and you put a pot on the scale that weighed 1000 grams and pressed the tare button the scale would display 0.0 and you would now have 4000 grams weighing capacity left (5000 - 1000 = 4000 g.) Then you put 500 gram olive oil in the pot and press the tare button the scale would display 0.0 and you would now have 3500 grams weighing capacity left (5000 - 1000 - 500 = 3500 g.) etc, etc.

Temperature Range

Digital scales are electronic devices. All electronic devices contain electronic components that have temperature coefficient. An example of this would be a resistor. If you measure a resistor with an Ohm meter and it measures 10 ohms at 0 degrees F and you then put that resistor in an environment chamber and bring the temperature up to 100 degrees F the resistance's value now could be 10.2 ohms. Since the resistance value has increased this is an example of a discrete component(a resistor) having a "positive temperature coefficient". Well, enough with electronics 101.

How does operating a scale above the manufacturer's operational temperature effect you? The temperature operational range is stated since the manufacturer has tested and confirmed that his scale will have an accuracy (scale manufacturers' use the word "LINEARITY") of +/- however many grams provided you operate the scale within the stated temperature. If you go outside of this temperature the scale may be off by a division (in the case of the i500 <500g x 0.1g> a division is 0.1 gram so that means it maybe off by +/-0.2 grams instead of the stated +/- 0.1 gram). Like all electronic devices it is not a good ideal to operate them all day at excessive temperatures. Will they operate?, Yes. Consider your computer, this also has a temperature operating range. Can you run your computer in an environment that is a 100 degrees?, Yes. Do you want to do this all the time?, No because it stress the components.

General Product Features

Power Up Test: When the scale is turned on, all display segments will appear for approximately 3 seconds before resetting to zero.

Stable Reading Indication: During weighing, a segment of the display activated once a stable reading has been reached.

Overload: If the applied load exceeds the capacity of the scale, an "E"' will appear on the display and the load should be removed immediately. The scale will return to normal operation. Excessive overloading the scale can destroy the load cell and this is not covered under warranty.

Negative Value: When a load is removed from the scale, any tared value will be displayed as a negative number. To return to normal operation, the tared value can be canceled by pressing the tare button.

Zero Function: Values can progressively be added to a sample. By pressing the tare key, the scale display returns to zero and an indication appears at the upper left corner of the display.

Auto Shut-Off: To extend battery life, the scale will automatically turn off after approximately two to five minutes (depending on scale model) if no active weighing is occurring.

Off: Pressing this key turns the scale off