To wrap up the discussion we’ve been having over the past several weeks, we’d like to discuss the different types of strain gauge load cells available and how to know which style is right for your application.

S-Beam Load Cells: These are used for tension applications, often when hanging or suspending a weighing system. They are so named because of their shape and are also sometimes called Z-Beam load cells. With high precision, low cost and easy installation, s-beam load cells are among the most popular, but they are designed solely for in-line applications and are very sensitive to off-center and extraneous loads. The LFS 210 is Cooper Instruments’ best-selling s-beam load cell.

Beam Load Cells: These load cells get their name from their shape, which is rectangular. Some typical applications include tank, hopper or silo weighing. Cooper Instruments’ most popular beam load cell offering is the shear beam LQB 610. Beam load cells are divided into several sub-categories.

Shear Beam: Shear beam load cells have an actual “shear” machined inside to protect the cell from side loading forces. ‘Single-ended’ shear beams are used for medium capacity weighing while ‘double-ended’ shear beams are used for higher capacities.

Bending Beam: Often used in bench scales, bending beams must be installed with care to avoid side-loading.

Canister Load Cells: These (shockingly) canister-shaped units were the first strain gauge load cells designed. Today they are typically used for high (100,000 lb+) capacity compression applications. See our LRCN 710 for an example of a canister load cell.

Pancake Load Cells: Also called Low Profile load cells, pancake load cells (like the LGP 310) offer high precision with less sensitivity to load condition. Sometimes they are designed with multiple shear struts and sometimes with bending beams. They typically have a female thread through the center and a ring of outer holes for mounting, so they can be used in either compression or tension (by using the outside mounting holes).

(Load) Button Load Cells: These are round load cells with a small, raised ‘button’ in the center, like our LKCP 410. They are comparatively small in size making them ideal for use in limited space and are used for compression applications. Common uses are in the medical and automation fields where space is often an issue and the load cell must be small in size.

Thru-Hole (Donut) Load Cells: Shaped like donuts, thru-hole load cells are used for bolt force measurement, clamping force, post or leg mount and many more applications. They offer high accuracy and high stiffness for applications requiring compression, off-center or press loading. Cooper Instruments’ LKCP 48X series are donut load cells.

As always, if you have any questions related to this material, our support staff at Cooper Instruments is available to help. Contact them by calling (800) 344-3921 or emailing This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

In the last two weeks, we explored strain and then strain gauges . Next in the series, we’ll discover how that strain gauge fits into a load cell.

We have this strain gauge and we know that subjecting it to force causes it to deform and its electrical resistance to change. Last week, we briefly mentioned that a Wheatstone bridge is used to help us read this change. According to Wikipedia, a Wheatstone bridge is “an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, on leg of which includes the unknown component.” A basic Wheatstone bridge circuit contains four resistances (four strain gauges), a voltage input*, and a voltmeter. This Wheatstone bridge configuration is used because the use of four strain gauges amplifies the amount of change in the resistance, making it easier to read and more accurate, as well as less sensitive to thermal changes.

*The voltage input into the Wheatstone bridge could be constant or not. If the amplifier is ratiometric (able to measure the input voltage relative to the output voltage) then constant input voltage is not necessary.

The four strain gauges are configured in such a way that the current from the power source splits, flows through the sequence of resistors, then recombines into a single conductor (image). Three of these resistors have known values. The value of the fourth resistor is not known. As the strain gauge is loaded, the bridge becomes unbalanced producing an indication at the voltmeter. So a known voltage is input into the circuit and then the output voltage should be slightly different due to the change in electrical resistance of the strain gauge when loaded.

(Click here for cool pictures of Wheatstone bridges.)

So when these elements, the strain gauges configured as a Wheatstone bridge, are bonded to an engineered physical structure, the overall package is called a load cell. Wikipedia defines a load cell as “a transducer that is used to convert a force into electrical signal.” Although other types of load cells exist, strain gauge load cells are the most popular and surpassing all other types because manufacturers continue to increase their accuracy and lower their costs.

As always, if you have any questions related to this material, our support staff at Cooper Instruments is available to help. Contact them by calling (800) 344-3921 or emailing This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

We’d love to hear your feedback regarding this or any other article we’ve posted. To leave feedback, ‘Like’ us on Facebook and then post your feedback on our wall.

Having read the previous blog entry, we can say we understand a little bit about strain. Now we’d like to look at how a strain gauge is used to measure the amount of strain on an object. Strain gauges are frequently used in mechanical engineering research and development to measure the stresses generated by machinery.

The most basic/common strain gauge construction involves affixing a metallic foil, usually in a zigzag pattern (this is the slinky from last week’s blog ) with an active area of about 2-10mm², to a flexible backing which is in turn affixed to the object being measured (image). When the object is loaded, it deforms and the foil pattern deforms as well. This deformation of the foil pattern causes its electrical resistance to change. That change in electrical resistance is what we can measure, usually using a Wheatstone bridge , by applying a known voltage to the input leads of the gauge and then reading the output voltage to determine the change in resistance.

Strain gauges operate on the principle that the changes to the foil due to strain will cause changes to the electrical resistance in a defined way. When the foil is stretched (tension application), it will get longer and narrower (image). This increases its electrical resistance. In the case of a compression application of load, the foil will get wider and shorter with the deformation of the object the strain gauge is attached to (image). This would decrease the electrical resistance. Using a zigzag pattern creates a multiplicatively larger change in resistance than using a single straight  piece of foil, because it foil is essentially being deformed in multiple places instead of one. This makes it easier to measure small changes and makes the measurement more accurate. Why are we measuring such small changes? Because forces great enough to induce greater resistance changes would likely cause permanent deformation to either the test specimen or the strain gauge thereby “overloading” or ruining the device.

So, to sum up: The metallic foil (slinky) is attached to the object being stressed. When load is applied, the object and, consequently, the foil deform. The deformation in the foil causes a change in the electrical resistance which we can then measure. Through mathematical calculations, we can then determine how much force was used to produce that change in electrical output. And that, in a nutshell, is how a strain gauge works.

As always, if you have any questions related to this material, our support staff at Cooper Instruments is available to help. Contact them by calling (800) 344-3921 or emailing This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

We’d love to hear your feedback regarding this or any other article we’ve posted. To leave feedback, ‘Like’ us on Facebook  and then post your feedback on our wall.

What do we mean when we use the term “strain” as in “strain gauge”?

Strain results in a physical dimensional difference from loading a material.  When the last time you played with a slinky? Do you remember stretching it too much? You became downhearted as your nice new slinky wouldn't retain its original shape.  As you stretch that slinky, the metal is getting thinner along the length that is stretched out (the small cross section of the metal).  Stretch it within its elastic range (the range in which it operates as a spring), it will come back to its original shape.  Stretch it too much (beyond its elastic range) and the slinky yields (bends).  Not only does it not go back to its original shape, but it also doesn't quite work correctly anymore.  It doesn't have its original "springyness" anymore.

Load cells operate on the same principle.  Operate them within their elastic range (maximum capacity) and they are like a nice new slinky.  Overload them, and they are like that slinky that is perfect out on the ends, but has that dumb wrinkle in the middle.

The slinky and anything else in this world including load cells, will never (yes, I know never say never, but in this case there are enough decimal places, as in infinite, to say never) return to its exact original shape.  This is called hysteresis.  If I deform material, even if it is well within its elastic range, the molecules are shifting, heat is occurring and everything on the atomic structure is just changing.  That is why you can't flex a slinky forever without it eventually breaking, even if you only stretch it out a couple of inches.  It will break, eventually.  Every time you let it go back to its unloaded state, it has changed a little bit.  In the case of load cells, the strain gage and the electronics are not able to typically resolve this change when loading and unloading loads less than the maximum rated capacity.  For example, if you load 10 lbs on a 1000 lb load cell, you will read zero before you put on the 10 lb weight and then zero when you take it off.  However, if you could measure the displacement with enough decimal places, .000000000000 etc, you would eventually see a change.  The strain gage and electronics cannot see it though, which is actually very nice for all of us.

Click here for more technical information about strain and deformation.

If you have any questions related to this material, our support staff at Cooper Instruments is always available to help. Contact them by calling (800) 344-3921 or emailing This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

We recently came across this white paper on the galling or "cold welding" of stainless steel, an issue that can occur when using threaded fixtures. This should interest any of our customers who use fixtures or attachments with their stainless steel equipment. This paper discusses:

• the definition of galling and how it happens
• suggestions for how to prevent galling
• questions you should ask if you think or know that galling is a problem in your application and corresponding suggestions for how to treat the issue

Click here to view the white paper.

We´ve put together a list of terms that are often seen in the force and pressure measurement industry as a reference for our customers and anyone else who might find it useful. Terms that are seen in blue text can be clicked to open the Wikipedia article if a deeper definition is required. As always, our knowledgeable technical staff at Cooper Instruments is available at (800) 344-3921 or This e-mail address is being protected from spambots. You need JavaScript enabled to view it   to help you with any clarification you might need.

Accuracy:

·         the degree of closeness of measurements of a quantity to that quantity's actual (true) value

·         a limit tolerance which defines the average deviation between the actual output versus theoretical output

·         precision in the measurement of quantities and in the statement of physical characteristics

Accuracy is expressed in terms of error as a percentage of the specified value (e.g., 10 volts ± 1%), as a percentage of a range (e.g., 2% of full scale), or as parts (e.g., 100 parts per million).

Ambient Conditions: The conditions (humidity, pressure, temperature, etc.) of the medium surrounding the measuring device.

Analog: Anything that corresponds, point for point or value for value, to an otherwise unrelated quantity; data represented by continuous values rather than in discrete steps.

ANSI: American National Standards Institute: a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States.

ASCII (American Standard Code for Information Interchange) : Pronounced "askee". a seven-bit plus parity character-encoding scheme based on the ordering of the English alphabet. ASCII was established by the American National Standards Institute (ANSI) to achieve compatibility between data services.

Axial Load: A load applied along a line concentric with the primary axis.

Baud: symbols per second or pulses per second. A unit of communications processing speed in digital data communications systems.

Calibration: comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device. The comparison of load cell outputs against standard test loads.

Capacity: The amount of weight the scale is capable of weighing accurately.

Combined Error (Non-linearity and Hysteresis): The maximum deviation from the straight line drawn between the original no-load and rated load outputs expressed as a percentage of the rated output and measured on both increasing and decreasing loads.

Compensation: The utilization of supplementary devices, materials or processes to minimize known sources of error.

Compensated Temperature: The range of temperature over which the transducer is compensated to maintain Rated Output and Zero Balance within specified limits.

Creep: The change in transducer output occurring with time, while under load, and with all environmental conditions and other variables remaining constant. Usually measured with Rated Load applied and expressed as a percent of Rated Output over a specific period of time.

Deflection: the degree to which a structural element is displaced under a load. It may refer to an angle or a distance.

Drift: A random change in Output under constant Load conditions.Also, a continuously upward or downward change in the number displayed on the digital readout. Drift could be due to temperature, static electricity or RFI (radio frequency interference).

Environmentally Protected: Device is protected from normal environmental factors in indoor or outdoor applications.

Error: the difference between a computed, estimated, or measured value and the accepted true, specified, or theoretically correct value. In load measuring terms, the algebraic difference between the indicated and true value of the load being measured.

Excitation: The voltage or current applied to the input terminals of the transducer.

Hermetically Sealed: airtight: impervious to moisture, air and gas.

Hysteresis: The maximum difference between the transducer output readings for the same applied load; one reading obtained by increasing the load from zero and the other by decreasing the load from Rated Output. Usually measured at half Rated Output and expressed in percent of Rated Output. Measurements should be taken as rapidly as possible to minimize Creep.

IP Ratings (Ingress Protection Rating): Consists of the letters IP followed by two digits and an optional letter. As defined in international standard IEC 60529, it classifies the degrees of protection provided against the intrusion of solid objects (including body parts like hands and fingers), dust, accidental contact, and water in electrical enclosures.

Linearity: Refers to the quality of delivering identical sensitivity throughout the weighing capacity of a scale or balance.

Load Cell: A device which produces an Output signal proportional to the applied weight or force. Types of load cells include beam, S-beam, platform, compression and tension.

Natural Frequency: The frequency of free oscillation under no-load conditions.

NEMA: National Electrical Manufacturers Association

NIST (National Institute for Standards and Technology): An agency of the federal government to which all precision measurements are traceable. Formerly the National Bureau of Standards (NBS)

Non-linearity: The maximum Deviation of the Calibration Curve from a straight line drawn between the no-load and Rated Load outputs, expressed as a percentage of the Rated Output and measured on increasing load only.

Non-repeatability: The maximum difference between transducer output readings for repeated loadings under identical loading and environment conditions.

NTEP (National Type Evaluation Program): A program of cooperation between the National Conference On Weights & Measures, NIST, state weights and measures officials and the private sector for determining conformance of weighing equipment with the provisions of H-44.

OEM (Original Equipment Manufacturer): A manufacturer who produces equipment for use or inclusion by another manufacturer in its product.

Operating Temperature: The extremes of temperature within which the transducer will operate without permanent adverse change to any of its performance characteristics.

Output: The signal (voltage, current, pressure, etc.) produced by a load cell. Where the output is directly proportional to excitation, the signal must be expressed in terms such as Volts per Volt, Millivolts per Volt, or Volts per Ampere, etc., of excitation.

Primary Axis: The axis along which the transducer is designed to be loaded; normally its geometric centerline.

Rated Load (Rated Capacity): The maximum Axial Load that the transducer is designed to measure within its specifications.

Rated Output: The signal (voltage) produced by the transducer; the algebraic difference between the Outputs at no-load and at Rated Load.. Where the output is directly proportional to excitation, the signal is expressed in terms of millivolts / volt (mV/V) of excitation.

Reference Standard: A force-measuring device whose characteristics are precisely known relative to a primary standard.

Repeatability: The maximum difference between load cell output readings for repeated loadings under identical loading and environmental conditions; the ability of an instrument, system, or method to give identical performance or results in successive instances. Also called reproducibility or repeatability.

Resistance: Opposition to current flow offered by a purely resistive component; simple opposition to current flow. Measured in ohms.

Resolution: The smallest change in mechanical input which produces a detectable change in the output signal.

Safe Overload: The maximum, temporary or accidental load in percent of Rated Capacity which can be applied without producing a permanent shift in performance characteristics beyond those specified.

Sensitivity: The ratio of the change in output to the change in mechanical input.

Shunt Calibration: Electrical simulation of transducer output by insertion of known shunt resistors between appropriate points within the circuitry.

Span: The difference between the highest value and the lowest value.

Strain Gauge: A device for detecting the strain that a certain force produces on a body. The gauge consists of one or more fine wires cemented to the surface under test. As the surface becomes strained, the wires stretch or compress, changing their resistance. Several strain gauges are used to make up a load cell.

Tare: The weight of an empty container or vehicle, or the allowance or deduction from gross weight made on account thereof.

Terminal Resistance, Input (Excitation): The resistance of the load cell circuit measured at the excitation terminals at standard temperature with no load applied and with the output (signal) terminals open-circuited.

Terminal Resistance, Output (Signal): The resistance of the load cell circuit measured at the output signal terminals at standard temperature with no load applied and with the excitation terminals open-circuited.

Tolerance: The amount of error that is allowed in a value. It is usually expressed as a percent of nominal value, plus or minus so many units of measurement.

Transducer: A device that converts energy from one form to another.