Strain gage

We introduce here answers to frequently asked questions.

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Q1Explain the different purposes of using 120 Ω and 350 Ω.


Generally 120 Ω type is used in stress strain measurement and 350 Ω type is used in the production of transducers.

Q2Why don’t they use more even numbers such as 100 Ω or 200 Ω?


Strain gage was first manufactured in the USA in 1938. Since then the 120 Ω and 350 Ω has become the standard resistance level and is complied internationally.

Q3Resistance value has been displayed on the package of the gage. What is the standard deviation?


Example of Single-axis Type Gage

Standard gage 120 ± 0.4 Ω, 350 Ω ± 1.2 Ω
Gage with lead wire 120 ± 0.8 Ω, 350 Ω ± 2.4 Ω

Q4What is the appropriate excitation voltage to apply to a 120 Ω or 350 Ω strain gage?


The maximum electrical current that can be applied to a strain gage bonded to metal material is 20 mA because of heat caused by Joule Heat. When considering the stability of zero balance, 10 to 15 mA is the maximum and the applicable excitation voltage for 120 Ω is 2 to 4 volts and for 350 Ω is 5 to 10 volts.

Q5I want to bond a KFGS gage to a pipe with less than 5 mm in diameter in the horizontal direction. Please tell me the applicable strain gage model and the minimum diameter needed to bond the gage. Also what are the characteristics of a strain gage when bonded to a round surface.


Model Name Applicable Diameter (mm)
KFGS-5-120-C1-11 φ4.5
KFGS-2-120-C1-11 φ3.5
KFGS-2N-120-C1-11 φ3.5
KFGS-1-120-C1-11 φ3.0
KFGS-1N-120-C1-11 φ3.0

If the radial curvature is too small, the change in resistance when bonding the strain gage may cause abnormal readings during measurement.
(1) Unable to get zero balance with the strain measurement instrument.
(2) The maximum strain limit of the gage will decrease and become less than 5 % strain
(3) The characteristics of the apparent strain caused by thermal expansion will change.
(4) A lack of consistency in the thickness of the adhesive layer, or the lack of required pressure applied for bonding the strain gage may cause a decrease in the fatigue life of the gage as well as decrease the creep characteristics of the gage.

Q6Do you issue an SDS Safety Data Sheet (formerly called an MSDS Material Safety Data Sheet)?


Yes. With a few exceptions, we can issue an SDS for all adhesives and coating agents in our catalogue.
Please contact us from here for more information.

Q7How do strain gages work?


A foil strain gage consists of a metal foil sandwiched between resin films. When metal materials are elongated or contracted, their electrical resistance changes. Since the relative change in elongation or contraction has a proportional relation with the resistance change, by measuring the resistance change, we can measure the strain induced in the material.
* Please see here for more information.

Q8What are the advantages of using strain gages?


Strain gages offer a relatively cheap and simple method for measuring strain induced in a target material. Small and simple in structure and easy to attach, they offer good frequency response and allow measurements of fast phenomena, localized strain, and multi-point strain distributions.
In addition, specialized strain gages are capable of measurements at high or ultra-low temperatures or under water or other conditions.
* Please see here for more information.

Q9What factors should I consider when selecting the most suitable strain gage?


Consider factors like your test environment (temperature, humidity, etc.), the coefficient of linear expansion of the target material, the space available for attaching the strain gage, and the magnitude of the expected strain.
* Please see here for more information.

Q10What are some of the applications for strain gages that feature long gage lengths?


Strain gages that feature long gage lengths are used to measure the mean strain of materials that exhibit non-uniform deformation under stress. Examples of such materials include concrete mixed with small pebbles and coarse-grained wood.

Q11How do I form bridges (Wheatstone bridges)?


There are basically three types of bridge circuits: 1-gage system, 2-gage system, and 4-gage system. (These are also called the quarter bridge, half bridge, and full bridge, respectively.) A bridge circuit has four arms. In the 1-gage system, one of the arms is replaced with a strain gage, 2-gage system with each of two arms is replaced with a strain gage, and 4-gage system each of the four arms is replaced with a strain gage.

Other modifications to the bridge circuits include using active gages and/or dummy gages.
* Please see here for more information.

Q12What is a 2-wire system and what are effects from the lead wires?


The 2-wire system is a method that connects a gage to the bridge circuit using two lead wires.
It requires fewer wire connections than a 3-wire system, but it should be used only in environments having relatively constant temperature conditions, because any changes in temperature during a measurement will change the electrical resistance of the lead wires, which will appear as apparent strain.
* Please see here for more information.

Q13What is a 3-wire system and what are effects from the lead wires?


The 3-wire system is a method that connects a gage to the bridge circuit using three lead wires. In this configuration, any changes in the electrical resistance of lead wires caused by temperature changes during the measurement will be canceled out, eliminating the problem of apparent strain.
* Please see here for more information.

Q14What are self-temperature-compensation gages?


Self-temperature-compensation gages are the gages that the temperature coefficient of resistance of the sensing element is controlled based on the linear expansion coefficient of the measuring object. Thus, the gage enables strain measurement without receiving any thermal effect if it is matched with the measuring object. In Kyowa’s self-temperature-compensation gages, apparent strain is kept to ±1.8 μm/m or less.
* Please see here for more information.

Q15What is the coefficient of linear expansion?


The coefficient of linear expansion is the rate of change in length or volume of a material per 1 °C change in temperature.

Q16What is Poisson’s ratio (ν)?


When a bar is pulled, it elongates by ΔL, and the ratio of this elongation ΔL, to the original length, L, is called longitudinal strain ε1. Meanwhile, the pulled bar becomes thinner while lengthening, and the strain from the thined in diameter called lateral strain ε2. Each material has a certain ratio of lateral strain to longitudinal strain, this ratio is called oisson’s ratio, which is expressed in ν (nu):
ν = |ε2/ε1|.
For common steel (mild steel), this value is 0.3.
* Please see here for more information.

Q17What is the difference between encapsulated strain gages and other types, such as foil strain gages?


There are various types of strain gages, such as the foil strain gage, linear strain gage, semiconductor strain gage, and encapsulated strain gage.
The encapsulated strain gage was developed for high-temperature applications. It is completely sealed inside a heat-resistant metal cover. The sensor consists of a tube (a sensing part) and flange. MI cable is used for the lead wire. Encapsulated strain gages are attached to the target object with spot welding.

Q18What is the shelf life of adhesives and how should they be stored?


In general, adhesives should be stored in a refrigerator or other cool, dry, dark place. Their "Use before" dates are printed on the packages. Some adhesives designated as harmful substances require a special storage facility that can be locked.

Q19To attach strain gages, can I use adhesives other than the recommended?


The characteristic tests for strain gages at Kyowa were performed using the recommended adhesives. We strongly recommend using the specified adhesives.

Q20There seem to be many different types of lead wires. What should I choose?


In general, choose a lead wire type based on the temperature conditions you expect during measurement, the required wire length, and the gage configuration (2-wire or 3-wire system). If preventing noise is a requirement, you may want to choose a low-noise type lead wire.
* Please see the following for more information.
Strain gages with Pre-attached Lead Cables
Lead Wire Cables

Q21What are some problems I might encounter when using gages with long lead wires?


First, using long lead wires will increase the resistance of lead wires, which will in turn reduce bridge excitation voltage. Thus, calibrations must be made to the output results.
Also, since the lead-wire resistance will be changed by temperature changes and will add the effect of apparent strain to the output, the precision of measurement will be affected. We recommend adopting the 3-wire system to eliminate the effect of apparent strain. Furthermore, when long lead wires are used in combination with carrier wave amplifiers, problems associated with line capacity may arise, and in some cases, balance may not be achieved.
* Please see here for more information.

Q22Please show me the formula for the rosette analysis.


The rosette analysis lets you calculate the maximum principal strain and angle based on data measured with a rosette gage (triaxial gage). You can perform these calculations easily using the data processing functions offered in the Kyowa Dynamic Data Acquisition Software DCS or Data Analysis Software DAS-200A.

* Please see here for more information.

Q23When attaching the strain gage, will the deviation in the angle of attachment cause errors in measurement results? Please show me how to calculate such errors and reference values.


Please see the Technical Notes of "Misalignment Effect of Bonding Strain Gage" for explanation of the effect of deviation in the angle of attachment on the measured strain.

Q24Can I purchase customized strain gages?


Yes. Please contact our sales representative.

Q25I want to measure strain on a PCB. Tell me about the minimum items required for measurements.


You need strain gages, a strain gage bonding kit, a Printed Circuit Assembly Stress Measurement Set PCAS-1000A, and a PC.

Q26What’s the difference between models with codes like -C1, -D9, and -D16?


Codes like -C1, -D9, and -D16 in the model name represent the pattern of the strain gage.
* See "Strain Gage Model Name Coding System" for more information.

Q27What do the numbers like -11 and -16 at the end of the model name mean?


Numbers like -11 and -16 in the model name represent the applicable linear expansion coefficient of the target's material.
* See "Strain Gage Model Name Coding System" for more information.

Q28I’ve never used a strain gage before. How should I select one?


Select one according to the measuring conditions, including the target's material, expected amount of strain, and operating temperature, etc.
* See "How to Select Strain Gages" for more information.

Q29I don’t know whether the strain working on the part is tensile or bending. How can I determine this?


You can distinguish tensile and bending strain by bonding two gages on the part. For example, in the case of a simple cantilever, you can measure either the tensile or bending strain with the two-active-gage method by changing the circuit wiring.
* See "How to Form Strain Gage Bridges No. 7 and 8" for more information.

Q30Regarding the linear expansion coefficient, if I measure the strain of copper with the strain gage for normal steel (KFGS-1-120-C1-11-L3M3R), can this be compensated for? If yes, what’s the compensation formula?


The measurement is possible. If the temperature remains constant during measurement, the data doesn’t require compensation.
If the temperature changes during measurement, the data must be compensated for the apparent strain. Since the linear expansion coefficient of copper exceeds that of normal steel, the difference must be subtracted from the measured data. The difference = (16.7-11.7) x (temperature change). Compensation requires data of temperatures during measurement.
The common dummy method is an alternative. Bond another gage of the same model not subjected to any external force to the same material and place the gages under the same temperature conditions. This will cancel the apparent strain.
* See "How to Form Strain Gage Bridges No. 5" for more.

Q31Does the strain gage have restrictions on frequency characteristics?


The response frequency of the strain gage differs depending on gage length and gage type. The response frequency of a general-purpose KFGS gage is several hundred Kilohertz. The response frequency also changes depending on the material to which the gage is bonded and the measuring instrument used.

Q32I’m considering using the KFGS-1.5-120-C20-11 strain gage to measure bolt axial force from -40 °C to 120 °C. Can I use the EP-340 adhesive for measurements in the temperature range from -55 °C to 150 °C?


Measurement simply by changing the adhesive is not possible. The temperature compensation range of KFGS-1.5-120-C20-11 is 20 °C to 50 °C.
We can provide a bolt tensile force sensor usable in the temperature range from -40 °C to 200 °C by bonding a special gage. Please contact our sales staff.

Q33Since the strain gage is soaked in water (only during the experiment), I’m considering applying a waterproof coating. Tell me about a coating material that doesn’t influence the rigidity, since I want to measure strain in a part with low rigidity.


Although it depends on the test temperature and time, if the test is performed at room temperature for a brief period, consider using a waterproof strain gage (KFWB, KFWS) with pre-applied protection against water. Alternatively, we recommend the C-5 coating material, since the part in question has low rigidity.

Q34Do you have a strain gage with dimensions small enough to be bonded to a chip capacitor?


We offer the KFRS-02-120-C1-13 strain gage with base dimensions of 1.2 mm x 1.1 mm for measuring strain in circuit boards.
Check if it can be attached.