ABC of strain gauges

Like any technology, strain gauges have specific terminology. In casual conversation words are often used synonymously when in engineering the same words have distinct, different meanings. For example, we speak of stress and strain as if they are the same, but strain is distinctly different from stress when it comes to materials and structures. To put this in simple terms, strain is a change of shape (deformation) whereas stress is the structure resisting the force (pressure). A piece of material that heats up and is free to expand changes shape (strain) but is stress-free. Conversely, if you heat up a material that is constrained it may not be able to change shape (expand) but it will be stressed.

Language is important when we want to communicate something clearly. Using the correct words with grammar and punctuation adds meaning; if you’ve ever tried to read a block of text without any punctuation you’ll be familiar with the difficulty in understanding it. Misused words such as your/you’re, there/they’re/their, its/it’s, to/too and “could of” instead of “could’ve”, a contraction of “could have”, are common (“I’ve” is never changed to “I of” even though it has the same structure).

The apostrophe is even more confusing, most commonly scattered around as if the rules are random. The possessive use can inform us whether there is one or more of something or someone (the Kings’ books or the King’s books?) Plurals do not normally need it; CDs, DVDs, 1960s are all plural and yet often have apostrophes, and sometimes we see both versions at the same time, for example “CD’s and DVDs” (Note: British English). Used for missing letters can add additional challenges – “it’s” rather than “it is” (not to confuse this with “its”, a statement of belonging) should be straightforward. But, common usage can change – 1960s is often contracted to 60s but should really be ’60s, and we all accept phone, a contraction of telephone, and almost never see the grammatically correct form ‘phone.

Sometimes we let this go as we have an implicit understanding of what the person means. “Can I get…” when asking to be served is so common (a US import to the UK) and yet the answer should almost always be “No” because you are not allowed to go into a restaurant kitchen and make you own coffee or sandwich. We just accept it as this is implied to mean the polite “May I have…”, not so easy if your first language is not English.

From an engineering perspective this all seems rather superfluous, and yet there have been many failures due to misunderstandings caused by sloppy language. Incorrect conversion from metric to imperial, poor material selection or a simple design change for convenience can lead to catastrophic failure (Hyatt Regency walkway collapse, for example).

Simple examples of this are suggested at the top of this article; strain gauges measure strain, not stress, load or pressure. Strain and microstrain are sometimes swapped, and sometimes we see millistrain too – if you are a factor or 1,000 or 1,000,000 out in a calculation this is the probable source of the error! Creep, hysteresis, zero shift and zero drift are distinct properties of a measurement and are often confused (a measurement might contain all four as well as noise and the actual measurement quantity) and must be understood for error correction or fault finding. Even identifying noise can be an issue – strain gauges pick up the minutest change, and discarding this as noise or the commonly-heard “the strain gauge isn’t working properly” can lead to discarding or misinterpreting perfectly valid data.

I often talk about language in my presentations. A measurement is a different language that requires interpretation – no sensor speaks English (or any other spoken word) but must be conditioned, converted, error corrected and calibrated before it can be interpreted in terms of the desired outcome. This assumes that it has been correctly specified and installed, a different discussion!

In summary, definitions are important to ensure unambiguous understanding, especially in science and engineering. The glossary on our website has many commonly-used definitions which may be useful when defining a problem or looking to communicate a solution. Alternatively you can call us to speak to one of our experts – there will be someone who speaks your language!

https://www.micro-measurements.com/glossary.