There were lots of fancy, expensive sensors measuring light, temperature and fuel use in the greenhouses I worked in, but my clear plastic standard classroom ruler was my most guarded tool during the past few years. Most people probably have a ruler of some sort floating around their house or office and I'm sure every scientist uses them rather regularly. So, the mechanism of using a ruler should be pretty well known.
The primary function of my oh-so-important ruler was measuring leaf length of growing salad greens. Two or four times a week I would measure leaves on hundreds of plants. That's primarily what my lab notebook is full of - 20,000 numbers from ranging from 0 to 22. You can kind of see a page full of these numbers in the picture above and you can see why I was doing all this measuring here.