Generally, the electronic digital plastic calipers has a movable plastic bar , which can reduce the chance of scratching . Therefore, it is good for artifacts. Additionally, the plastic calipers are quite cheap.
Measuring the Outer Edges
Do you know how to use a caliper correctly? First, please look at the bump on the bottom edge of the vernier calipers. It is a grip for your thumb. Then, slide it back towards the tail end of the calipers and the jaws open. If you are measuring the outer edge of an object, namely, the head of an ushabti figure, you would open the jaws of the calipers and slide them carefully over the head of the ushabti. Read the sliding scales along either top or bottom or read the dial to see the size of the head. Meanwhile, you must be sure that your calipers are straight for even a slight angle throws off your measurements. Remember this is highly precise work and readings must be taken very, very carefully to assure proper dimension and accurate drawings. Remember, it takes a lot longer to correct your work and find a mistake than to take the time to do it right for the first time!
Measuring the Inner Edges
Firstly, please take a look at the top edge of the dial calipers. Can you see those blade like edges sticking out the top? Those are to take measurements of the inside of holes like on the opening of a jar. Put them inside the opening and carefully spread them using the thumb grip. They will widen simultaneously as the jaws open. Be careful that you are taking your reading from the widest diameter (distance across). Please do bear in mind that handmade objects will have variance unlike those made by machines of our time. So you need to take several readings and make notes.
Measuring the Depth
Now look at the little thing sticking out the bottom at the tail end of the dial calipers. When you move the thumb grip, simultaneously as the jaws open and the blades spread, that rod will move out the bottom. This is a depth gauge which you can put into a vessel and measure it's depth. Open the calipers until you hit bottom and take your reading.