On calipers

I'll ramble on about calipers today, just because I like calipers. In my line of business I use calipers very frequently. And not just a normal handy 150mm caliper, but a larger 300mm caliper or an even larger 600mm calliper. So I'll just list my callipers in size order.

150mm glass fibre dial calliper

I really like this one, despite the fact that I bought it at a local hardware shop - if it's good enough to refurbish your kitchen or build a bed, then it's good enough for me. So far I've trusted my life with the works of carpenters/engineers so I don't see why I can't trust my measurements using their tools. To begin with, craniometrics are not the most precisely defined measurements and taking these at the precision of 0.01 mm is absurd - rounding to the closest mm is fine, at least it's accurate to the mm or maybe 0.1mm.

Anyway, I digressed. I like this dial calliper because first of all it is very easy to read. Unlike vernier calipers dial callipers are very straight forward; you can just simply read off the dials and there is no need to get confused with the vernier scale. Of course you sacrifice the precision - vernier scales typically can let you read to the precision of 0.02mm whereas dials only allow to the level of 0.1mm - but like I rambled on above, I don't really need that level of precision. Secondly, my dial calliper is glass fibre so it pretty much doesn't leave any scratches. This is kind of important when you're working with fragile or old specimens in bad conditions - sharp metallic callipers can leave scratches on specimen surfaces if you're not careful.

300mm digital calliper

After visiting a couple of museums it soon became clear that a 150mm calliper wasn't going to be enough for some larger specimens, for instance crocodilian skulls or all the mid- to large-sized cat skulls. So I got myself a decent 300mm digital calliper from ebay (I was a self-funded student so I didn't have the luxury of charging it to a grant). This calliper is very nice in that it has the precision at 0.01mm but also it is digital so the measurement gets displayed on a LCD which reduces transcription error considerably. I remember being very excited by this large calliper and I happily measured larger specimens. Again, I rarely read the measurement to the 0.01mm scale.

600mm vernier scale caliper

Despite my delight at the size of the 300mm calliper and its capabilities in measuring larger specimens, it became apparent once again that some measurements for very large specimens could not be measured by even my 300mm calliper. So I got myself a decent 600mm vernier scale calliper (I think I found this on ebay as well). This was good to some extent but this calliper was difficult to use in several ways. First, I could never read the vernier scale correctly, despite having had training in University (I just vaguely remembered I had to line up the scales). Second, and more importantly, I found out that the jaws were too short for any easy use against objects that had considerable depth, like a lion or tiger skull. Frankly, I don't really know what kind of object would be 600mm in length but can be measured by callipers with jaws that are only about 50mm. Skulls are typically obscure in shape and can be very awkward to measure direct distances between two landmarks, and too frequently the 600mm calliper proved to be a very clumsy tool for this. For instance, if you wanted to measure the greatest length of the skull taken as the direct distance between the tip of the snout to the tip of the occipital crest, then quite frequently, the skull roof would get in the way of the calliper. It was clear that I needed something with longer jaws.

600mm tree caliper

Initially I couldn't find any callipers with jaws that were long enough. However, after many hours of searching online using a variety of key word combinations, I finally came across something close to ideal: tree callipers. Tree callipers are commonly used by forestry people to measure tree trunk diameter and are available in very large sizes, even up to 2m in length. But most importantly tree callipers have very long jaws, exactly the kind of thing I was looking for. It is worth noting here that there was an option to buy an anthropometer but the price of such an instrument was way beyond my then financial capabilities (maybe now I can charge my grant). But in any case, a tree calliper is sufficient for my purposes. Unfortunately, my tree calliper only has the precision to a mm, but given the specimen size of 350 or 400mm (and sometimes even bigger) a mm precision is good enough; e.g. 404mm and 405mm is nearly identical, let alone 404.3mm and 404.4mm. In other words, a precision of less than a mm is overkill in these cases.

I am very happy with this calliper. The jaws are sufficiently long enough to bracket large cat skulls with ease so it allows me to measure distances more or less parallel to the direct distance between two landmarks by taking the jaws roughly perpendicular to that imaginary straight line. One drawback is that the jaws are too long for easy transport so I always have to take it apart when I leave the museum; fortunately, the jaws can easily be detached by unscrewing a couple of screws.