Imperial or Metric


As we prepare to release the hardware design for OpenROV 2.0, a question about what measurement system to use -imperial or metric- has come up. Because most parts in the United States (where presumably most of these will be initially built and used) are specified with imperial units, it's been easiest to design everything with the imperial system. Because we want OpenROV to be embraced internationally and by the scientific community, and because we think the metric system is generally a better way of doing things, however, it seems that releasing the design of OpenROV 2.0 with metric units may be a worth while choice.

We hope that OpenROV (the community) will end up playing a roll in the Open Hardware movement, so setting the right trends from the get-go is important.

I'd like to realease our design with metric units, but I wanted to see if anyone had points to make about the choice before doing so.

What do you guys think?


(Photo credit: "Biking Nikon OGG" from Flickr)


Definitely go with metric. It is much easier to get precise measurements across to others and just easier to measure out. I always go with metric if I can to make things simpler and especially mathematically easier on me.


Hi Eric. Metric is great! The only downside is with choosing hardware and being able to readily and cheaply obtain it. For instance, cup seals or rotary shaft seals are somewhat tricky to find in metric... though I was finally able to. Also, I was able to find stainless retaining rings to hold my seals in place but I wound up needing to buy a bag of 1000 instead of just the 4 that I needed ($80.00 for the bag). Self Sealing metric screws usually need to be sourced from a manufacturer even though sites like McMaster-Carr have loads of imperial.

The more that metric is used and demanded in the USA the more suppliers will carry what "we" want. I doubt the next generation will ever have this conversation when they start to tinker.


Kyle and Terry,

Thanks for the advice. Yeah- I think we are all on the same page: although it's much easier to find imperial parts in the US, metric is better in the long run. If we can help influence the demand for metric that Terry refers to, then we are also making the world a better place, right?! Fortunately, because of the primarily laser-cut design, the OpenROV 2.0 spec doesn't require very much (any?) high-tolerance hardware, so hopefully most parts will interchangeable with similarly sized pieces in the other unit system.

On my flight back from New York yesterday, I redesigned everything with metric units, and I should be testing the first laser-cut prototype of the new metric design by the end of this week!



I always regret when I go with imperial units, I definitely see what Terry is saying. I guess a flexible system of both imperial and metric would work when parts to be cheaper and easier to get must be imperial but if there is a choice between imperial and metric hardware that are close and the same price a metric priority would be better for the future design.

When designing my own ROV I never actually came across a pick between metric and imperial but also, at this time, it is still only academic, sitting on my computer.


So glad to see another openhardware project selecting metric system! Go for it :slight_smile: