For private use that would be a hassle indeed, although you could build yourself a test rig
Beam style versions do not need calibration
just have been through some documents, these small t shaped fixed torque wrenches typically have an accuracy of +/- 10%, ratchet style wrenches are usually +/- 3% for a quality tool
I like a beam wrench because it's reliable. However, it's not precise; that is, reading the pointer is not exact. A clicking wrench is precise but could be inaccurate (unreliable).
Last year I bought a set of 7 calibration weights which I could use for 1g, 188g, and 124 points between. I tested a cheap digital kitchen scale after 8 years of daily service. It was consistently accurate to the gram! It uses strain gauge technology, which is a bit like a beam wrench.
I was so impressed that I bought this because it seems to use a strain gauge:
It's advertised at 2% cw and 3% ccw, but the calibration certificate showed mine to be far more accurate. I think it will stay calibrated like my scale. It showed me that my cam-type torque tool varies wildly from one click to the next. I think that's because friction on the cam is inconsistent.
Over time, I've been so pleased with the digital torque driver that I've ordered this one, which also seems to use a strain gauge. The driver was good to 6 Nm. The wrench is good to 60.
However, compared to a set of ISO hex keys, I consider these digital tools toys. To use a digital tool, you have to have it with you. To use it well, you have to know the torque spec, and the spec has to be correct.
I prefer Allen keys. In 1909, Allen discovered that the socket in a screw could be sized for a key that would apply the desired torque. Using an Allen key is like walking barefoot up a driveway whose gravel is increasingly coarse and sharp. You want to go as far as you can and still be able to stand there for 5 minutes. Even blindfolded, if you walk slowly you'll stop at the same place every time.
Before I came to rely on proper hex keys, I would tighten by feeling the torque take up the elasticity. That didn't work on the front axle of my Abound because I didn't feel any elasticity. The head said "8-10 Nm." That was from the toolmaker who designed the part. Aventon said 10-15 Nm. You can't always count on what an e-bike manufacturer specifies.
The axle has a 6mm socket. I put a package scale under the center stand so I could calculate how much torque I was applying with an ISO H6 key. The first time, I could not comfortably maintain the pressure. That told me I'd been too aggressive, as could happen to a professional mechanic in a hurry. I wrote down the Newtons on the scale and tried again, this time taking a few seconds to feel my way as I increased pressure. That came out to 8.9 Nm, almost exactly in the center of what the toolmaker specified. The first try, which had felt too high, came out to 12 Nm, which wasn't as bad as relying on Aventon's recommendation.
Aventon recommends 2 to 4 Nm for three washerless screws that hold the chain guard. That's ridiculous. The chain guard is polyethylene, and two of the screws thread into soft sheet steel. The sockets are 3mm. I used my digital torque driver to test my H3 ISO key: 1.0 Nm. That's plenty to secure the chain guard. It and the sheet steel threads will last a lot longer than if I followed Aventon recommendations.
Louis Armstong said, "You can't go wrong with a good song and a sincere heart." I think I know what he really meant. You can't go wrong with an ISO hex key if you take a few seconds to feel your way.
