How accurate is the dial?
The pen is not where your error comes from. A click is a fixed mechanical increment of 0.01ml and it does not drift. Almost all real-world dosing error comes from the mixing arithmetic, from rounding a fraction of a click, and from not priming.
What the mechanism actually does
Inside the pen a threaded rod presses on the cartridge plunger. Each click advances it a fixed distance, chosen so the displaced volume is 0.01ml. It is a screw thread. It does not gradually become a different thread.
Where the error really comes from
The mixing arithmetic
Get the concentration wrong and every dose is wrong by the same proportion, perfectly repeatably. This is the big one, and it happens before the pen is involved.
Rounding to a whole click
If the true answer is 7.5 clicks you must dial 7 or 8. On a small dose that is a 7% error on every dose. Fix it by changing the water volume, not by trying to half-click.
Not priming
A fresh needle holds a small amount of air. Skip the two-click prime and the first part of your dose is pushing that air out.
Withdrawing too early
Pull the needle out before ten seconds and some of the dose follows it back out. Nothing to do with the pen at all.
Air in the cartridge
A large bubble occupies volume the dial counted as liquid, so the delivered amount is short.
Four of those five are technique, and the fifth is arithmetic. Chase those before you go looking for a more accurate pen.
Checking your pen is behaving
Dial 50 clicks and expel into a measuring container or onto a scale. You should see about 0.5ml, or half a gram if you are weighing water. If it is wildly out, the cartridge is not seated, the plunger has jammed, or there is a large air pocket. If it is close, the pen is fine and any dosing problem is upstream of it.
Questions
- Do pens lose accuracy over time?
- Not meaningfully. What degrades is the plastic cartridge holder and the cartridge seal, both of which are replaceable. The dial mechanism itself is a screw thread.
- My dose feels short. What do I check first?
- In order: did you prime, is there a bubble, is the cartridge properly seated, and did you hold for ten seconds. Then re-check your concentration maths.
Duihao V3 Reusable Pen
Reusable metal pen with a closed cap and pocket clip.