If you’ve ever drawn liquid into a small syringe, you know the routine. Hold it at eye level, tap out the bubbles, line the plunger up with the mark, and hope you read it right. A pen takes most of that judgment out of the process. Here’s why, and what the published testing shows.
How a pen dial works
A pen measures mechanically. Turning the dial sets how far a threaded screw (the lead screw) will travel. Pressing the button drives that screw forward, which pushes the cartridge’s plunger by exactly that distance.
A 1994 BD patent for a dial pen describes it plainly: the dial “creates a click for each unit,” and “the lead screw’s travel is proportional to the number of units displayed.”
The volume that comes out is just geometry: the inside area of the cartridge times how far the plunger moved. Since every 3 mL cartridge is made to the same inside diameter, the same number of clicks moves the same volume. On every PepPen, one click is 0.01 mL.
Where syringes go wrong
A syringe depends on you. Published testing points to a few repeat problems:
- Air bubbles. In a 2024 study, 33 nurses drew from vials with syringes across 198 trials. Errors were significantly larger among participants who didn’t check for air bubbles. The biggest misses were 2 to 4 units off target.
- Dead space. Some liquid always stays in the syringe hub and needle. A 2021 study measured 0.034 mL left in a standard 1 mL syringe after expelling, and about 0.05 mL in a separate 1-inch needle.
- Reading the scale. Small marks, curved liquid surfaces and viewing angle all add up, especially at small volumes.
A pen avoids most of these because it doesn’t rely on a visual line, and the needle screws onto a sealed cartridge instead of a separate hub.
What the head-to-head testing shows
The best comparisons come from insulin pens, since that’s where pens were first developed and studied. Treat these as context for how dial mechanisms perform, not as claims about any specific product.
In a 1998 study in Archives of Disease in Childhood, researchers dispensed 1, 2, 5 and 10 units, 15 times each, from two pens and a syringe, then weighed every one.
| Target | Pen A | Pen B | Syringe |
|---|---|---|---|
| 1 unit | 0.89 | 0.92 | 1.23 |
| 10 units | 9.8 | 9.9 | 10.1 |
At 1 unit, the syringe was off by 23%, against 8% to 11% for the pens. At 10 units, all three were close. The pens were more accurate than the syringe at 1, 2 and 5 units.
The lesson: the smaller the amount, the more the syringe’s human error shows up. That’s exactly where a mechanical dial helps most.
The standard behind it
Pen injectors sold as medical devices are tested under ISO 11608-1, which includes methods for measuring how accurately each dose is delivered. PepPens are research-use products and aren’t certified to that standard, so we don’t make accuracy claims. What we can say is how the mechanism works: a fixed screw travel per click, on a standard-size cartridge.
Getting the most out of a pen
- Purge the air after loading a new cartridge. Trapped air compresses instead of moving liquid. See our fill guide.
- Hold the button a few seconds after pressing so the plunger finishes its travel.
- Use a fresh needle. A clogged needle is the most common reason a press stops early.
Comparing models? See our pen comparison or the Compare PepPens page.
All PepPens products are sold for research use only.
