What a Click Is
A click is the dial locking into a detent - a spring-loaded notch that stops the threaded drive rod at one fixed travel distance. Turning the dial advances that rod, which pushes the piston forward inside the cartridge. Because every detent sits in the same place, each click moves the piston the same amount, every time.
In this article
The pen clicks when you turn the dial. Most researchers don't think much about that sound, but it isn't just feedback - it's the lock engaging.
- The sound is secondary. What you hear is the dial seating into a notch; the lock, not the sound, is what matters.
- The travel distance is fixed. Every detent stops the drive rod at the same point, click after click.
- The volume is reproducible. Because the stopping point never shifts, the piston moves the same amount each time that detent engages.
Key point: A click is a physical mechanical lock, not just a sound - it is what makes each dial setting correspond to the same exact volume, every time.
How the Pen Selects a Volume
The Piston and Drive Rod
A precision click-dial pen advances a piston — a rubber stopper sitting at the back of the liquid inside the glass cartridge — forward each time you press the dosing button.
The drive rod, a threaded metal shaft running through the pen body, controls that travel. Turning the dose dial moves a threaded collar along the rod; pressing the button pushes the collar, and the rod, forward exactly as far as the dial indicated.
Converting Travel to Volume
The number in the counter window represents a physical distance, not a volume by itself. Multiply that distance by the cartridge's inner bore cross-section and you get the volume dispensed. Because that geometry is fixed by the cartridge and pen design, the same dial setting produces the same travel distance every time the mechanism works correctly.
Which Cartridges Fit the Pen
This piston-and-bore system only works with a cartridge matched to the pen's stroke. Our pens are built for standard 3 ml glass cartridges with an 11 mm long plunger (stopper) — the cartridge we sell. 3 ml cartridges with a shorter, roughly 8 mm plunger also exist; the glass looks identical from the outside, but the shorter stopper sits at a different starting point in the barrel, so that format will not dose correctly in our pens. Sourcing cartridges elsewhere? Check for the 11 mm long plunger before loading one. Pen needles are more forgiving: our pens accept universal 28G-33G screw-on needles.
What a Detent Actually Is
How the Detent Mechanism Works
Detents are the small notches or pockets machined into a rotating part of the dial mechanism, paired with a spring-loaded ball or ridge that snaps into them as the dial turns.
Picture a round collar with a ring of shallow holes around its edge, and a tiny ball held against those holes by a spring. As you rotate the dial, the ball pops in and out of each hole. That is what creates the physical click.
Eliminating Intermediate Positions
The click means the dial has landed in a defined position, not between positions. Without detents, the dial would spin freely and stop anywhere, including halfway between two markings. You would have no reliable way to know what volume you had actually set.
With detents, every possible stopping point is a precise, repeatable spot. The spring force holds the dial there until you deliberately turn it further. Vibration, a slipped finger, or setting the pen down on the bench won't shift the setting off position.
For bench work, that reliability matters most when:
- You're dispensing small volumes where a fraction of a click is a measurable difference
- You're filling multiple vials from one cartridge and need each volume to match
- You need to record exactly what volume entered each sample
The Numbers Behind Each Click
The volume each click represents depends on two factors: the thread pitch of the drive rod (how far the rod advances per rotation) and the inner diameter of the cartridge bore.
The detents are spring-loaded notches that fix every dial setting to an exact travel distance inside the cartridge.
Our pens are built around a standard 3 ml glass cartridge with an 11 mm long plunger (stopper) — the dimension that sets each detent's travel distance. The same 3 ml volume is also sold with a shorter, roughly 8 mm plunger; the glass looks identical, but a pen's drive mechanism is built for one plunger height, so the wrong format won't dose correctly.
We engineer and test our pens against the 11 mm-plunger cartridges we sell, not against other manufacturers' hardware, so we can't speak to fit with third-party pens or cartridges — check plunger length before assuming compatibility if sourcing elsewhere. The exact per-click volume is listed in the product documentation; that figure is what you reference when calculating how much solution moves per click.
Mistakes That Undermine Click Accuracy
- Dialing past your target and backing up without checking the manual: Some pen mechanisms allow backward dialing to reduce a set count. Others don't, and forcing them backward can skip detents or leave the dial seated between positions. Check the specific instructions before correcting an overshoot.
- Losing count while dialing: If you're counting clicks to reach a target, dial slowly. Count out loud or use a tally mark, then verify the unit counter window before dispensing into your vial.
- Skipping the prime on a new or stored cartridge: Air in the tip or needle means the first volume you dial doesn't fully reach your vial. A short prime, dispensing a small amount into a waste container until liquid flows cleanly, clears that air before your actual samples.
- Ignoring low-cartridge behavior: Near the end of a 3 ml fill, the piston is close to the front. Many pens won't let you dial more than what remains in the cartridge. If the dial stops short of your usual setting, check remaining volume before assuming a mechanical problem.
- Loading a cartridge with the wrong plunger height: Our pens are built for standard 3 ml glass cartridges with an 11 mm long plunger, the cartridge we sell. Some 3 ml cartridges on the market instead use a shorter, roughly 8 mm plunger. The glass looks identical from the outside, but a pen built for one plunger height will not dose correctly with the other. Check plunger height before loading any cartridge you didn't source from us.
- Mixing up volume and concentration: The pen delivers a volume. The amount of peptide in that volume depends entirely on how you reconstituted the cartridge. Two cartridges at different concentrations will put different amounts of peptide into a vial at the same click setting. Label every cartridge with concentration and date.
The clicks are not decorative. They are the mechanism that makes your volumes repeatable and your bench records defensible.
Which cartridges fit the pens?
Standard 3 ml glass cartridges built with an 11 mm long plunger — the format we sell. A pen is machined for one plunger height, so matching that spec affects click accuracy, not just whether the cartridge loads.
do the cartridges work with other pens too?
We haven't tested our cartridges in other manufacturers' pens, so we can't confirm fit either way. The spec that matters is plunger height: ours is 11 mm long. Any pen you pair them with needs to be built for that same height, or the click count on the dial won't match the volume actually dispensed.
Frequently asked questions
What does a detent do in a metering pen?
A detent is a spring-loaded ball that snaps into machined notches on the dial, locking it at defined positions so the dial cannot stop between dose settings, keeping each dispensed volume precise and repeatable.
How do I calculate what volume each click on my peptide pen delivers?
Check the pen's product documentation for its per-click volume. Multiply that figure by the number of clicks dialed and divide by your cartridge concentration to find the amount of reconstituted peptide in each dispensed volume.
Can I dial backward on a metering pen to correct a dose I set too high?
It depends on the pen design. Some mechanisms allow backward dialing safely; others can skip detents or lose position accuracy if reversed. Always consult your specific pen's manual before dialing in the opposite direction.
Related from our lab: the pen · cartridges · bacteriostatic water · reconstitution calculators
Shared by PreppinPeppers for research, educational, and demonstration awareness only. We link to third-party coverage; we do not endorse it, and nothing here is medical advice or a recommendation to use any substance in humans or animals. Our products are sold for laboratory research use only.
More in our reusable reconstitution pens collection.
What the research community gets wrong about metering pen clicks and detents
Five mechanical misreadings keep surfacing in questions about dial mechanisms. None of them are about how much to use — they are about how the hardware itself behaves, and about whether a click count still means what the counter window says it means.
- The click is treated as a sound, not a lock. It is a spring-loaded ball seating into a machined notch that physically holds the dial at a defined position. Listening for the click without reading the counter window can miss a dial that seated between positions.
- A click is confused with an amount of peptide. A click sets a fixed volume only. The mass inside that volume comes entirely from how a given cartridge was reconstituted, so two cartridges at different concentrations deliver different amounts at the same click count.
- Every pen is assumed to dial backward the same way. Some mechanisms reduce a set count cleanly. Others can skip detents or leave the dial seated off position when forced backward. That behavior belongs to the specific pen, not to pens in general. Confirming the new setting on the counter window after any backward adjustment avoids relying on the click alone.
- Small settings are expected to be as reliable as large ones. Device studies of pen injectors report the widest relative error at the smallest settings, where a fraction of a click is a measurable share of the total volume (Gnanalingham 1998).
- Accuracy is assumed to be permanent. Dose-setting mechanisms have been tested across simulated lifetime stress, and durability is something verified, not assumed (Kristensen 2011). A pen that clicks cleanly can still drift after heavy use or impact.
From our bench: Place a tared vial or weigh boat on an analytical balance, dispense a fixed number of clicks of reconstituted solution into it, and record the mass. Repeat the same click count several times and compare the spread. Send us your click count, concentration, and mass readings, and we will add your real numbers to this page.
Sources
- Gnanalingham et al., Arch Dis Child 1998: Accuracy and reproducibility of low dose insulin administration using pen-injectors and syringes
- Clarke and Dain, Expert Opin Drug Deliv 2006: Dose accuracy of a reusable insulin pen using a cartridge system with an integrated plunger mechanism
- Kristensen and Lilleøre, Curr Med Res Opin 2011: Dose accuracy and durability of a durable insulin pen before and after simulated lifetime use
- Bohannon, Postgrad Med 1999: Insulin delivery using pen devices. Simple-to-use tools may help young and old alike
- Jonker et al., Int J Clin Pharmacol Ther 2021: First pre-filled pen device with highly purified human menopausal gonadotropin (HP-hMG, Menopur) in liquid is shown to be bioequivalent to powder for reconstitution
- Lingen et al., Endocr Connect 2023: Advantages and disadvantages of connected insulin pens in diabetes management
✔ Reviewed by Bryan Le, PharmD, RPh
Bryan is a licensed pharmacist (Doctor of Pharmacy, Registered Pharmacist). Reconstituting lyophilized preparations is core pharmacy practice, so he reviews The Lab’s content for technical accuracy and to keep it within a research-and-education scope, with no medical or dosing advice. View profile on LinkedIn.
Reminder: research and educational reference only. PreppinPeppers sells hardware and materials, not peptides. Not medical, dosing, or health advice, not evaluated by the FDA, and not intended for human or animal use.
