A 3 mL glass cartridge is the cheapest part of your setup, and it’s also the part most likely to cause a bad day. The pen around it is solid metal and will last for years. The cartridge is glass and rubber, and it doesn’t forgive much. This guide covers what happens to a cartridge after it’s filled: how to label it, where to keep it, what to check before each use, and how to swap one in the PepPen V3.
If you haven’t filled a cartridge yet, start with our vent needle fill guide and come back here.
Label it the day you fill it
Every cartridge looks the same once it’s full. Before it goes into the pen, put a small label or a strip of tape on the cartridge or the pen body with three things:
- What’s in it
- The concentration you mixed
- The date you filled it
It takes ten seconds. It also stops the most common mix-up we hear about, which is two pens sitting next to each other in the fridge with no way to tell them apart. If you run more than one project, give each project its own pen color. The V3 comes in seven colors, so that’s easy to do.
Store it the way your compound needs
The cartridge itself doesn’t have storage requirements beyond keeping it clean and unbroken. What’s inside it does. Follow the storage guidance from whoever supplied your compound, and assume it applies to the whole filled pen.
A few things apply no matter what’s in the cartridge:
- Don’t freeze it. Liquid expands as it freezes, which can crack the glass or push the rubber stopper out of position. A cartridge that froze once isn’t worth trusting.
- Keep it out of direct sun and away from heat. A dashboard or a sunny windowsill is the wrong place for a filled pen.
- If it needs to stay cold, keep it in a case. Our Travel Case holds a pen with an ice pack, and it’s padded so the glass doesn’t knock against anything in a bag.
Check it before every use
Hold the pen up to the light and look through the cartridge window. You’re checking for four things:
- Cracks or chips in the glass. Even a hairline crack can let liquid seep out under pressure. Replace the cartridge.
- A crooked stopper. The rubber stopper at the back should sit flat and square. If it’s tilted, liquid can get past it.
- Liquid behind the stopper. If you see liquid on the plunger side, the seal has failed. This almost always traces back to filling without a vent needle, which pushes the stopper out of position.
- Air bubbles. A small bubble is normal after you load the cartridge. If you see a large bubble before a use, clear it with a short purge the same way you did on day one.
How to swap a cartridge in the V3
The V3 has a classic exterior, but inside it uses the same layout as the V2. That surprises people the first time they change a cartridge, because the cartridge holder isn’t where they expect it to be.
- Remove the pen needle and dispose of it properly. Our needle disposal guide covers how.
- With the pen screwed together, pull the front metal cover straight off. Don’t twist it. The cartridge holder is underneath, attached to the threads that join the two halves.
- Take out the empty cartridge and set the new, labeled one in the open end of the holder.
- Reset the plunger rod and reassemble the pen as shown in our Preparing Your Pen video on the FAQ page.
- Attach a new pen needle and purge the air before the first use from the new cartridge.
If you unscrew the V3 in the middle and can’t find the holder, that’s normal, and it’s the most common question we get. Here’s exactly where it is.
Use a fresh cartridge for every fill
Our cartridges come sterilized in packs of five, and they cost about a dollar each. Unless you have a way to sterilize cartridges yourself, don’t reuse them. Refilling a used cartridge saves very little and adds risk: the rubber seal has already been pierced, and anything left inside the glass gets mixed into the new batch. Start each fill with a fresh cartridge.
Quick reference
- Capacity: 3 mL per cartridge
- Increments: 0.01 mL per click, so 300 clicks in a full cartridge
- Most per press on the V3: 0.60 mL
- Fits: every reusable PepPen (V1, V2, V2 Pro and V3)
Shop 3 mL glass cartridges or see the PepPen V3.
All PepPens products are sold for research use only. They are not medical devices.
