Read the Plunger: Measuring Accurately with Syringes

Read the Plunger, Not the Liquid Level

A syringe is read at the leading edge of the plunger—the edge closest to the tip. The visible liquid level inside the barrel does not indicate the volume drawn.

This is not a matter of opinion or technique. A syringe is a measuring instrument, and its graduations are read using the position of the plunger.

When the plunger of a dry syringe is fully depressed, air remains inside the tip, or attached fitting. This is normal and expected. As liquid enters the tip, that air is displaced into the barrel, where it will appear as a bubble between the liquid and the plunger.

Do not compensate for this bubble. Do not adjust the measurement or use the visible liquid level as the reading. The correct measurement is always taken at the leading edge of the plunger.

A longer tip holds more liquid and initially contains more air. Therefore, more air will be displaced into the barrel. This will make the visible liquid level appear lower and the bubble appear larger. The measured volume has not changed.

Read the plunger.

Do

• Submerge the tip before drawing liquid.
• Keep the tip submerged while drawing.
• Pull the leading edge of the plunger to the exact graduation.
• Read the plunger—not the liquid level.

Do Not

• Try to remove air from the tip before filling.
• Use the visible liquid level as the measurement.
• Adjust the reading to estimate or compensate for air inside the syringe.

If air must be expelled for a particular application, first draw and read the required volume. Then turn the syringe tip upward and expel the air.

The lesson is simple: read the leading edge of the plunger, not the apparent liquid level.

This article concerns syringes used for measuring liquids. It is not guidance for medical injections!