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The “Blow-Out” Rule: How to Tell if Your Serological Pipette is TC or TD

A serological pipette plays a critical role in cell culture, reagent preparation, and routine liquid transfer in molecular biology laboratories. While these instruments appear simple, misinterpreting their calibration markings can introduce significant volume errors into your data.

One of the most common points of confusion involves the markings TC (To Contain) and TD (To Deliver). Many researchers also encounter the double ring marking, which indicates whether a pipette requires a blow-out step.

Knowing how to read these markings correctly ensures accurate liquid handling and prevents systematic pipetting errors during experiments.

What Is a Serological Pipette?

A serological pipette functions as a graduated tool for transferring millilitre-scale liquid volumes. Laboratory staff typically pair these pipettes with electronic pipette controllers or manual aids to ensure precision and safety, moving away from outdated manual mouth pipetting.

Typical applications include:

• Cell culture media transfer
• Reagent preparation
• Buffer dilution
• Sample mixing in biological experiments

Manufacturers typically produce serological pipettes from high-quality plastic or glass in various sizes, including 1 mL, 5 mL, 10 mL, 25 mL, and 50 mL.

To ensure accuracy, each pipette follows specific calibration standards for precise liquid delivery.

Understanding TC vs TD Calibration

Serological pipettes are calibrated in one of two ways.

TC: To Contain

A TC pipette is calibrated to hold the exact volume indicated on the scale.

If a TC pipette reads 10 mL, it means the pipette contains exactly 10 mL of liquid. However, when the liquid is dispensed, a small amount will remain inside the pipette due to surface tension.

Because of this, TC pipettes are typically used when the liquid is later rinsed out or when the remaining liquid does not affect the final measurement.

TD: To Deliver

A TD pipette is calibrated so that the volume indicated on the scale is the amount delivered when the liquid drains normally.

This means that the calibration already accounts for the small liquid residue left inside the pipette.

Most modern serological pipettes used in laboratories are TD pipettes, which are designed to deliver accurate volumes without rinsing.

The Meaning of the Double Ring Marking

To identify how to use a serological pipette correctly, check for two small rings near the top of the pipette.

This double ring marking indicates that the pipette is a blow-out pipette.

In a blow-out pipette, a small amount of liquid remains in the tip after gravity drainage. To deliver the full calibrated volume, the user must expel the remaining liquid using the pipette controller.

This final step ensures the correct volume is transferred.

The Blow-Out Rule Explained

When using a blow-out pipette, the final step of dispensing requires a short burst of air to remove the remaining liquid from the tip.

This process is called blowing out.

The rule is simple:

• Double ring present: blow out the final drop
• No double ring: do not blow out

Ignoring this rule can affect measurement accuracy.

Why Blowing Out a TD Pipette Can Ruin Accuracy

Some pipettes are designed to deliver their calibrated volume without forcing out the remaining liquid.

If a user blows out a pipette that is not designed for it, extra liquid will be delivered beyond the calibrated volume.

This causes:

• Over-delivery of liquid
• Incorrect reagent concentrations
• Inconsistent experimental conditions

For experiments that depend on accurate reagent volumes, even small deviations can affect results.

This is especially important in procedures such as:

• Cell culture media preparation
• Enzyme reactions
• Buffer dilution
• Quantitative assays

Correct interpretation of pipette markings helps avoid these systematic errors.

Other Markings to Look for on a Serological Pipette

In addition to the double ring marking, serological pipettes usually include other useful indicators.

Volume Scale

The graduations on the pipette indicate the volume of liquid delivered. Some pipettes use reverse scales, where the numbers increase toward the tip to help users track the delivered volume.

Maximum Capacity

The largest number on the pipette indicates its total volume capacity, such as 5 mL or 25 mL.

Colour Coding

Many manufacturers use colour coding to identify pipette size quickly.

Examples include:

Colour coding helps laboratory staff quickly select the correct pipette during routine work.

Practical Tips for Accurate Serological Pipetting

To maintain accuracy when using serological pipettes, researchers should follow a few simple practices.

• Check for the double ring marking before dispensing
• Use the correct pipette size for the required volume
• Keep the pipette vertical while draining
• Allow the liquid to drain naturally before blowing out
• Use compatible pipette controllers for smooth dispensing

   

Following these practices reduces measurement errors and improves consistency across experiments.

Conclusion

Serological pipettes may appear straightforward, but their calibration markings play a critical role in accurate liquid transfer. Understanding the difference between TC and TD pipettes, as well as recognising the double ring blow-out marking, ensures that the correct volume is delivered during laboratory procedures.

Applying the blow-out rule correctly prevents systematic pipetting errors and helps maintain reliable experimental results.

Bio Basic Asia Pacific’s pipettes are manufactured to meet strict laboratory standards and are suitable for a wide range of applications in research and cell culture workflows. If you need assistance selecting the right pipette for your laboratory, our team is ready to help.