OxiTop® - Respirometers

A 510 ml measuring bottle is used as standard for BOD determination with OxiTop®.
However, the decisive factor is the sample volume, which is selected based on the expected BOD value.
Typical volumes are:

• 432 ml → low BOD
• 164 ml → medium to high BOD

The selection is made to ensure that there is sufficient oxygen in the system and that the measurement is not limited.

Practical Tip:
The bottle volume always remains the same - only the sample volume is adjusted.

BOD is a biological parameter, not a purely analytical value.

Factors that influence it:

• Microorganisms (biology)
• Nutrient conditions
• Temperature
• Possible inhibitors

Therefore, fluctuations in the range of approximately 10–20% are normal.

Accuracy depends not only on the device, but also significantly on the sample.

• Measurement principle: Pressure measurement (manometric)
• Limitation: Biological variability

There is no “exact” accuracy in the traditional sense - the method is process-dependent.

The methods are fundamentally different:

Therefore, deviations are completely normal due to the differences in the methods.

Recommended methods:

• Calibration tablet (chemical)
• Glucose-glutamic acid standard solution (biological)

These can be used to check both:

• Device function
• And biological activity

This is often due to the AutoTemp feature:

• Measurement starts only after the temperature has stabilized
• Typically takes 1–3 hours

This ensures that the measurement begins correctly.

• OxiTop i:
  • 1 reading per day
• OxiTop IDS:
  • up to 360 readings per measurement

Typical battery life:

• Several years (approx. 3 years in practice)

Yes - definitely in the following cases:

• Wear and tear
• Hardening
• Leaks

Even small leaks can lead to inaccurate measurement results.

Recommendation:

• Mechanical cleaning with a brush
• Diluted acid, if necessary
• No dish soap residue!

Residues can affect the biological process and alter the BOD value.

Common causes:

• Incorrect measurement mode
• Measurement not completed
• No compatible controller (for IDS)

The recommended target concentration is:

• 5 mg ATH per liter of sample

When using WTW NTH600:

• approx. 20 drops per liter of sample

Reason:

• new communication standards (e.g., IDS, Bluetooth)
• different electronics

Therefore, there is no full backward compatibility.

No, ATH is not always required.

• With ATH → Measurement of carbon BOD (C BOD)
• Without ATH → Additional oxygen consumption due to nitrification

This can lead to significantly higher measured values.

In short:
ATH determines whether nitrogen processes are included in the measurement.

Yes - NaOH is absolutely essential for the OxiTop® measurement principle.
Function:

• CO₂ is chemically bound
• This results in a measurable drop in pressure

This is the only way to correctly determine oxygen consumption.

Typical usage:

• 1–3 NaOH pellets per measurement

These are placed in the rubber tube.

This is a normal effect:

• CO₂ reacts with NaOH → water is formed
• Additionally, NaOH absorbs moisture

Therefore, the pellets are often partially or completely dissolved by the end.

The BOD measurement is only standard-compliant at:

• 20 °C ± approx. 1 K

Reasons:

• Microorganisms are highly sensitive to temperature
• The OxiTop measures pressure → which is also temperature-dependent

Important:
Even small temperature deviations lead to measurement errors.

Possible causes:

• Temperature fluctuations in the incubator
• Leaks in the system
• Natural decline in biological activity

Since the system is extremely sensitive to changes in pressure, even small temperature changes have a direct effect on the curve.

An erratic measurement curve is often caused by:

• Opening the incubator
• Unstable temperature conditions
• Inconsistent mixing

Opening the incubator, in particular, affects the pressure and thus the measured values.