Respirometer

Quick Answer: A respirometer is a sealed apparatus that measures the rate of respiration of small organisms, such as germinating seeds or woodlice, from the volume of oxygen they take in. The reading comes from a coloured liquid index moving along a graduated capillary tube.

Principle of the Respirometer

In the simple design, the organisms sit in a boiling tube or small chamber above a layer of soda lime or potassium hydroxide solution, separated from it by a wire gauze platform. A capillary tube with a scale leads from the stopper, and a drop of coloured liquid is introduced into it. As the organisms respire they take in oxygen and release carbon dioxide; the carbon dioxide is absorbed, so the gas volume falls and the liquid index moves towards the organisms. The distance moved in a set time, multiplied by the cross-sectional area of the capillary bore, gives the volume of oxygen used.

Temperature matters more than students expect. A slight change in air temperature expands or contracts the trapped gas and moves the index as much as the organisms do, so the tube is usually stood in a water bath and run beside a control tube holding glass beads of equal volume. Differential versions join the two tubes through a U-tube manometer so that temperature and pressure effects cancel out.

The image file name refers to a simple type, but the design is not described in the listing, so confirm whether the item has a single tube or a control tube, and whether a scale, syringe or three-way tap for resetting the index is included.

Specifications

Care & Handling

• Handle soda lime and potassium hydroxide with gloves and eye protection, and keep the organisms off the absorbent with the gauze platform.
• Let the apparatus stand in the water bath for several minutes before sealing it, so the gas inside reaches bath temperature.
• Grease tapered joints and stoppers lightly to keep the system airtight, and wipe the grease off before washing.
• Return invertebrates unharmed to their habitat after the experiment.

Applications

• Measuring the rate of oxygen uptake of germinating seeds at different temperatures
• Comparing respiration rates of woodlice or maggots with those of seeds
• Estimating the respiratory quotient by running the same organisms with and without the carbon dioxide absorbent

Why Choose LabEquip

Respirometers are ordered by senior secondary schools and colleges that teach respiration rate as a quantitative practical. LabEquip lists this one in the Biology Lab Products range next to the qualitative respiration apparatus, which suits earlier classes; send your practical requirements through the contact page.

Frequently Asked Questions

Why does the liquid index move towards the organisms?

The organisms take in oxygen, and the carbon dioxide they give out is absorbed by the soda lime or potassium hydroxide. The volume of gas in the tube therefore falls and the index is drawn inwards.

How do I convert the distance moved into a volume?

Multiply the distance moved by the cross-sectional area of the capillary, which is pi times the square of the bore radius. Divide by the time taken, and by the mass of the organisms, to express it as a rate.

What is the purpose of the control tube with glass beads?

It experiences the same temperature and pressure changes but contains no living material. Movement in the control is subtracted from the test reading, or cancels out in a differential set-up.

How can the respiratory quotient be found?

Run the organisms once with the absorbent to measure oxygen uptake, then with water in place of the absorbent to measure the net change. The difference gives the carbon dioxide released, and RQ is carbon dioxide released divided by oxygen taken in.

Should I use soda lime or potassium hydroxide?

Both absorb carbon dioxide. Soda lime granules are dry and easier to handle, while potassium hydroxide solution works well but is corrosive and must be kept away from the organisms.

Why must the apparatus stand in a water bath?

Gas volume changes with temperature, so even slight warming from a hand or a sunny window moves the index. A water bath keeps the temperature steady during the readings.

Last Updated: September 2026