Benedict Roth Spirometer Parts, Functions and Principle

BENEDICT-ROTH-SPIROMETER-parts

Benedict Roth spirometer is used to measure the oxygen consumption for measuring the basal metabolic rate and for measuring the lung’s volumes and capacities. This device can be used for measuring Vital capacity (VC), tidal volume (VT), inspiratory and expiratory reserve volumes (IRV ERV), as well as Forced Expiratory Volume (FEV1). This spirometer cannot measure Residual volume, functional residual capacity, and total lung capacities.

Benedict Roth’s spirometer parts and functions are

1. Inverted cylinder: It is also called Bell. It is a cylinder of 9 liters capacity made of lightweight metal which dips into the water from above. The top of the bell carries a hook to which a chain is attached. The chain passes over a frictionless pulley and has a counterweight and a pen. This pen moves up and down as the volume of air in the bell decreases or increases, thus the bell displacements are recorded on the kymograph.

2. Inner cylinder: it is composed of soda lime that is to remove the carbon dioxide from expired air. The end of the cylinder is attached to the one-way valve so that no gas can exit.

3. Soda lime tower: It is fitted within the spirometer and removes carbon dioxide from the expired air so that one can breathe in and out through the mouthpiece.



4. Outer cylinder:  A double-walled cylindrical chamber containing water is formed with the help of an outer cylinder that maintains an airtight seal.

5. Breathing assembly, i.e. breathing valves with mouthpiece: There are two unidirectional breathing valves, one for inspiration and the other for expiration. These are connected with the help of a Y-shaped piece to a free-breathing valve.  Free-breathing valve is a bore metallic tube having a bi-directional tap that can be turned to allow the subject either to breathe room air or spirometric air.

Benedict Roth Spirometer showing different parts

6. Pulley: Bell is hooked with the chain that passes through the pulley and attached with the writing pen that will make the graph according to a change in volumes in the kymograph.

7. Writing pen: Traditional instruments will have a writing pen that should be filled with ink.

8. Tap: for the drainage of the water.



9. Gas inlet: There is a slot for the gas inlet usually oxygen is passed through that inlet

10. Kymograph:  Graph paper is attached in this segment.  There is an on/off switch and speed regulator that will revolve the kymograph with the different speed marks in the instrument. Change in different volumes is plotted on the graph paper.

Speed chart of Benedict Roth Spirometer

11. Graph paper: it varies according to the model of the Benedict Roth spirometer.

Chart paper: calibration

Y-Axis represents volume and 1 row represents 100ml

The X-axis represents time.

For eg. at the drum speed (kymograph) of one revolution in 24 sec, it will take 2 sec to cover 1 column.

Sample of graph paper used in Benedict Roth spirometer

How does Benedict Roth’s spirometer work?



Benedict Roth spirometer Principle: Bell is dipped into the water for an airtight seal and oxygen gas is inserted from the gas inlet. The bell began to rise and stop inserting the gas until 1/3 of the bell remained in the water.   The top of the bell carries a hook to which a chain is attached. The chain passes over a frictionless pulley and has a counterweight and a pen. During inspiration, the bell moves down and the pen moves upward whereas during expiration the bell moves upward and the pen moves down. This pen moves up and down as the volume of air in the bell decreases or increases, thus the bell displacements are recorded on the kymograph.

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