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General equipment information only, not medical advice. This page does not say what flow a person should receive; that is a decision for a prescriber or the person’s care team.
Short answer
To read an oxygen flow meter, make sure it is upright, then read the number on the scale that lines up with the float: the centre of the ball for a ball float, or the top of the bobbin for a bobbin float, with your eye level and parallel to the float. The scale is in litres per minute (L/min, often written LPM). That is the method WHO and UNICEF describe for the glass-tube type of flow meter (WHO/UNICEF, 2019, section 4.2.4).
The device is usually a Thorpe tube flowmeter, also called a rotameter: a clear upright tube holding a float that rises and falls with gas flow, a control valve, and an outlet port. It is a different part from the oxygen regulator that reduces cylinder pressure, although the two are often mounted together.
What you need
- A Thorpe tube flow meter connected to a suitable gas source. WHO/UNICEF state this type is used only with a 345 kPa (3.45 bar, 50 psig) source, such as a piped terminal unit or a regulator on a high-pressure cylinder.
- A way to keep the flow meter vertical. WHO/UNICEF: “This type of flowmeter must be used in a vertical position.”
- Good light and a position where your eye can sit level with the float.
Steps: how to read the float
- Check that the flow meter is vertical. WHO/UNICEF: “To read accurately, a Thorpe tube must be vertical.”
- Identify the float shape. A ball float is a sphere. A bobbin is a shaped float with a flat or tapered top.
- Find the widest part of the float. WHO/UNICEF say the tube “should be read at the largest diameter of the float”.
- Hold your eye parallel to the float, level with it, not looking up or down at it.
- Read the scale line at the reading point: the middle of a ball, or the top of a bobbin.
- Read the unit printed on the tube. For oxygen it is normally L/min.
Ball float versus bobbin: where to read
| Float type | Read at | Source |
|---|---|---|
| Ball float | The centre (middle) of the ball | WHO/UNICEF 2019, 4.2.4 and Figure 4.2 |
| Bobbin | The top of the bobbin | WHO/UNICEF 2019, 4.2.4 and Figure 4.2 |
The WHO/UNICEF figure shows both float types, each depicting 4 L/min, with the reading point differing between them.
How to read an oxygen flowmeter or an O2 flow meter
“Oxygen flowmeter”, “oxygen flow meter” and “O2 flow meter” all describe the same task, and the method above applies to each. The device name on a label or instructions for use may read “Thorpe tube flowmeter”, “rotameter” or “flowmeter”; use the scale and unit printed on your own meter.
What is LPM in medical terms?
LPM means litres per minute, the volume of gas that passes through the meter each minute. WHO/UNICEF write it as L/min and define it as “litres per minute”. On an oxygen flow meter it is the flow rate of the gas through the tube.
What can go wrong
| Situation | What it can do to the reading | Source |
|---|---|---|
| Flow meter not vertical | Reading may be inaccurate; a non-vertical meter is listed as a cause of an inaccurate reading | WHO/UNICEF 2019, 4.2.4, Table 4.3 |
| Reading the wrong point on the float | Ball: read the middle. Bobbin: read the top | WHO/UNICEF 2019, 4.2.4 |
| Eye not parallel to the float | WHO/UNICEF say the eye should be parallel to the float | WHO/UNICEF 2019, 4.2.4 |
| Uncompensated meter with a downstream obstruction (for example kinked tubing) | Back pressure makes the float rise and show an erroneously high flow rate | WHO/UNICEF 2019, section 4.2 |
| Compensated meter with a downstream obstruction | The displayed flow falls as flow is restricted, down to zero on complete blockage | WHO/UNICEF 2019, section 4.2 |
| Float not moving although oxygen is flowing | WHO/UNICEF list a leak too small to hear, or a faulty flow meter | WHO/UNICEF 2019, Table 4.3 |
| No flow at all | WHO/UNICEF list a closed knob or a closed cylinder valve | WHO/UNICEF 2019, Table 4.3 |
Compensated and uncompensated flow meters
In a compensated flow meter the float sits upstream of the control valve, at source pressure, so it stays accurate when something downstream is obstructed. WHO/UNICEF note it is preferred because it can show when the system is delivering insufficient oxygen if the line becomes obstructed or pinched. In an uncompensated flow meter the float is downstream of the valve at atmospheric pressure, so back pressure makes it over-read. US regulations treat the two as separate devices: 21 CFR 868.2320 (uncompensated Thorpe tube flowmeter, calibrated when the outlet is open to the atmosphere) and 21 CFR 868.2340 (compensated Thorpe tube flowmeter). Both are Class I, exempt from premarket notification subject to the limits in 21 CFR 868.9 (Cornell LII copies of the CFR, checked 2026-11-01).
WHO/UNICEF also note that the float of a pressure-compensated meter will “jump” when first connected to a source and the tube is pressurised.
When to get professional help
- If the float does not move while gas is flowing, WHO/UNICEF point to a leak or a faulty flow meter. Ask the equipment owner or supplier to check it.
- Calibration checks are done by a trained biomedical engineer or technician using an oxygen flow analyser (WHO/UNICEF 2019, Table 4.3).
- For any question about how much oxygen a person should receive, contact the prescriber or care team. This page cannot answer it.
FAQ
How do you read an oxygen flowmeter?
Keep it upright, put your eye parallel to the float, and read the scale at the middle of a ball or the top of a bobbin. The value is in L/min (WHO/UNICEF 2019).
What is LPM in medical terms?
Litres per minute, the flow rate of the gas through the meter. WHO/UNICEF write it as L/min.
Is an oxygen flow meter the same as a regulator?
They are different parts. The regulator reduces source pressure; the Thorpe tube flow meter indicates the flow. See the oxygen regulator guide.
Does it matter which way the flow meter sits?
Yes. A Thorpe tube flow meter must be vertical to read accurately (WHO/UNICEF 2019).
Related equipment
See oxygen tubing, nasal cannulas, oxygen concentrators and other types of flowmeter.
Sources
- WHO/UNICEF, Technical specifications and guidance for oxygen therapy devices (WHO medical device technical series, 2019), sections 4.1-4.2 and Table 4.3. Checked 2026-11-01: https://nhsrcindia.org/sites/default/files/2021-05/WHO-UNICEF-TECHNICAL-SPECIFICATIONS-AND-GUIDANCE-FOR-OXYGEN-THERAPY-DEVICES.pdf (PDF copy of the WHO publication).
- 21 CFR 868.2320, Uncompensated Thorpe tube flowmeter. Checked 2026-11-01: https://www.law.cornell.edu/cfr/text/21/868.2320.
- 21 CFR 868.2340, Compensated Thorpe tube flowmeter. Checked 2026-11-01: https://www.law.cornell.edu/cfr/text/21/868.2340.
