Thorpe tube flowmeter

A Thorpe tube flowmeter is an instrument used to directly measure the flow rate of a gas. It consists of a connection that may be fitted to a gas source, a needle valve opened and closed by turning an attached dial for control of flow rate, a float resting in a clear tapered tube, and an outlet port. It is primarily used in health care institutions during delivery of medical gases, often in conjunction with other devices such as pressure gauges or pressure reducing valves.

Contents 1 Function 2 Safety 3 See also 4 References

Function

When a driving pressure is applied to the inlet of a Thorpe tube flowmeter, the ball rises in the tapered tube until the flow rate creates an applied pressure on the ball equal to its weight. The tube's shape, that of a slender cone, decreases the pressure behind the ball as it rises. A cylindrical tube would not permit driving pressure to decrease with flow rate, resulting in the ball rising to the top of the tube, and allowing for no variance in readings. The flow rate of a specific gas necessary to cause the float to rise to a given height is precalculated in order to calibrate a tube.

A variety of float shapes may be seen with older Thorpe tube flowmeters, and all floats should be read from the top of the float, except for the ball float, which is read from its center. Floats should rotate in the airstream, and the absence of rotation may indicate faulty readings resulting from the float catching on the tube.

The needle valve may be located proximal or distal to the inlet port; these two types of flowmeter are respectively called 'non-compensated' or 'compensated'.

• The original Thorpe tube flowmeter is the non-compensated type: it works with a fixed orifice and variable pressure. The non-compensated type is more accurate for low flow rates, such as are used in neonatal units, laboratory experiments, or anaesthetic machines.

• Compensated flowmeters work with a variable orifice and fixed pressure. They read back pressure, and take into account resistance changes downstream from the needle valve. If pressure exceeds 50 psig downstream, flow ceases.

An easy means of distinguishing between the two types, besides reading the labels, is by fitting them onto a gas outlet. In compensated flowmeters, the ball will initially jump as the gas flows through the tube before being released through the needle valve just before the outlet. No result will be observed in the non-compensated flow meter, as the gas will release before reaching the tapered tube containing the float.

Although Thorpe tube flowmeters are very similar in design and function to rotameters, the latter are more accurate. As a result, Thorpe fube flowmeters are often checked against rotameters to ensure their accuracy. Rotameters are also used in series on anaesthesia machines, where precise measurement of gas delivery is crucial to the wellbeing of the patient.

Safety

Thorpe tube flowmeters should be connected to a source pressure not exceeding 50 psig, as their inlet connections generally will not hold past this pressure, and if loosed, they may become dangerous projectiles.

General Safety Features:

1. DISS connections are located on both the inlet and outlet ports of the flowmeter.
2. Flowmeters will be labelled for the gas they are specific to. Additional color-coding may be used, for example, O₂ flowmeters may have white and green labels, since white and green are respectively the Canadian and American colours identifying this gas.
3. They possess a safety relief mechanism to relieve excess pressure.

Inaccurate flow readings may occur for several reasons:

1. Mechanical damage to the instrument, resulting from contamination with water or debris, cracking, faulty O-rings, or worn needle valves and seats. Regular comparison of flowmeter flow rates against a rotameter will identify damaged equipment.
2. When flowmeters calibrated for one gas are used with another, as commonly occurs due to budgetary restraints. Recognizing that Actual Flow = Indicated Flow x (Density of Calibrated Gas)^1/2(Density of Gas Being Used)^-1/2 we can solve for the actual flow rate.
3. Changes in pressure or temperature. As we can guess from the gas laws, barometric pressure, and temperature will also affect the density of a gas. The error rate for indicated flow with increases in altitude is 1% for every 1000 ft, and may be significant in air transport, or cities at high altitudes.
4. Readings will be affected by turning the flowmeter sideways or upside-down, since the calibrations depend on the height at which the drag force on the float and its weight are equal.
5. Increasing the flow rate past the 15 LPM (litres per minute) which is the normal maximum flow rate recorded on a flowmeter.

See also

• Flow measurement

References

• Branson, Richard D., et al. 'Respiratory Care Equipment, 2nd Edition'. Cont. Banner et al. pp. 58–62. (Philadelphia: Lippincott Williams & Wilkins, 1999).
• Cairo, J.M., et Pilbeam, Susan. 'Mosby's Respiratory Care Equipment'. Ed. Kelli Chronister et al. pp. 61–62. (St. Louis, Missouri: Mosby Elsevier, 2010).
• Wilkins, Robert L. et al. 'Egan's Fundamentals of Respiratory Care, Ninth Edition'. Ed. Lucy Kester et al. pp. 862–865, 898-899. (St. Louis, Missouri: Mosby Elsevier, 2009).