# Thorpe tube flowmeter

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A Thorpe tube flowmeter is an instrument used to directly measure the flow rate of a gas in medical instruments. It consists of a connection 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.

A needle valve is a type of valve with a small port and a threaded, needle-shaped plunger. It allows precise regulation of flow, although it is generally only capable of relatively low flow rates.

Pressure measurement is the analysis of an applied force by a fluid on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure and display pressure in an integral unit are called pressure gauges or vacuum gauges. A manometer is a good example, as it uses a column of liquid to both measure and indicate pressure. Likewise the widely used Bourdon gauge is a mechanical device, which both measures and indicates and is probably the best known type of gauge.

A pressure regulator is a control valve that reduces the input pressure of a fluid to a desired value at its output. Regulators are used for gases and liquids, and can be an integral device with an output pressure setting, a restrictor and a sensor all in the one body, or consist of a separate pressure sensor, controller and flow valve.

## 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 anaesthetic machine or anesthesia machine is a medical device used to generate and mix a fresh gas flow of medical gases and inhalational anaesthetic agents for the purpose of inducing and maintaining anaesthesia.

The types can be distinguished by their response when gas starts to flow. 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 on anaesthesia machines, where precise measurement of gas delivery is crucial to the wellbeing of the patient.

A rotameter is a device that measures the volumetric flow rate of fluid in a closed tube.

## Application

Thorpe tube flowmeters are designed for use only on systems not supplying more than 50 PSI ( about 3 bar). The 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. The flowmeter will have a safety Relief valve to relieve excess pressure.

A relief valve or pressure relief valve (PRV) is a type of safety valve used to control or limit the pressure in a system; pressure might otherwise build up and create a process upset, instrument or equipment failure, or fire. The pressure is relieved by allowing the pressurised fluid to flow from an auxiliary passage out of the system. The relief valve is designed or set to open at a predetermined set pressure to protect pressure vessels and other equipment from being subjected to pressures that exceed their design limits. When the set pressure is exceeded, the relief valve becomes the "path of least resistance" as the valve is forced open and a portion of the fluid is diverted through the auxiliary route. The diverted fluid is usually routed through a piping system known as a flare header or relief header to a central, elevated gas flare where it is usually burned and the resulting combustion gases are released to the atmosphere. As the fluid is diverted, the pressure inside the vessel will stop rising. Once it reaches the valve's reseating pressure, the valve will close. The blowdown is usually stated as a percentage of set pressure and refers to how much the pressure needs to drop before the valve reseats. The blowdown can vary from roughly 2–20%, and some valves have adjustable blowdowns.

Inaccurate flow readings may occur if the device is damaged, or contaminated with water or debris. Flowmeters are only calibrated for a specified gas and will not directly read accurately on gases of different density. Changes in pressure or temperature will also affect the accuracy, and may be significant in air transport, or cities at high altitudes. Flowmeters are only calibrated in the vertical position, and for flow rates less than 15 litres per minute.

## Related Research Articles

Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways. The common types of flowmeters that find industrial application can be listed as below: a) Obstruction type(differential pressure or variable area) b) Inferential(turbine type) c)electromagnetic d)Positive-displacement flow meters accumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow. e)fluid dynamic(vortex shedding) f)Anemometer g)Ultrasonic H)Mass flowmeter(Coriolis).

A float chamber is a device for automatically regulating the supply of a liquid to a system. It is most typically found in the carburettor of an internal combustion engine, where it automatically meters the fuel supply to the engine. However, this arrangement is found in many automatic liquid systems, for example the cistern of most toilets could be said to be a type of float chamber.

An orifice is any opening, mouth, hole or vent, as in a pipe, a plate, or a body

A diving regulator is a pressure regulator that reduces pressurized breathing gas to ambient pressure and delivers it to the diver. The gas may be air or one of a variety of specially blended breathing gases. The gas may be supplied from a scuba cylinder carried by the diver or via a hose from a compressor or high-pressure storage cylinders at the surface in surface-supplied diving. A gas pressure regulator has one or more valves in series which reduce pressure from the source, and use the downstream pressure as feedback to control the rate of flow and thereby the delivered pressure, lowering the pressure at each stage.

The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of a pipe. The Venturi effect is named after Giovanni Battista Venturi (1746–1822), an Italian physicist.

A gas meter is a specialized flow meter, used to measure the volume of fuel gases such as natural gas and liquefied petroleum gas. Gas meters are used at residential, commercial, and industrial buildings that consume fuel gas supplied by a gas utility. Gases are more difficult to measure than liquids, because measured volumes are highly affected by temperature and pressure. Gas meters measure a defined volume, regardless of the pressurized quantity or quality of the gas flowing through the meter. Temperature, pressure, and heating value compensation must be made to measure actual amount and value of gas moving through a meter.

The nasal cannula (NC) is a device used to deliver supplemental oxygen or increased airflow to a patient or person in need of respiratory help. This device consists of a lightweight tube which on one end splits into two prongs which are placed in the nostrils and from which a mixture of air and oxygen flows. The other end of the tube is connected to an oxygen supply such as a portable oxygen generator, or a wall connection in a hospital via a flowmeter. The cannula is generally attached to the patient by way of the tube hooking around the patient's ears or by elastic head band. The earliest, and most widely used form of adult nasal cannula carries 1–5 litres of oxygen per minute.

Choked flow is a compressible flow effect. The parameter that becomes "choked" or "limited" is the fluid velocity.

A thermal expansion valve or thermostatic expansion valve is a component in refrigeration and air conditioning systems that controls the amount of refrigerant released into the evaporator thereby keeping superheat, that is, the difference between the current refrigerant temperature and its saturation temperature at the current pressure, at a stable value, ensuring that the only phase in which the refrigerant leaves the evaporator is vapor, and, at the same time, supplying the evaporator's coils with the optimal amount of liquid refrigerant to achieve the optimal heat exchange rate allowed by that evaporator. Thermal expansion valves are often referred to generically as "metering devices".

The term separator in oilfield terminology designates a pressure vessel used for separating well fluids produced from oil and gas wells into gaseous and liquid components. A separator for petroleum production is a large vessel designed to separate production fluids into their constituent components of oil, gas and water. A separating vessel may be referred to in the following ways: Oil and gas separator, Separator, Stage separator, Trap, Knockout vessel, Flash chamber, Expansion separator or expansion vessel, Scrubber, Filter. These separating vessels are normally used on a producing lease or platform near the wellhead, manifold, or tank battery to separate fluids produced from oil and gas wells into oil and gas or liquid and gas. An oil and gas separator generally includes the following essential components and features:

A positive displacement meter is a type of flow meter that requires fluid to mechanically displace components in the meter in order for flow measurement. Positive displacement (PD) flow meters measure the volumetric flow rate of a moving fluid or gas by dividing the media into fixed, metered volumes. A basic analogy would be holding a bucket below a tap, filling it to a set level, then quickly replacing it with another bucket and timing the rate at which the buckets are filled. With appropriate pressure and temperature compensation, the mass flow rate can be accurately determined.

Custody Transfer in the oil and gas industry refers to the transactions involving transporting physical substance from one operator to another. This includes the transferring of raw and refined petroleum between tanks and tankers; tankers and ships and other transactions. Custody transfer in fluid measurement is defined as a metering point (location) where the fluid is being measured for sale from one party to another. During custody transfer, accuracy is of great importance to both the company delivering the material and the eventual recipient, when transferring a material.

A pressure carburetor is a type of fuel metering system manufactured by the Bendix Corporation for piston aircraft engines, starting in the 1940s. It is recognized as an early type of throttle-body fuel injection and was developed to prevent fuel starvation during inverted flight.

Of the three types of carburetors used on large, high-performance aircraft engines manufactured in the United States during World War II, the Bendix-Stromberg pressure carburetor was the one most commonly found. The other two carburetor types were manufactured by Chandler Groves and Chandler Evans Control Systems (CECO). Both of these types of carburetors had a relatively large number of internal parts, and in the case of the Holley Carburetor, there were complications in its "variable venturi" design.

Station Outlets (US) or Terminal Units consist of an outlet port with color-coded faceplate attached to a medical gas supply line, and primary and secondary check valves which open and close automatically upon use and disengagement from the system. In most jurisdictions, Station Outlets are required to be equipped with safety systems that prevent cross-connection errors, such as connecting a Medical air flowmeter to an Oxygen port. Hospitals mainly use DISS connections, though a wide variety of quick-connect adapters, based on male strikers and female outlet ports, are still in use today. These latter types are easier to connect, but are more prone to leaks and failure.

The Dräger Ray is a semi-closed circuit recreational diving rebreather designed to use standard nitrox breathing gas mixtures.

Work of breathing (WOB) is the energy expended to inhale and exhale a breathing gas. It is usually expressed as work per unit volume, for example, joules/litre, or as a work rate (power), such as joules/min or equivalent units, as it is not particularly useful without a reference to volume or time. It can be calculated in terms of the pulmonary pressure multiplied by the change in pulmonary volume, or in terms of the oxygen consumption attributable to breathing. In a normal resting state the work of breathing constitutes about 5% of the total body oxygen consumption. It can increase considerably due to illness or constraints on gas flow imposed by breathing apparatus, ambient pressure, or breathing gas composition.

## 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).