Thorpe tube flowmeter
Encyclopedia
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
.
A variety of float shapes may be seen with older 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 flowmeters are respectively called 'non-compensated' or 'compensated'. A non-compensated flowmeter is the original type, and works with a fixed orifice and variable pressure. It 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 will take into account resistance changes downstream from the needle valve. If pressure exceeds 50 psig downstream, flow will cease.
An easy means of distinguishing between the two, 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.
General Safety Features:
Inaccurate flow readings may occur for several reasons:
Needle valve
A needle valve is a type of valve having 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.- Construction and operation :...
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
Pressure measurement
Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges....
or pressure reducing valves
Pressure regulator
A pressure regulator is a valve that automatically cuts off the flow of a liquid or gas at a certain pressure. Regulators are used to allow high-pressure fluid supply lines or tanks to be reduced to safe and/or usable pressures for various applications....
.
Function
When a driving pressure is applied to the inlet, 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 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 flowmeters are respectively called 'non-compensated' or 'compensated'. A non-compensated flowmeter is the original type, and works with a fixed orifice and variable pressure. It is more accurate for low flow rates, such as are used in neonatal units, laboratory experiments, or anaesthetic machines
Anaesthetic machine
The anaesthetic machine is used by anaesthesiologists and nurse anaesthetists to support the administration of anaesthesia...
. Compensated flowmeters work with a variable orifice and fixed pressure. They read back pressure, and will take into account resistance changes downstream from the needle valve. If pressure exceeds 50 psig downstream, flow will cease.
An easy means of distinguishing between the two, 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.
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:
- DISS connections are located on both the inlet and outlet ports of the flowmeter.
- 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.
- They possess a safety relief mechanismRelief valveThe relief valve is a type of valve used to control or limit the pressure in a system or vessel which can build up by a process upset, instrument or equipment failure, or fire....
to relieve excess pressure.
Inaccurate flow readings may occur for several reasons:
- Mechanical damage to the instrument, resulting from contamination with water or debris, cracking, faulty O-ringsO-ringAn O-ring, also known as a packing, or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a disc-shaped cross-section, designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface.The O-ring...
, or worn needle valves and seats. Regular comparison of flowmeter flow rates against a rotameterRotameterA rotameter is a device that measures the flow rate of liquid or gas in a closed tube.It belongs to a class of meters called variable area meters, which measure flow rate by allowing the cross-sectional area the fluid travels through to vary, causing some measurable effect.-History:The first...
will identify damaged equipment. - 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.
- Changes in pressure or temperature. As we can guess from the gas lawsGas lawsThe early gas laws were developed at the end of the 18th century, when scientists began to realize that relationships between the pressure, volume and temperature of a sample of gas could be obtained which would hold for all gases...
, 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 transportAir ambulanceAn air ambulance is an aircraft used for emergency medical assistance in situations where either a traditional ambulance cannot reach the scene easily or quickly enough, or the patient needs to be transported over a distance or terrain that makes air transportation the most practical transport....
, or cities at high altitudes. - 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.
- Increasing the flow rate past the 15 LPM (litres per minute) which is the normal maximum flow rate recorded on a flowmeter.