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RT Corner.net |
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Terms: · Gas regulator: pressure-reducing valve that uses opposing forces on two side of a diaphragm. Regulators are used in piping systems, on cylinders and inside ventilators to: o Maintain a constant pressure because the pressure from the tanks will decline with use o Reduce a high pressure to a lower one. Many regulators reduce the cylinder's 2000 psig to the 50-60 working pressure needed for respiratory care equipment · Direct action regulators: this regulator operates with a spring-loaded diaphragm that opposes the pressure of the gas. The opposition of the force of the spring and the pressure of the gas keeps the pressure down stream from the regulator at a constant pressure. o When the poppet is at rest, it is closed against the door that leads to the higher pressure chamber. The lower pressure chamber starts to empty o Once the pressure in the lower chamber drops below the spring tension, the diaphragm moves the poppet into the high-pressure chamber. This opens the door. Gas flows from the higher pressure chamber into the lower pressure chamber until it equals the spring tension. o Then the poppet closes and the flow stops o To keep the pressure constant: if the pressure upstream drops, the poppet opens to allow gas to enter until the pressure equals the spring tension o The diaphragm position and its regulation of the pressure are determined by the position of the spring. § Indirect action regulators: works the same way, the gas flows differently § Single stage regulators: the regulator reduces the pressure only in one step § One determines how many stages a given regulator has by counting the pressure relief valves. § Dual stage regulators: the regulator reduces the pressure with two single stage regulators in series so that the pressure goes from highest pressure to a working pressure. This will have two pressure relief valves § Multistage regulators: more than two single stage regulators in series will have as many stages as there are pressure relief valves. § Adjustable regulators: Some springs are preset at the factory and others are set by the operator. The regulator is adjustable because the spring is attached to a handle that can be turned to tighten or loosen the tension § Preset regulators: will not have the adjustment handle. The tension of each spring will never vary. § Pressure relief valves: when the pressure in a regulator exceeds the appropriate pressure by 50%, the spring disc opens to vent the chamber to the atmosphere. There is one pressure relief valve per reducing stage. Over-pressure tends to happen when there is trash downstream from the regulator causing decreased flow out of the regulator. § Pressure gauges: manometers are devices used to measure pressure in line or in tanks. They are generally calibrated in psig or in kPa.
Structure and function of the pressure gauges called the bourdon gauge
Bourdon Gauge The Bourdon gauge works on the same principle as that of the snakelike, paper party whistle you get at a New Year party, which straightens when you blow into it. Within the Bourdon gauge is a thin-walled metal tube, somewhat flattened and bent into the form of a C. Attached to its free end is a lever system that magnifies any motion of the free end of the tube. On the fixed end of the gauge is a fitting you thread into a boiler system. As pressure increases within the boiler, it travels through the tube. Like the snakelike paper whistle, the metal tube begins to straighten as the pressure increases inside of it. As the tube straightens, the pointer moves around a dial that indicates the pressure in psi. The Bourdon gauge is a highly accurate but rather delicate instrument. You can easily damage it. In addition, it malfunctions if pressure varies rapidly. This problem was overcome by the development of another type of gauge, the Schrader. The Schrader gauge is not as accurate as the Bourdon, but it is sturdy and suitable for ordinary hydraulic pressure measurements. It is especially suitable for fluctuating loads. In the Schrader gauge, liquid pressure actuates a piston. The pressure moves up a cylinder against the resistance of a spring, carrying a bar or indicator with it over a calibrated scale. The operation of this gauge eliminates the need for cams, gears, levers, and bearings.
In 1849 the Bourdon tube pressure gauge was patented in France by Eugene Bourdon. It is still one of the most widely used instruments for measuring the pressure of liquids and gases of all kinds, including steam, water, and air up to pressures of 100,000 pounds per square inch. Eugene Bourdon founded the Bourdon Sedeme Company to manufacture his invention. The American patent rights to Eugene Bourdon's gauge were bought by Edward Ashcroft in 1852. Ashcroft played an important role in the widespread adoption of steam power in this country. He renamed Bourdon's gauge, the Ashcroft gauge.
· Some bourdon gauges have soft copper coils more responsive to lower pressure changes. If these are attached to a dial face that is calibrated in liter/minute, the pressure changes can be converted into a liter flow. o For it to work properly to display the flow rate, it must have a fixed orifice downstream from the bourdon gauge. § A fixed orifice would allow more gas through it as the pressure rises.
§
The gauge actually measures the back-pressure created by the
restricted orifice.
Is the pressure accurate in all conditions?
Over the last few years, bourdon gauges on ventilators are being replaced by digital manometers with a faster response time. "If the digital manometer has been calibrated, it will always be more accurate than any bourdon gauge." · Does this device work upside down? Yes, there is no problem with changes in gravity like there are with the other common type of flow meter · Could Bourdon gauges work in the international space lab? Yes, as long as the cab was pressurized to 760 mmHg [one atmosphere] & if the space lab were heated to room temperature. Explain the structure and the function of the Thorpe Tube. In this device a variable orifice and a constant pressure will control the flow rate. The 02 flowmeter must be placed downstream from a regulator that has reduced the pressure to 50 psig. At a constant pressure, the gas flow will rise as the orifice opens to allow more gas to enter. The orifice is controlled by a needle valve.
Thorpe Tube · The actual flow rate is measured by the rising of a float. The gas molecules push up on the float until the tube widens enough for them to get around it. As the flow rate increases, the float rises higher. As long as the flow is stronger than gravity the float will stay suspended. · Can this Thorpe tube be used as a flow meter? Yes, if it is at sea level and room temperature and the appropriate gas is used. · Is the flow rate accurate in all conditions? No. · Different gases have different densities and these densities will cause inaccuracies. Flow meters are gas-specific. An 02 flowmeter cannot be used for compressed air or for C02 delivery. If one uses an 02 flowmeter to deliver Heliox [02/He mixture] the RCP must make conversions for accuracy. · Multiply the 02 liter flow by 1.8 to correct for 80% Heliox gas & by 1.6 for 70% Heliox mixture o The Thorpe tube must be attached to 50 psig, if it is not, the flow rate will be inaccurate o Does this work up side down? While the Thorpe tube may not be read when it is turned upside down or on it's side, because the float will move, its flow rate will be constant, but one must turn the Thorpe tube back to upright to read the flow rate. o The Thorpe tube would be a poor choice for the international space lab because it requires gravity to work. While the needle valve actually changes the flow rate, the float would not be responding properly and the flow meter would not be functional as a monitoring device. o One would do better with a bourdon gauge type of flow meter or one would use a flow restrictor rather than a flow meter. The flow restrictor has several adjustable orifices that can be selected by turning a knob. Also see Heliox
Trouble shooting the Thorpe tube
Differentiate between compensated and non-compensated flow meters · Compensated Thorpe tube flow meters are compensated for the back pressure which in the case of the bourdon gauge will cause inaccurate readings · The compensated Thorpe tube will read the accurate flow rate as restriction downstream from it rises. As the flow is restricted, the flowmeter will display lower and lower flows. If there is complete occlusion, there will be a zero displayed. If the resistance is removed, the flow will increase. For example: you place a cool humidifier on the nipple of a flowmeter set at 5 lpm. Because the humidifier's outlet is not patent, it causes a complete occlusion and the flow rate will drop from 5 lpm to zero. As you adjust the flow rate nothing happens; the float stays at the bottom. Once the humidifier is removed, the flow rate rises back to 5 lpm. · The placement of the float inside the Thorpe tube will determine whether your flowmeter is compensated or non-compensated. o An uncompensated flowmeter will have a float that is downstream from the valve. This float is subjected to atmospheric pressure § if there is an obstruction down stream such as attaching an entrainment device onto the flow meter, the back pressure builds up and effects the float. § It will rise & record an erroneously high flow rate because the density of the gas under the float has been increased. o A compensated flowmeter will have a float that is upstream from the valve so that the float will be in contact with the wall pressure rather than the atmospheric pressure. o A quick way to remember the compensated flow meter:
What are the advantages of having a compensated flow meter? § The flow is accurate in the face of an obstruction down stream. Imagine what would happen if your 02 line became occluded or pinched off? Your patient could become hypoxic. o The compensated flowmeter would show that the flow rate is zero o The uncompensated flowmeter would display an erroneously high flow rate § If the ordered flow rate is too high for your device [such as a cool humidifier or an entrainment device], as you increase the flow, the indictor would show that the flow is not increasing as you turn it…this means there is restriction down stream
· Read the label on the flow meter · If there's nothing written, you would attach the flow meter to the wall or the tank. As soon as the inlet is in contact with the high pressure, the float [ball] will leap to the top then fall back to the correct flow. § The uncompensated flow meter's float will not leap up Why can you not use an air flow meter for 02 delivery?
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Because the DISS connection will not allow you to attach the
air flow meter to the 02 outlet · Because the air flowmeter is dirty and can be a fire hazard as gas enters the regulator · Because Thorpe tubes are calibrated for their gas and the flows will not be accurate.
Differentiate between flow restrictors and flow meters When some persons go home on 02, you may not trust them to not change the prescribed liter flow. The flow restrictor can be preset & to get a different flow rate, the home care RCP needs to change the flow restrictor's outlet. Some flow restrictors are operated off 50 psig for the hospital or for cylinders but other flow restrictors use 20 psig pressures such as used with LOX [liquid 02 systems]. Flow restrictors are accurate only at their rated pressures What would be an advantage to a flow restrictor? A patient with chronic hypercapnia cannot change his 02 to a dangerous level where he is no longer breathing off his hypoxic drive. With a flow restrictor, the patient will get only his prescribed liter flow--- not more--- not less Because they are so simple to operate, flow restrictors are getting a lot of use in air transport and these are the flow meters of choice in the international space station's hospital. Five types of floats used in Thorpe tubes · Ball float: read in the middle · Non-rotating float: read at the top · Skirted float: read at the top · Plumb bob valve: read at the top · H type float: read at the top
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