Kingwood College Respiratory Program
RSPT 1429: Unit 5: gas regulators & flow meters Lecture
by
Elizabeth Kelley Buzbee A.A.S.,  R.R.T.-N.P.S.,  R.C.P


READ:
Cairo's Equipment page 59-65 Chapter 3
read tutorial on tank duration: file:///G|/programs/respcare/duration.html


1. Define these terms:
a. 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
i. Maintain a constant pressure because the pressure from the tanks will decline with use
ii. 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
b. 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 downstream from the regulator at a constant pressure.
i.     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

 
ii.      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.


iii.     Then the poppet closes and the flow stops

 
iv.     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


v.     The diaphragm position and its regulation of the pressure are determined by the position of the spring.

c.     Indirect action regulators: works the same way, the gas flows differently

d.     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.
e.     Dual stage regulators: the regulator reduces the pressure with two single stage regulators in series so that the pressure goes from highest to a working pressure. This will have two pressure relief valves

f.     Multi-stage regulators: more than two single stage regulators in series will have as many stages as there are pressure relief valves.

g.     Adjustable regulators: Some springs are preset at the factory and others are set by the operator. The regulator in pp. 60 Figure 3-2 is adjustable because the spring is attached to a handle that can be turned to tighten or loosen the tension. 

h.     Preset regulators: will not have the adjustment handle. The tension of each spring will never vary. The regulator in pp. 60 Figure 3-2 is pre-set at the factory.

i.     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.

j.     Pressure gauges: manometers are devices used to measure pressure in line or in tanks.  In medical gases, these are generally calibrated in psig or in kPa.


2. Explain the structure and function of the pressure gauges called the bourdon gauge [page 58 fig 3-4.]
Bourdon gauge: http://www.tpub.com/machines/9c.htm
History of the bourdon gauge: http://inventors.about.com/library/inventors/blbourdon.htm
Regulators:
http://www.gehealthcare.com/usen/suction_oxygen/oxygen_therapy/images/sotgsrgO2_flwgg_us_l.jpg
 

The Bourdon Gauge pressure manometer consists of a coiled hollow tube with a closed end, which straightens out as gas pressure inside the tube rises. The tube is attached to a series of gears which in turn are finally to a dial that points to a pressure on the dial face.



3.     Can the bourdon gauge be used as a flow meter?
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.
a.     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.


b.     The gauge actually measures the back-pressure created by the restricted orifice.



4.     Is the Bourdon gauge an accurate flow meter?
The bourdon gauge is an accurate flow meter as long as there are no restrictions down stream.  If the line downstream from the regulator was to become occluded the pressure would rise. This rise in pressure would straighten out the copper coil and an erroneously higher flow rate would be displayed.

5.     Is the pressure accurate in all conditions?
Bourdon gauges on ventilators are exposed to rapid pressure changes. At a respiratory rate of 12 bpm the gauge is rising and falling every 5 seconds. Bourdon gauges will display less accurate pressures in the face of respiratory rates in excess of 12 bpm. [refer: private conversation with electronic experts]

Over the last few years, bourdon gauges on ventilators are being replaced by digital monometers with a faster response time. "If the digital monometer has been calibrated, it will always be more accurate than any bourdon gauge."

a. 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

b. 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.


6. Explain the structure and the function of the Thorpe Tube.
Regulator with thorp tube flow meter: http://www.westernmedica.com/medicacatalog/pdfs/WG8.pdf
In this device, a variable orifice and a constant pressure will control the flow rate. The 02 flow meter 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.
a.     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.

b.     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.

c.     Is the flow rate accurate in all conditions? No.
i.     Different gases have different densities and these densities will cause inaccuracies. Flow meters are gas-specific. A 02 flow meter will not be used for compressed air or for C02 delivery. If one uses a 02 flow meter to deliver Helox [02/He mixture] the RCP must make conversions for accuracy.
1. Multiply the 02 liter flow by 1.8 to correct for 80% Helox gas or by 1.6 for 70% Helox mixture
ii.     The Thorpe tube must be attached to 50 psig,---if it is not, the flow rate will be inaccurate
d.     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.

e.     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 devise.

i.     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.
Flow meter/regulator 'click style' or flow restrictor: http://www.westernmedica.com/medicacatalog/pdfs/WG6.pdf
Thorpe tube Flow meters  http://www.life-assist.com/flowmeters.html

Trouble shooting the Thorpe tube
           Problem is:                                                                      try this:
The station outlet or the regulator is leaking

You hear a hissing sound around the connection 
& feel cold gas around the connection. 

The flow may or may not enter the flow meter based on the degree of the leak 
 
 
 
 
 
 
 

1. Have you selected the wrong connector for the gas?
a. Check the DISS connections of both flow meter and station outlet
2. Is the quick-connect getting worn?
a. Get a new flow meter
3. Is the station outlet quick-connect getting too old?
a. Call maintenance to replace the station outlet. 
 The flow meter displays inaccurate reading: 

you set it up & check the flow with an Erie test flow meter to see that the flow is off
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

1. Is the flow meter compensated for back pressure?
a. Occlude the outlet and see if the flow meter indicates that the flow is higher than before. If the flow rises erroneously this is an uncompensated flow meter. Change to a compensated flow meter.
2. Is there an obstruction downstream such as an entrainment device or a cool humidifier? 
a. Remove the obstruction
b. Check that the humidifier's inlet and outlet are both patent
c. Change from a low flow to a high flow humidifier
d. Adjust the incoming flow rate based on the entrainment device's specifications.
3. If the flow meter is a Thorp tube, is it up right?
a. Turn it upright to read & adjust the flow rate
4.  Is the flow meter running off 50 psig?
a. Use correct reducing valve/ regulator to get to 50 psig
5. Is the flow meter at sea level?
a. If you are in a un-pressurized air plane cabin, change to a flow-restrictor type of flow meter or bourdon gauge type of flow meter
6. Is it at room temperature?
a. Temperatures where the flow meter are inaccurate would be intolerable for other reasons so that unlikely
7. Is it attached to Helox rather than 02?
a. Calculate the correct flow rate by multiplying the set flow rate by 1.8 to get the accurate flow rate.
the gas is leaking from the tube: 

you feel a leak or you hear a hiss around the flow meter's body

1. the integrity of the tube is disrupted
a. replace the flow meter & send it out for repair
the flow meter is leaking from the nipple adaptor 1. tighten the nipple adaptor
2. check that the nipple adaptor is not cross-threaded

7.    Differentiate between compensated and non-compensated flow meters

1. Compensated Thorpe tube flow meters are compensated for the back pressure which in the case of the bourdon gauge will cause inaccurate readings

2. 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 flow meter 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.

 

3. The placement of the float inside the Thorpe tube will determine whether your flow meter is compensated or non-compensated.
a. An uncompensated flow meter will have a float that is downstream from the valve. This float is subjected to atmospheric pressure
i. 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.
ii. It will rise & record an erroneously high flow rate because the density of the gas under the float has been increased.
b. A compensated flow meter 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.
c. A quick way to remember the compensated flow meter:
 
Wall... ball...valve.... patient



8.    Identify  the advantages of having a compensated flow meter?
a. 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.
i. The compensated flow meter would show that the flow rate is zero
ii. The uncompensated flow meter would display an erroneously high flow rate
b. 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


 15.    How can I tell I have a compensated flow meter?
a.     Read the label on the flow meter
b.     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.
i. The uncompensated flow meter's float will not leap up

16.    Why can you not use an air flow meter for 02 delivery?
you cannot because the DISS connection will not allow you to attach the air flow meter to the 02 outlet
you must not:
i.  because the air flowmeter is dirty and can be a fire hazard as gas enters the regulator
ii.. because Thorpe tubes are calibrated for their gas and the flows will not be accurate.



17.    Differentiate between flow restrictors and flow meters
When some persons go home on 02, you may not trust them to not change the proscribed 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


18.    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 proscribed 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.


List the five types of floats used in Thorpe tubes [

a. ball float: read in the middle
b. non-rotating float: read at the top
c. skirted float: read at the top
d. plumb bob valve: read at the top
e. H type float: read at the top



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