Blenders & Flowmeters

Types & accuracy of blenders: Mechanical; Electronic

1. Sechrist blender + dual flow meter

a) Blender

i) Mechanical dial

ii) Air/oxygen

iii) Receives air & O2 operating satisfactorily with inlet pressures ranging from 30 - 70 psi

iv) If the pressures are > 25 psi of each other an alarm occurs

v) Bleed flow 7.5 to 9.5 lpm @ 15 lpm flow [bleed flow is needed to keep the FiO2 accurate at very low flows]

b) Flowmeter

i) stage: 0 - 10 l/min; 0 - 1 l/min

ii) Ball bobbin

iii) Poor resolution at 0 - 2 l/min

2. Bird Low flow microblender

a) FiO₂ 0.21 — 1.0

b) Output range: 0 — 30 LPM

c) Gas supply pressure: 30 — 75 PSI

i) (Air & oxygen must be between 20 PSI)

ii) Accuracy: FiO₂ will remain constant ± 1%

Electronic versus mechanical blenders

1. Electronic [eg Sarns 9000]

a) Design

i) Is a monitoring device, not a setting device?

b) May offer

i) Better alarm states

ii) Better recording

iii) Higher resolution

iv) Feedback control

v) But dependent on power supply

Mechanical still operates during power failure

?May revert to 100% O2; ?like electronic flowmeter, the electronic component may simply be monitoring flow & % ie may be mechanically adjusting flow & %

Describe & test a flowmeter by underwater collection

· Flowmeters are individually calibrated by the manufacturer against a master flowmeter for each gas

· Each master flowmeter was in turn calibrated with a bubble meter system

· In clinical practice, flowmeter calibration is checked by setting gas flows to produce desired NO₂-O₂ concentrations and checking with a gas analyser the composition of the gas mixture emerging from the gas outlet

· Fill up a bag with gas via a flow meter at a set flow rate and time & then determine volume of gas

· Use of spirometry - mechanical flow meter

Hospital medical gas pressure rating for gas flow meters

Piped oxygen/air at 60 PSI

Describe how mechanical blenders use carrier gases for mixing & alarms

1.

Mixing [Needle valve design]

a) The ventilating gas used is a mixture of 100% oxygen and room air, allowing alteration in FiO2

b) The mechanical blender mixes the two gas sources (oxygen & air) to achieve FiO2 values between 1.0 (100% oxygen) & 0.21 (100% room air)

c) Screwing in the knob reduces the diameter of the air entry port but simultaneously increases the diameter of the oxygen entry port thereby increasing the percentage contribution of oxygen to the final gas mixture

d) Formula used:

ii) Q_oxy: flow of oxygen

iii) Q_air: flow of air

2. REED alarm

a) Alarm is activated when there is a differential pressure of 20 PSI between the oxygen & air

b) Should both gas pressures simultaneously increase or decrease & a 20 PSI differential is not seen, there will be no audible alarm

c) Once a 20 PSI gas differential is seen by the poppet, the higher gas pressure will overcome the spring and pressure at its opposite end thus creating a path for the gas to flow into the alarm channel creating an audible alarm

d) The FiO₂ during the alarm state may vary from 0.21 to 1.0 depending on which gas supply is failing ie the blender delivers only the gas available at the higher inlet pressure

Filters required for blenders & why they are used

1. Filters in Regulator

a) System is protected from dust & grit by 3 filters

i) Sintered metal filter in inlet nipple

ii) Glass fibre filter inside body of regulator

iii) Sintered metal filter in encapsulated seat assembly

2. Mechanical blender

a) Each gas inlet connector (O₂ & Air) incorporates a 30 micron particulate filter

b) Blender must be operated with clean & dry medical grade gases. Water, oil or particulate matter may cause defective operation (occlude valves etc)

· Looks like a coarse brass filter

How to trouble shoot problems with blenders: gas flow; FiO₂ inaccuracies

· Use of a oxygen monitor distal to the blender; relatively inexpensive device that is used to validate blender performance. Commonly emits an audible alarm during a low oxygen gas scenario

· Note that for the Bird mechanical blender, 0.21 room air may be delivered during an oxygen gas supply failure

Problem	Potential Cause	Action
Discrepancy between blender & analyser	Analyser out of calibration	Calibrate analyser
	Blender out of calibration	Send to service department
	Gas supply failure	Correct gas supply
	Air entrained into circuit	Correct
	Blender contaminated by poor quality compressed gas	Send to service department
Alarm sounding	Inlet pressure discrepancy > 20 PSI	Correct pressure difference
	Reed alarm module not correctly calibrated	Send to service department
	Alarm module malfunction due to inlet gas contamination	Correct contamination problem & send to service department
Blender only accurate when inlet pressures are exactly equal	Both air & oxygen sources are below minimum operating inlet pressures of 30 PSI	Correct low pressure condition

1. Testing overall accuracy (Sechrist)

a) Both gas supply pressures to 50 psi

b) Flow set at 15 l/min

c) Compare O2 analyser readings to mixer settings of 0.21, 0.4, 0.6, 0.8, 1.0

d) Accuracy ± 3%

2. Accuracy with varying inlet pressures (Sechrist)

a) Select FiO2 of 0.6

b) Vary inlet pressures

i) psi O2 + 60 psi Air

ii) psi O2 + 40 psi Air

c) Note O2 analyser readings [±1%]

3. Alarm Module Function (Sechrist)

a) FiO2 0.6

b) O2 & Air both at 50 psi

c) Reduce air inlet to 24 psi

d) Alarm should sound

e) O2 analyser should remain at 100%

f) Increase air inlet pressure slowly to 50 psi

g) Alarm should turn off

h) Repeat reducing oxygen inlet pressures

i) O2 analyser should indicate 21%

4. Failure of any of above tests

a) Return to biomed dept or manufacturer

Brands of flow meters & blenders: advantages & disadvantages