Oxygen sensors for Medical application

Oxygen sensors for Medical Applications

 

Introduction:

Oxygen Sensors are used to measure and monitor the level of oxygen concentration, (i.e) oxygen which is inhaled and exhaled by the patient who is connected to ventilator or anesthesia machine.

Oxygen Sensor incorporated in a respiratory gas monitors (RGM) is used to measure oxygen concentration (or) partial pressure of oxygen in a breathing gas mixture.

Oxygen sensor is also called as FiO2 sensors or O2 Cells, The fraction of inspired oxygen (FiO2) is the concentration of oxygen in the gas mixture. The gas mixture in atmospheric room air has a fraction of inspired oxygen of 21%, meaning that the concentration of oxygen at room air is 21%.

Why RGM’s need Oxygen sensor?

All Respiratory gas monitoring are designed to move a combined mixture of air and oxygen into and out of a patient’s lungs to either assist in breathing or, in some cases, do the mechanical breathing for a patient who is breathing insufficiently or is physically unable to breath.

During ventilation, the mixture of breathing gas needs to be measured precisely.In particular, the measurement of oxygen during ventilation is most critical because of its importance in metabolism.

In this oxygen sensors are implemented to control and detect the calculated oxygen supply to the patient.

The major requirement is to provide highly accurate measurement of the oxygen content of the breathing gas.

 

Different mechanisms used in medical oxygen sensor

1. Electrochemical sensors
2. Ultrasonic Sensors

 

1.Electrochemical oxygen sensors

Electrochemical oxygen sensing element are primarily employed in the activity of measuring the oxygen level in ambient air .These sensors are incorporated in the RGM machines to measure the concentration of O2 supply. They leave the chemical change with in the sensing element that create an electrical output proportional to the oxygen level. An electrochemical sensor converts chemical energy into electrical energy through the processes of oxidation and reduction. It provides an electrical output to the equipment that is directly proportional to the percentage of oxygen in the Cathode and an anode. The oxygen sensor acts as a current source, so the voltage measurement is carried out through load resistor. The output current from the oxygen sensor is proportional to the rate of oxygen consumed by the oxygen sensor.

The output of an electrochemical sensor is a current most commonly in microamperes (µA).This current is generated when electrons pass from the oxidation process at the anode and the diffusion of ions into the electrolyte solution from the  process of oxygen reduction at the cathode.

 

 

Fig: 1 Mechanism Illustration

Important elements in oxygen sensor:

Plastic body encapsulate the following parts

Anode (-),

Cathode (+),

Base electrolyte,

PCB , Resistors, Thermistor

For controlled oxygen supply electrochemical oxygen sensors are used, these are relatively low cost and dependable, machines detects 0 to 100% of oxygen concentrations.

Shelf Life:

Storage life or “shelf life” is important for each electrochemical oxygen sensors. These oxygen sensors work by a chemical reaction happen inside the sensor. As the sensor is exposed to the atmospheric gas , the chemistry which happen within the sensor  gives off an electrical output (voltage or amperage) that is proportional to the gas level. This means that even if the sensor is not  being used, if it is exposed to air it is still using up the chemicals inside.

Cell reactions are described as follows:

O₂ + 2H₂O + 4e  4OH                       Cathode reaction

2Pb + 4OH  2PbO + 2H₂O +4e           Anode reaction (METAL) (METAL OXIDE) (ELECTRONS)

The overall cell reaction is as follows:

                                   

O2 + 2Pb = 2PbO

                                               

 

Fig: 2  Oxridized sensor

 Under high temperature and low humidity, the electrolytes will evaporate or dry out. So that the chemical reaction with the sensor will slows down to the point where the sensor no longer gives an accurate reading.

 A fresh electrochemical sensors will give a better usage. Most electrochemical oxygen sensors have a stated shelf life of 6 months. This is why it is important to order the oxygen sensor from suppliers who store the sensors in a controlled environment and regularly rotate their stock.

For better sensor life you should only order sensors when you are ready to use them. If you have several gas detection devices in the field using the same sensor, keep a few extra sensors on hand, and mark the delivery date on the packages so the oldest replacement sensor is the next you use. For better usage install the sensor within 4-6 months from the date of manufacture.

Sensor Lifetime:

Oxygen Sensor can have both single and multiple patient use application since it can work without any hindrance for few hundred thousands of oxygen hours which can be used for a single patient or it can be used for multiple patients within that cycle. For better understanding of the device lifetime in oxygen hours has been converted into years. A normal oxygen Sensor has a Service Life time of 24 months in air at 25°C and 60% RH. Regardless of whether or not the oxygen sensor is used, life of the sensor starts deteriorating immediately after its manufacture. As a rough criterion for the sensor life, the periods till the sensor output in the atmospheric at an ordinary pressure and temperature lowers below 70% of the initial output can be defined as the sensor life. The theoretical designed life (life expectancy) of the sensor can be expressed by (Oxygen Concentration %) X (time h), provided the sensor is used under constant condition of 25̊ C temperature with 1 atm pressure. The expected life of a sensor is greatly affected by the amount of anode material and signal output.A higher signal output yields a shorter life because the anode is being consumed at a faster rate. In reality, the expected life span of sensor considers the signal output range.

In general, sensor life is inversely proportional to changes from specified parameters:

• Oxygen concentration
• Atmospheric pressure
• Temperature

The life of OS series Oxygen Sensor is more than few hundred thousands of O2 Hours. The End life of the OS Series is specified as point at which output Voltage is reduced to 30 % from its initial output value.

 Calculated O2 Hours = 100 X 24 X 180

 O2 Hours = 4, 32,000 O2 hours

In general the lifespan of O2 sensor will be few hundred thousand of O2 Hours.

Calibration of oxygen sensor

Based upon the model of the oxygen sensor, the sensor output voltage varies.

To obtain oxygen concentration in percent, the oxygen sensor's output voltage (mv) is multiplied by a conversion factor. The conversion factor is determined by the user during calibration.

To calibrate divide 20.95 percent by the sensor's mV output in open air. Where 20.95 percentage is the conversion factor (% of oxygen supply to the ventilator) which can be set in ventilator by the user.

For example, 20.95 % O2 (% of oxygen supply to the ventilator) / 12 mV (oxygen sensor's output voltage) = 1.746 % O2 / mV. The oxygen sensor’s mV output multiplied by 1.746 will yield the percentage of oxygen present for the current humidity, temperature, and elevation.

Similarly to calculate 100% O2(% of oxygen supply to the ventilator) / 12 mv (Oxygen sensor’s output voltage) = 8.33 % O2 / Mv. The Oxygen sensor Mv outputmultiplied by 8.33 will yield the percentage of oxygen present for the current humidity, temperature , and elevation.

To avoid error reading calibration is done by keeping known value

 

As a result output is calibrated in the form of signal

Fig: 3 Linearity in O2 sensors

Reasons for aging / failure of oxygen sensors:

• Mechanical problems like Leaking of electrolyte - Damage of the sensor
• Improper fixing or assembling of oxygen sensors in the ventilators and anesthesia work stations
• These oxygen sensors does not work when it reaches its shelf-lifetime period when the sensors are used or not used.
• Improper storage.

How can O2 Cell life be improved ?

• For better usage install the sensor within 4-6 months from the date of manufacture.
• Perform a pre use check / calibration of the sensor in the required interval as per the supply device instruction.
• If the sensor falls more than 3 feet give 30 mins timr to warm- up and recalibrate the sensor. if the sensor fails , Kindly discard the sensor.
• Do not immerse the oxygen sensor in water or any other liquid

Manufacturers:

There are very few manufacturers for these type oxygen sensors presently,The authentic few are Analytical instruments U.S.A,Maxtec U.S.A,City technologies U.K,Envitec Germany,Biometric cables India.

2.Ultrasonic oxygen sensors

Ultrasonic oxygen sensor is an economical gas sensor used to detect oxygen concentration in two different gas mixture.

 By using ultrasonic detecting technology, these sensors have very stable performance and there is no need to calibrate the oxygen sensor. It is mainly designed for ventilator and anaesthesia machines oxygen concentration measurement with high flow range.

Compared with electro-chemical oxygen sensors, these sensors have much longer lifespan.

Principle of Ultrasonic Oxygen sensor:

Ultrasonic Concentration detection theory: Binary gas mixture composition has molecular weight difference, when sound travels the speed varies for different gas composition.

Features of Ultrasonic oxygen sensor:

No need calibration

Long lifetime  > 5 years

Compact size easy to install

Gas concentration & flow can be measured together

Advantages of ultrasonic oxygen sensor:

Stable ,

No Calibration,

High accuracy,

Long life time,

To measure concentration and flow together.

Disadvantages:

Only measure binary gases and influenced by moisture

High initial costing

Applications:

▪ Medical ventilator

▪ Respiratory devices

 

Characteristic difference between Electrochemical Oxygen sensor & Ultrasonic Oxygen sensor

Electrochemical Oxygen sensor	Ultrasonic Oxygen sensor
If the Oxygen is dissolved through the diaphragm in an electrolytic solution, in which the base metal (anode) & Noble metal (Cathode) are adjacent to each other, a current proportional to the dissolved oxygen is generated. The amount of o2 passing through the diaphragm is is proportional to the partial pressure of the gas, therefore the concentration of o2 can be determined by measuring the current	Ultrasonic concentration detection theory is used in ultrasonic oxygen sensors.When sound waves passing through Binary gas composition the sound travelling speed varies for each different gas composition due to molecular weight of each gas composition. The travelling speed of the sound wave is converted to electrical signal, that is used to determine the concentration of O2.
Gas concentration measurement only done	Only measure binary gas Concentration and Flow together  which is influenced by moisture
Mechanism used: Electrochemical reaction	Mechanism used: Ultrasonic Concentration detection theory
Least expensive type of sensor.	High Cost when compared to electrochemical sensor
Less accuracy whe compared to other sensor	Quick Response , Stable measurement, high accuracy
Life span is typically measured in months, storing the sensor will not adds the life span	Long life span > or equal to 5 years
As the sensor ages it require frequent calibration	self calibration & no need routine calibration
A challenge for electrochemical sensor is they depend on a chemical process that they are temperature dependent	Full temperature compensation
Applications: Hospital Ventilators,Anaesthesia machines, Hand-held gas analyzers	Applications: Hospital Ventilators,Gas analysis or application involving oxygen concentrators or portable oxygen generators

 

 

Please do write to us for any queries regaring Oxygen sensors to info@biometriccables.com

 

This Article was written by M.Keerthana,Biomedical Engineer,Customer Care Specialist