Sergio Quintana T., Ricardo Damián Z., José Castillo H., Gibrán Mejía T.
Laboratorio de Electrónica, CCADET UNAM.
In this work, we present the development of a final prototype of flow spirometer, made with low-cost
devices and easy acquisition in the national market for its easy maintenance and fixing. The flow
spirometer is an equipment that measure the quantity of flow and air volume expired by a patient. It
consists of a sense stage composed by a pneumotachograph and a differential pressure sensor, a
signal conditioner stage composed by an instrumentation amplifier and a low pass filter. It also gets
a voltage integration stage, and a coupling stage to an acquisition system based in a PIC16F877
microcontroller which processes and sends the signal to a LCD display or to a Personal Computer
for its storage, analysis or transmition.
Respiration is the process by which gas is exchanged across cell membranes in all living systems.
At the cellular level, oxygen enters the cell and carbon dioxide is excreted. This process occurs
even in dormant systems such as seeds. In human beings, the lung transfers O2 from the ambient
air to the blood and exhaust CO2 into the atmosphere. The blood in turn carries O2 to and CO2 from
In this process, contraction of respiratory muscles such as the diaphragm and intercostal muscles
between the ribs expands the thorax, creating a negative pressure in the lung, and drawing in
oxygen-rich air. The alveoli exchange O2 for CO2 in the blood flowing into the lung. The output blood
then stimulates CO2-sensittive cells called CO2 receptors in the arteries near the carotid sinus.
These cells, along with stretch receptors in the respiratory muscles, send out nerve impulses to the
medulla oblongata region of the brain stem. The output from the brain stem is fed back to the
respiratory muscles. This controls the breathing rate. Measurements of blood partial pressure of
CO2 called P
CO2, or partial pressure of O2, called P
O2, show that the respiration rate is controlled by
these factors. An increase in P
CO2 increases the breathing rate, as illustrated in Figure 1.
CO2 is a waste product of respiration that must be swept away as it builds up in the lung. On the
other hand, as p
O2 increases, the breathing rate slows down, as indicated in the figure. In this case,
the demand for oxygen-rich fresh air decreases.
Figure 1. The effect of blood P
CO2 and P
O2 on the respiration rate.
In order to diagnose diseases of the lung such as emphysema or bronchitis, clinicians need to
measure air volumes and flow rates. The nominal volumes, illustrated in Figure 2, are measured by
spirometers and plethysmographs, such as are described further on this paper. Commonly
measured volumes are defined as follow:
• TV – Tidal volume: The volume of air exchanged in relaxed breathing, nominally 0.6 liters.
• IRV – Inspiratory reserve volume: The additional air one can inhale with maximum inspiratory
effort above a relaxed inspiration, nominally 3 liters.
• ERV – Expiratory reserve volume: The additional air one can exhale with maximum effort
beyond a relaxed expiration, nominally 1.2 liters.
• VC – Vital capacity: The total volume of air one can exchange with maximum effort, nominally 5
• RV – Residual volume: The air that remains in a normal lung after full expiratory effort,
nominally 1 liter.
• FRC – Functional residual capacity: The amount of air remaining in the lung after a relaxed
expiration, nominally 2.2 liters.
Figure 2. Lung air volumes important to clinical diagnosis.
Parameters that relate to the airway resistance are defined as follows:
• FVC1 – Fractional volume capacity (1 second): The amount of air a subject can force into a
spirometer chamber after taking a maximum inspiratory breath, and exhaling with full force for 1
• FVC2 - Fractional volume capacity (2 seconds): The same as FVC1, except it is measured for 2
• FEF1 – Forced expiratory flow (1 second): The average flow over 1 second.
To evaluate the efficiency and the possible detection of respiratory disorders is needed clinical
testing in order to know the patient's condition. Ventilatory Functional Test (VFT) is a practice that
• Measuring lung capacity or, alternatively, the patient's respiratory impairment
• Diagnose different types of respiratory illnesses
• Assess the patient's response to specific therapies and disorders
• Preoperative diagnosis to determine if the presence of a respiratory illness increases the risk of
The VFT techniques that commonly are used are Spirometry and Plethysmography.
Respiratory function we can consider as a physical event in two stages, inhalation (fresh air to the
lungs) and exhalation (hot air out the lungs). Generally, when a doctor wants to establish whether or
not a patient has a respiratory illness, need to know certain parameters, such as the pressure
difference between the environment and the lung, and thanks to this pressure difference, the lung
can make the exchange of gases, the flow of exchange during the respiratory process, and the
volume of air entering and leaving the lungs. To measure these parameters during the exhalation
process, we designed a Flow Spirometer, which is an instrument that measures the instantaneous
change in volume and flow of air entering the lungs during ventilation, across the trace and/or a
register of volume-time and flow-volume of respiration .
Flow spirometers obtained directly ventilatory flow and air volume is obtained by electronic
integration of flow .
American Thoracic Society (ATS) in its recommendation suggests standardized spirometry  the
basic structure of the spirometers. The block diagram of the architecture of a spirometer flow is
shown in Figure 3. It consists of four stages divided as follows: sensing stage, signal conditioning
stage, voltage integration stage and signal processing.
Figure 3. Block diagram of the architecture of Flow Spirometer
2.1 Sensing Stage
The sensing stage is formed by a flow transducer type pneumotachograph and differential pressure
Depending on the mechanical principle used, different pneumotachograph types, such as turbine or
thermal gradient, but the most widely used, for its practicality and low cost are the differential
pressure pneumotachograph .
In the differential pressure pneumotachograph, exhaled gas passes through a mesh whose
resistance generates a pressure difference that is proportional to the flow F of gas passing through
it, as shown in Figure 4.
Figure 4. Differential pressure pneumotachograph
This proportional relationship is given according to the following expression:
Where K1 is the proportionality constant of the pneumotachograph.
This pressure difference is measured by a differential pressure sensor type NPC-1210 (Figure 5),
which generates an electrical signal output from which the assets of flow. This sensor is a "strain-
gauge," and its output a voltage signal, that is proportional to the differential pressure between its
inlets, according to the following expression:
PKVsensor Δ= 2
Where K2 is the proportionality constant of the differential pressure sensor
Figure 5. Differential pressure sensor type NPC-1210
2.2 Signal Conditioning Stage
Because the pressure differences are very low and hence the output voltage is very small, we need
a signal conditioning stage in which it uses a type AD620 instrumentation amplifier whose main
features are to have a high input impedance and a high rejection of common signs, and a low-pass
filter Butterworth 2nd order with a cutoff frequency of 5 Hz. The figure 6, show the schematic
diagram of this stage.
Figure 6. Schematic diagram of signal conditioning stage.
On the other hand, for a fixed gain amplifier circuit in the AD, and substituting (1) and (2), we have:
Where 12 KKAK D=
The conclusion of this result is that the voltage VF is proportional to air flow F that passes through
the mouthpiece of the pneumotachograph.
Figure 7. (a) Flow through the pneumotachograph. (b) Voltage out of the differential amplifier. (c)
Voltage out of the integrator.
2.3 Voltage Integration Stage
To measure the volume exhaled by a patient through the flow spirometer, we must follow the
following methodology: First, we close the reset switch in Figure 8 of the voltage integrator circuit for
a few moments to have zero initial conditions, then the capacitor C1 is downloaded, which will have
an output voltage VOUT zero. After that prompted the patient to exhale through the mouthpiece of the
pneumotachograph . The resulting change in ΔΡ generate a voltage VF as a function of time in
proportion to the airflow passing through the pneumotachograph, as shown in Figure 7(b).
The volume of air exhaled by the patient, starting at time t = 0, when the reset switch is activated,
will equal the area under the curve of the VF graph against time, as shown in Figure 7(c).
Mathematically this area is calculated by integration. In the circuit of Figure 8, the output voltage of
integrator circuit, VOUT is proportional to the volume of air exhaled by the patient from the time t = 0
to time t, desired.
After the patient has stopped breathing out, the voltage VOUT remains constant in proportion to the
total volume of air exhaled until the Reset switch is closed again to perform another measurement.
However, when the cycle starts, you open the Reset switch and initiate the exhalation at time t = 0,
the output voltage VOUT of the circuit of Figure 8, is given by:
Substituting equation (3) in this last expression and taking into account the total volume, V0, is:
This last equation means that the output voltage VOUT is proportional to the volume of air that has
passed through the pneumotachograph from the time t = 0 to time t of observation. The flow, F, is a
function of time that could increase, decrease or remain constant, provided it is in one direction.
Figure 8. Final circuit for measuring flow and volume.
2.4 Processing Stage
The instrument has a data acquisition system for display on a LCD screen or transfer to a computer.
The data acquisition system is shown in Figure 9. This system is based on Microchip PIC16F877
microcontroller, which has a 10-bits analog to digital converter and 8 kB FLASH memory for storing
the program .
Figure 9.Data acquisition, processing and data display.
The system receives the signal from the spirometer through a buffer to avoid applying it directly to
the analog-digital converter of the microcontroller. The microcontroller is responsible for generating
the base time of sampling frequency. The conversion result is sent to a 16x2 LCD display or a
personal computer via serial port for storage.
The advantage of using a microcontroller like data acquisition system is its low cost, since the
microcontroller is a cost in the national market about $ 100 pesos. In addition, you can easily adapt
to any computer including laptops. Furthermore, any changes to the programming of the
microcontroller can be done without being removed from the card, because the microcontroller has
the ISP programming option ('In System Programming').
The programming from analogue to digital and data display was developed in C
language using the MPLAB program.
To begin the processing of data in the microprocessor, the integer variables, the
floating point constant values and the resolution which will be the conversion are
defined. Then, to convert A / D, are defined as input ports and
channels where it will convert.
The deployment of data is sent to the display screen of the flow and volume values obtained after
digital A / D conversion thereof. If required to perform another measurement, the integrator is reset
(the switch is closed discharging capacitor, to return it to original condition) to repeat the process.
2.5 Hardware Design
The final schematic design of the flow spirometer is showed in Figure 10.
Figure 10. Final schematic diagram of the flow spirometer.
To design the PCB was made with the PCAD program. Figures 11 (a) and (b) show
the TOP and BOTTOM layers of printed circuit, respectively.
In Figures 12 (a) and (b), show some pictures of the hardware flow
spirometer that was developed.
Figure 11. Print circuit of the flow spirometer; (a) Bottom layer , (b) Top layer.
Figure 12. Final prototype of flow spirometer; (a) Inside instrument, (b) Final instrument.
3. TESTS AND RESULTS
The Department of Biomedical Engineering of the National Institute of Respiratory Diseases, INER,
provided us a calibration syringe with which the tests were conducted to determine and validate the
precision and accuracy of the spirometer developed.
The tests consisted of injecting air at three different speeds on the spirometer, in order to compare
results between turns fast and slow exhalations, as the amount of volume should not depend on the
speed of air intake. Moreover, one of the design criteria established by the American Thoracic
Society indicates that the spirometer should be able to accumulate air for at least 10 seconds. On
this basis, 20 tests were conducted at different times (5, 10, and 15 seconds). The table shows the
values obtained for the three test times, and the figure 14 shows the graph of these tests.
Figure 13. Calibration and measurement tests carried out on the final prototype of the spirometer.
5 [s] 10 [s] 15[s]
1 3.02 2.99 3.02 3.00
2 3.02 2.99 3.02 3.00
3 3.02 3.01 3.01 3.00
4 3.02 3.00 3.00 3.00
5 3.02 3.01 3.02 3.00
6 3.01 3.01 3.01 3.00
7 2.99 3.00 2.99 3.00
8 3.00 2.99 2.99 3.00
9 3.02 2.99 2.99 3.00
10 3.02 2.99 2.99 3.00
11 3.01 2.99 2.99 3.00
12 3.01 3.00 2.98 3.00
13 3.02 3.01 2.99 3.00
14 3.02 3.01 3.00 3.00
15 3.02 3.01 3.02 3.00
16 3.01 3.01 3.01 3.00
17 3.02 3.01 3.02 3.00
18 3.00 3.01 3.02 3.00
19 3.00 3.01 3.01 3.00
20 2.98 3.02 2.99 3.00
Average 3.01 3.00 3.00 3.00
Table 1. Readings taken with the final design Spirometer.
Figure 14. Values recorded with the final prototype. a) Test at 5 seconds; b) Test at 10 seconds;
c) Test at 15 seconds.
Now, with the values in Table 1, we proceed to calculate the accuracy of the instrument and validate
compliance or otherwise of that criterion as established in rules.
For volume, the minimum value recorded was 2.98 [L], and the maximum value was 3.2 [L],
therefore, the precision is ± 20 ml. The rule states that for volume measurements, the accuracy
must be ± 50 ml.
As the flow signal, the spirometer was developed, is capable of measuring values ranging from 0.01
[L / s] to 10 [L / s].
Based on these results, we can see that the criteria of resolution and precision of the standards
established, and the technical and operational recommendations are met satisfactorily in the final
prototype of the flow spirometer that was developed.
With the results obtained in the previous section, we show that the flow spirometer built also meets
the criteria for linearity, precision and accuracy established by the ATS with respect to the
parameters of flow and volume of expired air.
The flow spirometer developed in this work meets the basic measuring parameters of a commercial
spirometer. Due its low cost, it can encourage and promote the use of spirometry in our country as a
method of detecting and monitoring the health status of the airways, facilitating the acquisition of
such equipment to hospitals and national clinical. However, to have an equivalent to a commercial
spirometer, with which it can have a full spirometric diagnosis, requires some improvements that
could be made in later works.
Therefore, if one tries to optimize this design, is necessary to work on the following points: Having a
spirometer-computer interface, using the USB protocol in order to observe the register, in real-time,
of volume-flow graph and flow-time graph of breath. To obtain specialized spirometric parameters,
such as: forced expiratory volume in 1 second (FEV1), forced expiratory flow measured during the
middle half of the Forced Vital Capacity (FEF25-75) and Pico Forced Expiratory Flow (PEF). That
the system could be powered by batteries for to do a portable spirometer. Implement a program of
self-calibration and, offset compensation, since in this work this was done manually, and includes
the normal reference values for compared with measurements made, for to send warnings or alerts
to computer when the results are outside the standard parameters.
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