Most of the respiratory measurements can be adequately carried out by the classic water-sealed spirometer . This consists of an upright, water filled cylinder containing an inverted counter weighted bell. Breathing into the bell changes the volume of gases trapped inside, and the change in volume is translated into vertical motion, which is recorded on the moving drum of a Kymograph. The excursion of the bell will be proportional to the tidal volume. For most purposes, the bell has a capacity of the order of 6–8 l. Unless a special light weight bell is provided, the normal spirometer is only capable of responding fully to slow respiratory rates and not to rapid breathing, sometimes encountered after anaesthesia. Also, the frequency response of a spirometer must be adequate for the measurement of the forced expiratory volume. The instrument should have no hysteresis, i.e. the same volume should be reached whether the spirometer is being filled or being emptied to that volume.

As the water-sealed spirometer includes moving masses in the form of the bell and counter- weights, this leads to the usual problems of inertia and possible oscillation of the bell. This can lead to an over-estimation of the expiratory volume. A suggested compensation is by the use of a spirometer bell having a large diameter and which fits closely over the central core of the spiro- meter, so that the area of water covered by the bell is small in relation to that of the water tank. If the spirometer is used for time-dependent parameters, then it must also have a fast response time, with a flat frequency response up to 12 Hz. This requirement applies not only to the spirometer, but also to the recorder used in conjunction with the recording device.

The spirometer is a mechanical integrator, since the input is air flow and the output is volume displacement. An electrical signal proportional to volume displacement can be obtained by using a linear potentiometer connected to the pulley portion of the spirometer. The spirometer is a heavily damped device so that small changes in inspired and expired air volumes are not recorded. The spirometers can be fitted with a linear motion potentiometer, which directly converts spirometer volume changes into an electrical signal. The signal may be used to feed a flow-volume differentiator for the evaluation and recording of data. The response usually is ± 1% to 2 Hz and ± 10% to 10 Hz.

A wedge spirometer consists of two square pans, parallel to each other and hinged along one edge. The first pan is permanently attached to the wedge casting stand and contains a pair of 5 cm inlet tubes. The other pan swings freely along its hinge with respect to the fixed pan. A space existing between the two pans is sealed airtight with vinyl bellows. The bellows is extremely flexible in the direction of pan motion but it offers high resistance to ‘ballooning’ or inward and outward expansion from the spirometer. As a result, when a pressure gradient exists between the interior of the wedge and the atmosphere, there will only be a negligible distortion of the bellows.

As gas enters or leaves the wedge, the moving pan will change position in compensation for this change in volume. The construction of the wedge is such that the moving pan will respond to very slight changes in volume. Under normal conditions, the pressure gradient that exists between the wedge and the atmosphere amounts to only a fraction of a millimetre of water.

Volume and flow signals for the wedge are obtained independently from two linear transducers. The transducers are attached to the fixed frame and are coupled to the edge of the moving pan. One transducer produces a dc signal proportional to displacement (volume), while the other has a dc output proportional to velocity (flow).

The transducer outputs are connected to an electronics unit, which contains the power supply, an amplifier, and the built-in calibration networks.

A pointer attached to the moving pan and a scale affixed to the frame, combine to provide a mechanical read out for determining the approximate volume position of the spirometer. When open to the atmosphere and standing upright, the wedge will empty itself due to the force of gravity acting on the moving member. An adjustable tilt mechanism provides the means for changing the resting point of the moving pan to any desired volume point. An adjustable magnetic stop insures a more highly defined resting position.

Neither the tilt nor the magnetic stop has any noticeable effect on the moving pan position once it is connected to a closed system. This is primarily due to the large surface area of the pans, which serves to convert small pressures into large forces.

Thus, the relatively small forces due to gravity and the magnetic stop are overcome by a negligi- ble rise in pressure in the patient’s lungs. When gravitational return of the moving pan to the resting position is deemed undesirable, the wedge may be turned on its side so that at any point, the pan will be in a state of equilibrium. The wedge may be calibrated with a selector switch, which determines the magnitude of the calibration signal. The volume may be calibrated with a signal corresponding to either 0.5 ml or 5 l. The flow calibration signals for each particular wedge are adjusted, using special fixtures. A volume of one litre is introduced at a certain point and a flow rate of 1 l/s is introduced at another point, with the calibration signals then being adjusted to produce equal signals.

As on conventional spirometers, all standard pulmonary function tests may be performed on the wedge. X-Y recorders featuring high acceleration slew rates may be used in recording flow/ volume loops.

Ultrasonic spirometers depend, for their action on transmitting ultrasound between a pair of trans-ducers and measuring changes in transit time caused by the velocity of the intervening fluid medium (McShane, 1974). They employ piezo-electric transducers and are operated at their characteristic resonant frequency for their highest efficiency. Gas flowmeters generally operate in the range from about 40 to 200 kHz. At frequencies higher than 200 kHz, absorption losses in the gas are very high whereas sounds below 40 kHz are audible and can be irritating.

Ultrasonic spirometers utilize a pair of ultra- sonic transducers mounted on opposite sides of a flow tube . The transducers are capable of both transmitting and receiving ultrasonic pulses.

In conventional ultrasonic flowmeters, pulses are transmitted through the liquid or gas in the flow tube, against and then with the direction of flow.

The velocity of sound, C, does not appear in the final equation. Thus, the output accuracy is unaffected by fluid density, temperature, or viscosity. In gas flow measurements, pulmonary function tubes larger than 3 cm in diameter must be used; the single frequency systems that measure time delay directly must be able to resolve nanoseconds since the total transit delay, t, is usually measured in microseconds. This technique is not easily implemented because of the difficulty in measuring these small time differences.