Medical City · Device Lab 53 of 69
Spirometer
40 mm in radius and 40 mm long, held above the dew point by a small heater. Gas passing the bundle produces a pressure drop across it that is proportional to the flow, because each capillary is running laminar; two small-bore lines carry that difference back to a differential transducer, and the flow signal is integrated in software to produce a volume. The head sits on a handle at mouth height with a one-way barrier filter and a disposable mouthpiece in front of it.
- 10 parts
- 2 configurations in the Device Lab
- Also called: Spirometry
Configurations
The versions in the Device Lab.
Flow-sensing pneumotachograph head, integrated to volume
40 mm in radius and 40 mm long, held above the dew point by a small heater. Gas passing the bundle produces a pressure drop across it that is proportional to the flow, because each capillary is running laminar; two small-bore lines carry that difference back to a differential transducer, and the flow signal is integrated in software to produce a volume. The head sits on a handle at mouth height with a one-way barrier filter and a disposable mouthpiece in front of it.
Volume-displacement rolling-seal spirometer, differentiated to flow
A horizontal cylinder 180 mm across and 320 mm of stroke, closed by a rolling diaphragm seal on a light piston, so that gas entering the chamber moves the piston without any sliding seal to leak past. 636 mL a count. Position is sampled at 100 Hz and differentiated in software to produce a flow.
How it works
Explore it in the Device Lab.
Two gains whose product is exactly one
Leave every instrument alone and read two numbers off the bench error board: the integrator gain and the differentiator gain, at a tenth of a hertz, at one hertz, at five and at fifty. Multiply the two together at each frequency before reading on.
A constant, integrated, with no ceiling
This experiment needs the configuration that measures flow. Set the zero offset to the declared default of ten millilitres a second and read what it represents as a flow. Then read the volume error it becomes at one second, at six, at fifteen, and across the whole session.
A count, differentiated, multiplied by a hundred
Now the configuration that measures volume. Read the volume worth of one encoder count, which is the drawn seal area times the declared pitch and comes to six hundred and thirty-six microlitres. Then read what one count of jitter between two consecutive samples represents as a flow.
A correction of ten per cent that nothing displays
Read the correction factor on the BTPS board at twenty degrees, then at twenty-four, then at thirty-four, then at thirty-seven. Write the four figures down before reading on.
Infection control, in centimetres of water
This experiment is about a consumable and it is the only one here that is about the patient rather than the arithmetic. Read the device total resistance with the barrier filter fitted and with it removed, then read the pressure the filter consumes at the declared peak flow.
The parts
Part by part.
Disposable mouthpiece and the lip seal
A short flanged tube of moulded plastic, twenty-five millimetres in the bore and thirty-five long, with a rim the lips close on and a flange the teeth sit behind. It is thrown away after one patient.
One-way barrier filter and the resistance it adds
A flat cylindrical cartridge about forty-one millimetres across, with a pleated medium inside it, a coloured rim at each end and an arrow moulded on the side. It is fitted between the mouthpiece and the machine, it is changed for every patient, and it is the largest single resistance anywhere in the breathing path.
The volume between the lips and the sensing element
Not a part. The enclosed gas inside the mouthpiece, the filter housing and the adapter, added together: forty-eight point nine one millilitres.
Nose clip and the leak it exists to close
A sprung wire with two soft pads on it, pinching the nose closed. It costs almost nothing, it is the least technical object in the room, and it is the only thing preventing a parallel path around the entire instrument.
Ambient temperature, pressure and humidity sensing
A small module on the trolley top with a grille and three sensing elements behind it: a thermistor, a barometric element and a humidity element. It reports the state of the air in the room, and it is the only instrument in the room that measures the room.
Bench BTPS board and the conditions nobody breathed at
A board on a stand carrying two bars, a dial with a needle that moves, and a screen. It exists because the largest single number in this instrument is applied invisibly in software, and it says so on itself: no instrument of either class shows you this factor.
Signal conditioning, anti-alias filtering and the sample rate
A board inside the host enclosure carrying an instrumentation amplifier, a handful of capacitors that form a low-pass filter, and the clock that decides when the converter is read. The last of those three is the one that turns up in the arithmetic.
Analogue-to-digital converter and its least significant bit
A chip a centimetre and a half square with a voltage reference beside it. It turns a continuous voltage into a whole number, and the size of one step in that whole number is the finest distinction the instrument can make about anything.
Host computer and the software that owns the conversion
A small enclosure on the middle shelf with a power key, a lamp and a few ports. There is nothing remarkable inside it.
Display, the flow-volume loop and the volume-time curve
A flat panel on a mast, angled towards the operator rather than the patient. It carries two plots and a few numbers, and both plots are drawn from the same pair of converted signals, so neither of them is a measurement in its own right.
Model notes
Real models, reviewed.
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