Advice on the UK purchase of equipment, most probably involving either a thermistor, thermocouple or strain gauge and some form of data logger, would be appreciated.
A good answer to this needs a bit more information first.
Is this a stand-alone system or does it need to function with, port into, or communicate with another system?
How much do you care about tidal volume, if at all?
Also, how 'low cost' is low cost?
(The cheapest method if you're adding respiratory frequency measurement to other non-invasive measurements is to monitor respiratory cycles from ECG R-wave height, plethysmography or thoracic impedance signals... )
((OH, I should add: the respiratory signal is very strong in ear PPG signals, if you're measuring those.))
If you need integration with other data, then poligraphy systems used e.g. for sleep disorder study may be a choice - my friends in hospital use Embletta. If you need breathing frequency alone, then either look at some ECG based measures suggested by James (R height, RR variability, presence of diaphragm muscle activity or intracostal muscle activity in the ECG) - then any ECG with direct access to the data would do.
Alternatively you can build a simple customized system (eg Arduino based) with either a thermocouple or thoracic belt or both. Also it is a matter of accuracy: some people have changes in thoracic belt, some in abdominal. A thermistor signal will fail if the person speaks or if he/she breathes using mouth (can you force it in examination protocol?).
@James: thanks for the ear PPG remark, interesting. I've read that thoracic impedance also changes a lot, especially if you place lower pickup electrode(s) close to the diaphragm level.
Yeah, this is how impedance cardiographs pull the respiratory signal. It works well.
I have attached an ear PPG signal - you can very clearly see frequency of the pulse, and the frequency of breathing (the x-axis is still in samples @1khz). Detrended and band-passed, this resp signal compares favorably to other indirect methods.
If you want a once off or intermittent measurement, borrow a stop-watch
If you want to monitor and record, borrow a capnograph and sample from a prong of a nasal oxygen delivery system. Most if not all capnographs have a printer port or digital data output.
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