Another one of those stories about what is truly, a technological marvel: shrinking a gas chromatograph/mass spectrometer down to the size of an iPod, with the target size being that of a matchbox. Designed by MIT engineers, the device which can analyze the air for hazardous gases (and could be adapted for other media like water) is touted as a possible distributed sensor for water supplies to protect us against chemical attacks or in subway systems to warn of terrorist attacks. I think this is bullshit and I’ll explain why after a description of this ingenious device:
Their detector uses gas chromatography and mass spectrometry (GC-MS) to identify gas molecules by their telltale electronic signatures. Current versions of portable GC-MS machines, which take about 15 minutes to produce results, are around 40,000 cubic centimeters, about the size of a full paper grocery bag, and use 10,000 joules of energy.
The analyzer works by breaking gas molecules into ionized fragments, which can be detected by their specific charge (ratio of charge to molecular weight).
Gas molecules are broken apart either by stripping electrons off the molecules, or by bombarding them with electrons stripped from carbon nanotubes. The fragments are then sent through a long, narrow electric field. At the end of the field, the ions’ charges are converted to voltage and measured by an electrometer, yielding the molecules’ distinctive electronic signature. (Science Daily)
Sounds great, right? Especially if it is very sensitive, that is, it is highly accurate in detecting the hazardous material when its there, even in very small amounts. But therein lies the rub. Because to do this it is likely the device will suffer at least a little bit in specificity, that is, it will sound false alarms. It doesn’t take much. Even if it is highly and unbelievably accurate in this regard, say, 99.99999% accurate, there is a big problem.
Let’s do some back of the envelope calculations. Let’s say a single small sensor is 99.99999% specific (i.e., its false alarm rate is only .0000001, one in ten million). This device is said to perform an analysis in about 4 seconds, which is to say it could, with ease, report a reading at least once a minute, something it would need to do if it were to continuously monitor the environment for a terrorist threat. Let’s put it in a water distribution system. That’s 3600 readings a day or about 1.3 million reads a year from that sensor. But a water distribution system is big and you would probably have at least 100 of these sensors in your water supply. So that’s 130 million reads a year. Now most years there will be no terrorist attack on any water supply. There has never been such an attack and there are over 150,000 community water supplies in the US. But even at the rate of one attack a year (an absurdly high frequency) that means the risk for any water supply is one in 150,000 per year. If you insist that some supplies are more likely to be hit, say the top 1000 supplies, it is still a chance of only one in a thousand per year.
But how many false alarms will we have? For a single water supply (a supply that has only one chance in a thousand (at best) of being hit), the number of false alarms will be 13. So year after year this extraordinarily accurate sensor will spew out false alarms at the rate of more than one a month. How long to you think it will be before they are no longer maintained and in fact are just disregarded or disconnected?
These tech stories are great. It may well be that a tiny GC-MS will find an application somewhere but putting them into the subway system or our water supply isn’t likely to be one of them.