On Wed, Jun 22, 2011 at 10:54 PM, Mark Iverson <[email protected]>wrote:
> Abd wrote: > "Basically, the device does some math for you, based on certain > assumptions. Unfortunately, the > assumptions are the very issue here!" > > I don't' think that's correct... Not assumptions. > The instrument does calculations based on scientific laws and uses what > measured variables it does have to calculate different units... For example, > Relative humidity is calculated from Absolute humidity and temperature and > pressure. It sounds like you're just making shit up. The instrument doesn't have a way to measure absolute humidity directly. It measures capacitance, which varies with relative humidity. So, they have to calibrate it using known humidities (usually with different salts that have a known vapor pressure). So, humidity h1 in air corresponds to capacitance c1, and humidity h2 in air corresponds to capacitance c2, and so on. Then they make a graph of capacitance vs humidity, and use the graph to determine an arbitrary humidity from an arbitrary capacitance. Now, if it's calibrated in air, as it is, then the assumption they make when they report humidity is that you are using the device in air. If you use it in a mixture of steam and mist, the capacitance measurement may mean something, but using the calibration curve generated in air will not be meaningful. I have never seen an instrument that bases the display of other units on > assumptions. I certainly wouldn't buy one! > Most speedometers make assumptions about tire diameters. Many mass scales assume a value for g, and wouldn't read the correct mass on the moon, or in orbit. A barometric altimeter assumes you are in earth's atmosphere (and the weather is fair for good accuracy), and will not work in the space shuttle in orbit, or on another planet. Etc. > > >
