Josh wrote: "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.." Yes, agreed that at the most fundamental level it is making an electrical measurement, that being capacitance. However, since relative humidity is a moving target depending on the temperature, RH is usually calculated from absolute humidity and temperature. I'm not making this up... this is from direct experience... a few years ago we were using a temp/humidity sensor in the lab and I wrote the code to query it and get its data. I believe it too was a polymer/capacitive sensor and what it measured was absolute humidity (which doesn't change with temperature), and the user manual provided an equation to convert that into RH given the temperature, which it also measured. Perhaps they are a bit more sophisticated these days and they've incorporated the adjustment for temperature in order to get RH...
-Mark _____ From: Joshua Cude [mailto:[email protected]] Sent: Wednesday, June 22, 2011 11:07 PM To: [email protected] Subject: Re: [Vo]:E-Cat vs. Water Heater for coffee/tea... 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.
