On Thu, Jun 23, 2011 at 1:50 AM, Finlay MacNab <finlaymac...@hotmail.com>wrote:
> From this document http://www.macinstruments.com/pdf/handbook.pdf, from a > website trying to sell absolute humidity gauges, it would appear that a > relative humidity sensor can give accurate reading up to the boiling point > of water and that the measurement of humidity decreases in dry steam as the > temperature of the super heated vapor increases. > But RH measures the vapor content. It does not give a measure of the liquid content in the form of droplets. > > The delta ohm probe in question is rated to 150C with an accuracy of +/- > 3.5% above 95% RH from this spec sheet > http://www.deltaohm.com/ver2010/uk/st_airQ.php?str=HD37AB1347. > Right. But we don't need that because we already know (as is clearly spelled out in your reference) the RH of steam at 100C is 100%. That doesn't tell us how much liquid is present. > If the temperature of the vapor is above 100C and the pressure is 1 atm, > then an an examination of the phase diagram of water suggests that no liquid > water can be entrained in the vapor. > True. But if the pressure is slightly higher, then the boiling point will be slightly higher, and then it is possible for liquid droplets to be entrained above 100C. We know the pressure inside the conduit is above atmospheric pressure, because the pressure difference is what pushes the fluid out. Without a pressure difference, there is no flow. There is a good reason the pressure is higher. The pump is pushing water in from one side, so that increases the pressure. Then some of the water is changing phase, and that will increase the pressure. So the boiling point inside that reactor is going to be a little above 100C. It is obvious that the fluid is at the boiling point, because the temperature is completely flat. As the power increases, the temperature of the water increases, but when it reaches the bp, it is flat. When it first hits the bp, it can't possibly produce 100% dry steam because that would require an instantaneous 7-fold increase in the power transfer. So, for at least a short time, there must be a mixture of steam and droplets at the boiling point. That value is already above 100C. It remains flat after that, so there is no evidence it ever goes above the bp. If the steam were dry, then there would be nothing stopping it from going to a higher temperature. I have said repeatedly, if the steam temperature were 120C for example, the question of steam quality would be put to rest. The fact that the steam exiting the hose in the video is invisible is very > strong qualitative evidence that the steam is relatively dry. > It's not invisible. Against the black t-shirt, it is entirely visible. And it is moving far too slowly to be consistent with a 5 kW ecat. Since the steam can only become wetter after it's exit from the chimney, it > must be more dry when it is produced than when it exits the hose. If the > temperature and pressure were measured accurately then the steam is likely > to be significantly dry. > No. It is almost certainly very very wet. > > In summary, it would appear that if the water is superheated then the steam > is dry. > Yes, but it's not.
