Some rheobus designs do provide for fan monitoring, usually through a panel display in a drive bay.
It's not dangerous to plug fans into the rheostatic speed controllers, you run a small risk of fans not starting (insufficient current), but a good rheostat system will be designed to minimize the risk.
--->It ain't better if it don't work<---
I have the Vantec Nexus rheostat, it has no monitoring.
When you say that the fans may not start, if I see that they start one time, can I infer that they will start all the time? Or do you mean that you never know when it will not start up. I dont want to fry my cpu with no fan running..
EDIT: I may not have to worry about this, please help me understand this:
"Temp. Control, Fan Speed Auto Control Setting:
1300rpm at 20 ºC~ 6000RPM at 55 ºC " (thermaltake, spark 7)
Does this mean that it goes at 1300 at 20 and stays at that rpm untill it reaches 55 degrees? or does it gradually get faster?<P ID="edit"><FONT SIZE=-1><EM>Edited by backflash001 on 03/14/03 07:06 PM.</EM></FONT></P>
First of all... DO NOT connect your CPU fan to a rheostat buss. Grab the wrong knob by mistake and your whole investment goes down the toilet, especially without spin-up monitoring.
As for the risk of fans not starting.... Seeing it start nice and slow once means exactly that you were lucky that time. In the extremes it may be reliable, or it may never start again. Most fans can't be counted on to start every time below about 7 volts... some of those rheostat gadgets are very poorly designed and can take a fan down to 3 or 4 volts.
About the spark 7... Themostatically controlled fans use temperature sensors to determine the correct minmum fan speed to maintain a given temperature... It's a linear relationship between temperature and speed, usually done through a simple 1 or 2 transistor circuit. On CPU fans this tends to be an electronic guessing game in which the fan speeds up until the temperature stops going up. But it will never stay at speed while the temperature goes down a small drop in temperature will be met with a small change in fan speed. So, whatever temperature Thermaltake thinks is good for your CPU is what you'll end up with... It is likely that fan will run at the same speed almost all of the time.