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Here are a few commercial flow meters that are on the low end of the cost scale (but still not that cheap).
If you find something better, please let me know.
| Venturi with Target | |
|
www.watermeters.com -- home of the dlj meter
|
 They
sell a wide variety of water meters, including hot water meters, and meters
that also output a pulse count for loggers. As low as $45.(thanks to Chuck for suggesting this) |
| Sources:
www.kingsolar.com
www.Omega.com
http://www.dwyer-inst.com |
 In
these meters, the flow pushes a target up a tapered flow -- the greater the
flow, the higher the target rises. Easy to read the flow at a glance.
Normally no electronic output. Mount vertically.Moderate pressure drop. Some models can handle the full temperature and pressure range encountered in solar thermal projects. $75 and up -- over $200 for full temperature and pressure capability. |
| Sources:
www.Omega.com
http://www.dwyer-inst.com
|
  In
these meters, the flow deflects a turbine or disk which is restrained by a
spring.
High operating temperature and pressure capability. Moderate to high internal pressure drop. Mount in any orientation. $65 and up |
| Paddle Wheel
Meters
http://www.gemssensors.com/TOCproducts.asp
http://www.dwyer-inst.com
|
 In
these meters, the flow rotates a paddle wheel which in turn produces an
electronic output signal that is proportional to the flow rate.Some models can handle the full range of solar thermal temperatures and pressures. $150 and up -- maybe the cheapest flow meters with electronic output? |
| Clamp on Flow
Meter
|
 Drill
a hole in the pipe that is to have its flow measured, and clamp this meter
on.
$50 -- up to 120F. |