Subject: How can I detect liquid movement in a pipe without piercing it.

The liquid has the viscosity of water with or without antifreeze.  The pipe 
is steel of 1 to 3 inch diameter.  The liquid may be stationary and flow 
must be detected or the flow may be monitored and a lack of flow must then 
be detected. This is a closed system where ejected fluid cannot be measured 
or used to indicate flow.  Flow rate is in the order of 4-5 gpm.

Re: How can I detect liquid movement in a pipe without piercing it.