The usefulness of a head level sensor

Over the past ten days we've had a fair amount of rain and I've been gradually "upping" the nozzles to make the most of the increasing flow as it becomes available.

This morning at nine o'clock, I "upped" from a nozzle combination which delivered 2.47 lps to one delivering 2.78 lps, - but it was too big a step to take.  By midday, the header tank had stopped overflowing, the 'spare' flow falling over the quarry face below my catchment site had reduced to a trickle: 

...the online trace of live power showed a falling level of output as the water level in the tank fell:

...and the level sensor sited in the connection pillar by the turbine registered that the tank was 150mm below being full:

I relate all this to illustrate two things:

1. How much more useful it is having a head sensor at the top of the penstock compared with just having a pressure gauge at the bottom. Powerspout provide such a gauge in the kit of parts which comes with each turbine and it does have its uses.  But the pressure gauge for my site was calibrated to a full scale reading of 750 kPa, which is about 76 metres head of water.  There is no way it could have conveyed the relatively miniscule drop in pressure described in the above situation.

The head sensor is very simple: it's powered by three 9v batteries, which need replacing at yearly intervals.  It's connected to a pressure transducer at the top of the penstock, by a 3 core, 1.5 mm², SWA* cable running alongside the penstock. The pressure transducer sits in a vertical pipe, where the water is still and not flowing, with its reference mark exactly 1000 mm below the water level when the tank is full.

This vertical pipe also acts, incidentally, as a vent to let air out of the pipeline when filling it, and air into the pipeline when emptying it.

The sensor was provided by SCS Ltd, a company making turbine control and grid connection equipment for micro-hydros bigger than Powerspout size.  It's a manually operated version of the sensor they use to regulate the flow to a turbine by servo control of its inlet valve.

2. How handy it is to have a good amount of water storage: despite 3 hours operation during which flow out from the tank has exceeded flow in, the tank water level has dropped only 150mm and the turbine is happily still putting 714 W into the grid.  

From the power trace above, it looks as if the water level may have stopped dropping: the trace from 11.30 onwards seems to have levelled out, meaning inflow and outflow may have come into equilibrium.

All this being so, I am not going to rush to reverse the nozzle change I made this morning.  The weather forecast is for more rain, albeit not very much, and my hope is that the yield will pick up sufficiently for me to keep operating with the present nozzle combination.

Perhaps this account will convey something more than I initially intended, which was just to say something about the level sensor.  It will, doubtless, also convey that a micro hydro needs supervision, at least on days like today !

* steel wire armoured