The WX1 and WX2 Tap Timers feature a flow counting wheel on the outbound side of the timer. This wheel spins during watering offering a flow reading, the measurement is published as 'Last Water Flow' onto the status page for the tap timer after watering.
The accuracy of the flow meter is dependent on operational conditions for the timer, as well as your watering set up.
- 50-800 kPa working pressure
- 3oC - 50oC working temperature
- Maximum flow rate from your tap of 25L/min
Flow reporting from a normal system (not pressure compensating system)
You need to achieve a system output at/above 7L per minute (LPM). Anything less than 7LPM from the watering system will be inaccurate and then below 5LPM may even be non-existent.
This is due to the speed of the flow counter. If it slows too much (or stops) due to low flow (and lack of water movement) then it can’t report.
This can happen on a very short system (below 10m) or one with a low output dripper (for example watering a few pot plants).
Flow reporting from a pressure compensating system
Pressure compensating watering products deliver a set amount of water per outlet, per hour, and there will almost always be more water available to the system than it can use.
When you connect a pressure compensating system, your flow will be inaccurate and, in some cases, non-existent - depending on the total output from the system.
When the water is being released, it's being released in small increments which don't generate any movement on the flow wheel. What happens in these systems is that the poly/hose fills with water and then the water just sits - there's far more water on offer than is being let through the drippers, and the flow wheel of the tap timer is stopped in this process.
A good example of this is Techline tubing which has 3L/hour drippers, set 30cm apart in a bed (pre-set in the tube). In this example the bed is within 3m of the tap, which is serviced by a 12mm hose. The small square raised bed (3m square) is set up using these drippers.
The layout for the Techline is 2 x 13mm poly pipe sections (3m long) which have 4 x runs of Techline between them. The system may contain 40 (or so) drippers per bed, which will be an output of 120L per hour, or is 2LPM for the system. The timer on this system reports 0L as the flow, as there’s not enough movement in the flow wheel during watering.
Set up example
To get accurate reporting we’d need to have a system with 140 Techline emitters running at once, which would give us a flow of 7LPM and allow reporting.
What happens in short run, low output, or pressure compensating systems is effectively a pressure differential. Understanding how differential pressure can effect what’s going on in a watering system is helpful when we want to have everything work as expected.
Pressure and flow are inversely proportional to one another, and if there isn’t a difference of pressure across the valve or the flow meter, the results that you will get will never be satisfactory.
There is a little touch of physics in this, stated simply, Bernoulli's law says that the pressure of a fluid decreases as its velocity (speed) increases, and vice versa.
So any restriction along a the line will change the balance on the other side of the flow.
This is a problem when you significantly reduce flow as a valve or pressure compensator does.
The differential pressure drops as well and without it the valve won’t open and shut, and a flow meter has no means to rotate - as there is no negative pressure on the forward edge of the blade to allow it to move.
Effectively it’s like an airplane flying into a wind that is blowing at the same rate as the forward movement of the plane. The plane can’t move forward, because it doesn’t have enough speed (pressure) to overpower the wind it is flying into. When you increase the power to the engines, you can overcome the force of the wind blowing and move forward.
It’s the increase in pressure exerted by the plane which overcomes the flow of the wind and creates a differential that allows the plane to travel forward. Without a differential, the flow and pressure are in a static balanced state.
In our garden, reducing flow restrictions on the system increases the accuracy and function of your flow meter in the WX Tap Timers.