Water Quality Last update: 2019-08-13
Water quality primarely relates to the
electrical conductivity of the cooling water.
Water conductivity is expressed in μSiemens/cm. Siemens is the
inverse of Ohms so 1 μS/cm means that a column of water with a
cross section of 1 cm2 and a length of 1 cm has a
resistance of 1 MegOhm.
Many sources of information suggest that the conductivity should
be 2 μS/cm or less. At 20+ μS/cm the risk of flashing becomes
I found an interresting article about the subject (throug an
Israelian university site, but I could not find that back) from a
Measuring the conductivity of the cooling water is done by a
simple device consisting of two electrodes measuring 40 x 100 mm
on a distance of 10 mm. The electrodes were made of FR4 PCB
material with a continuous copper layer, the copper layers facing
One of the electrodes is periodically (33 Hz) set to GND or +5
Volt. Just before the change we measure the current into a
reference voltage of 2.5 V. From this we calculate the
conductivity in μS/cm. We periodically reverse the voltage to
prevent polarisation effects.
The Water Quality Probe normally hangs on its wires in the
buffer vessel, completely submerged.
This schema is an excerpt from the SafetyBoard schema.
The probe is connected between T71 and T69, represented here as
the water-resistance and an inevitable parasitic capacitance.
T71 carries a squarewave of 33 Hz, between GND and Arduino-VCC, so
is on average half VCC.
R4 and R20 keep T69 also on average half-VCC, so the probe has on
average no DC, which prevents polarisation effects.
R3, D1 and D6 protect the ADC input from noise spikes.
With our probe with a surface of 40 cm2 and a targeted
conductivity of 2μS/cm R-Water would be 12.5 kOhm. Together with
C1 estimated <= 1 nF we have a timeconstant of maximal 12.5
μsec, extremely shorter than the 30 ms measuring interval. The
effect of C1 can be neglected completely.
The Arduino code handling the WQ probe is executed each 30 ms (33
Around 5 ms before this the A/D converter was started for channel
6 and now has the conversion result ready in the register ADC:
((PIND & 0x80) == 0x80) WqPos = ADC; else WqNeg = ADC; //
from channel 6, Water Quality
PIND = 0x80; // toggle WQDRV
WaterQuality = WqPos - WqNeg; // take pp-Voltage
As we apply a voltage of 5V-pp and we also measure the pp-voltage
we may reduce the circuit to:
Now VQPP = WaterQuality(ADC-value) * 5V / 1024. [Volt]
(we do 10 bit conversions)
It is not difficult to see that RX = (5Volt - VQPP) * 500R - 100R.
The resistance of a watercolumn of 1cm2 and 1 cm length
will then be RX * 40. [Ohm]
and the conductivity 1 / (RX * 40) * 10E6.
These calculations are done in the LaserControl Program running on
Experiments learned that the conductivity is also quite dependent
on the temperature. The lower the temperature the lower the
conductivity, which is to our advantage.