Arduino control of the Kapta2000-AC2 water chlorine and temperature probe via an SPI interface.
The KAPTA™ 2000-AC2 sensor solution has been specifically developed to measure active chlorine (HOCl) and temperature directly within a pressurised pipe. It works in either of two modes: 0-5V and 4-20mA.
A MAX31865 RTD-To-Digital converter is used to interface the probe to the arduino which is optimised for platinum RTDs. The probe wires are connected to the MAX31865 PMB as follows:
- Yellow & Pink -> F+ and F-
- Green & Grey -> S+ and S-
This is a 4-wire configuration which allows for more accurate measurement.
The capability of the MAX31865 is determined by the onboard resistor, in that an onboard 400Ω will support a PT100 RTD while an onboard 4000Ω a PT1000. In case a PT1000 RTD is to be used with a board having an onboard 400Ω resistor, a resistor can be connected in parallel to the RTD S+ and S- wires to lower the resistance read by the board. Remember, R = (R1 x R2) / (R1 + R2). This means that once the value of R has been read by the board, the value of the RTD can be calculated backwards, i.e. R2 = (R1 x R) / (R1 - R).
The MAX31865 has 8 8-bit registers that can be accessed by SPI: configuration, RTD MSBs, RTD LSBs, High fault threshold MSBs, High fault threshold LSBs, Low fault threshold MSBs, Low fault threshold LSBs and Fault Status.
Calculating the temperature from the resistance is easily calculated by use of the Callendar-van-dussen equation:
(-RTD_A + sqrt (RTD_A * RTD_A - 4 * RTD_B * (1 - (rtd / RNOMINAL)))) / (2 * RTD_B)
where
- RTD_A = 3.90830e-3
- RTD_B = -5.7750e-7
- RNOMINAL = RTD resistance at 00C
The following steps are followed to read the temperature:
- initialise SPI
- write configuration settings to the MAX31865
- read RTD registers
- use the Callendar-van-dussen equation to calculate the temperature
- read fault registers
- read status register
Chlorine measurement is done via the analog pins. The brown and white wires from the sensors are connected in series to a resistor and to a battery. The voltage is read across the resistor. The value of the resistor can be calculated as using Ohm's law as follows:
20mA × R = 5V -> R = 5V / 20mA -> R = 250Ω.
A 250Ω resistor is not readily available, so a 220Ω resistor will be used. This means that at the 'zero' i.e. 4mA, a voltage of 0.88V will be read while at 'maximum',a voltage of 4.4V will be read. These values are within the allowable range of an Arduino Mega.
The input from the sensor will be connected to one of the analog pins of the Mega. The value read from the pin will be the result of analog to digital conversion.
The ADC step voltage is calculated as follows:
Resolution of the ADC / System Voltage = ADC Reading / Analog Voltage Measured
Making Analog Voltage Measured the subject of the formula:
Analog Voltage = ADC Reading x System Voltage / Resolution of the ADC
where
- ADC Reading = analog pin reading. This is the voltage due to chlorine
- System voltage = 4.4V - 0.88V
- Resolution of the ADC = 1023
From the Kapta2000 AC2 datasheet, the amount of chlorine in the water is calculated as follows:
[HOCL] = ((VHOCl – Vzero without chlorine) x 1000) / Ssensor where
- [HOCl] : HOCl concentration in ppm or mg/L
- VHOCl : HOCl output voltage (V)
- Vzero without chlorine : HOCl output voltage without chlorine (V)
- Ssensor : Sensitivity of the chlorine sensor (mV/mgL-1)