Unstable Voltage from pH Sensor

Other Hardware Sensors
1

So I've been using this pH sensor for my water quality monitoring system, the problem I am getting is the voltage measured at the analog input of an Arduino is not stable.

Sometimes it gives 2.7V for 7.5pH (mineral water), sometimes it jumps to 3.1V and sometimes 2.5V, it may stay stable on certain reading for 10-15 mins then start to fluctuate again. It never stabilize on a certain output that I can rely on, and that of course effect the pH readings for my system.

I've tried many things such as using RC filter at the input, using 5V external power supply and using software filtering. Nothing seems to be the solution of this problem.

The pH sensor is connected like illustrated in here:

image
image918×371 109 KB

And here's the code I am testing on:

void setup() {
  Serial.begin(9600);
}

void loop() {
  int pHsensorvalue = analogRead(A0);
  float pHvolt = pHsensorvalue*(5.0/1024);
  float pH = -5.6*pHvolt + 21.188; //equation from datasheet
  Serial.print(pHvolt);
  Serial.print(" | ");
  Serial.println(pH);
}

Anyone experienced with these type of sensors can help?

2

What specific pH sensor is it?

4

This Sensor
[PH Sensor E-201-C.pdf (e-gizmo.net)](https://www.e-gizmo.net/oc/kits%20documents/PH%20Sensor%20E-201-C/PH%20Sensor%20E-201-C.pdf

5

can you monitor the voltage with a meter independent of the Arduino? This is to establish if the PH module is the cause or the Arduino.

1 Like
6

Good question.
The sketch from e-Gizmo makes this absolute sensor readout not only dependent on PH, but also on supply voltage of the Arduino.
They should have used the 3.3volt supply as Aref, both for stability and higher resolution.
Leo..

3 Likes
7

Actually that's right. When I use 3.3V to power this sensor I get a much stable readings. But still with some fluctuations every now and then.
I might try @stevemj suggestion and see what I will get.

8

Have you got a filtered supply to the sensor. I would suggest a low value resistor and decoupling capacitance; say 10 ohm and 10uF plus 100nF as a start.

9

You know a pH sensor circuit needs to be isolated from the rest of the world to a very high standard? 10^11 ohms or better... The sensor is extremely high impedance and is electrically connected to the solution being measured and its voltage needs to float with the voltage of the solution.

10

I think you didn't understand what I said.
Leo..

1 Like
11

Hi
check the power connections between the arduino and the sensor.
If you use wires with connectors, they can give bad contact and cause variations in the supply voltage, which is also used as a reference.

12

Yes I have tried this, it made a slight change in the fluctuations but the problem is that the output voltage itself is increasing and decreasing on its own.
I think that's because of the high impedance the sensor have ..

13

I use simple jumper wires, is that may cause problems?

14

Can you tell what should I do to isolate the sensor? Sorry I am newbie to the sensor signal conditioning issue and get lost in some topics.

15

The data sheet states:
image

There is a contradiction here; spec says 5V you are supplying 3v3.

16

Yes, insulate it completely using materials that are good insulators (wood would be hopeless for instance, polythene or glass or teflon are OK) pH sensors have impedances measured in gigaohms or higher.

17

There is a module which includes the isolation:
image

18

From a quick look at the data sheet for the module, it's not clear if the the probe itself has the pre-amp built in or if it's just a basic probe with a lead and BNC connector.
If the pre-amp, an op amp suited to high impedance inputs, is in the probe itself, there should be no problems, but if the pre-amp is on the separate board, then you could be looking at fluctuations simply down to flexing the lead.
As others have said, the materials used, the coax lead and connectors all need to be high quality.

1 Like
19

For Mahmoud's info, the flexing of the cable for high impedance circuitry can induce charge due to tribolectric charging and also through moving the conductors through a magnetic field.

20

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