pH sensor measurements unstable

[MisterF]

First of all, thanks for the response. Sorry, but I'm rather new with noise filtering etc.

@DrDiettrich

Do you use analog or digital sensors?

If analog, what's your ADC reference voltage? It should be very stable, not depending on Vcc noise and fluctuations.

The temperature sensor is digital, the pH sensor is analogue. The ADC reference voltage is 4.95V if I'm not mistaken.

@Grumpy_Mike

That 100pF is going to do nothing it should be a 100nF ceramic capacitor, sometimes called a 0.1uF capacitor.
The long leads on the capacitor make them very much less affective. They should be placed on each module.

It is interference from your mains operated devices that is causing the problem. This is down to u tidy wiring and lack of noise suppression on these devices especially the motor,

So if I'm reading correct, I should place multiple 100 nF ceramic capacitors? Should I place them between the VCC and GND or should I place them over the data line from sensor to the arduino? If I remember correctly, the ceramic capacitors should be placed closely to the arduino?