The voltage measured from the remaining water from the washing by hand, and the remaining water from the washing machine will have the same value. Although the quantity of detergent is different.
The potential difference is measured from the water left from washing clothes by hand and the remaining water from the washing machine will have the same value. Although the detergent is different.
Voltage measured by the remaining water from washing by hand and the remaining water from washing with air is all the same to you! Although the amount of detergent is different.