Etape liquid level measurement not working as expected

Question

I am building a monitor to measure how much water is in a tank.

I started using 12" Standard eTape Liquid Level Sensor with Plastic Casing connected to mcp3008 and it's not working great, well not working how I would expect it to.

When it's out of the water it returns a result of water being at 27cm! This cant be right

I wanted to know if there is another device that is more accurate as in my user case, it needs to be within 1cm accuracy.

That said I am wondering if there is a problem with the etape or the mcp3008 chip because the other issue is, I can not get the python code to return any value which I wasn't worried about at first as I building the application in Ruby and it returns values with ruby code using pi-piper gem

Any suggestions on devices would be appreciated

EDIT:

Sorry should have added this in the first place:

this is how it is wired up:

I am using a Rotary shaft potentiometer as the etape as I couldn't find a Fzitxing part to match the etape I have.

Then the code I am using is:

require 'pi_piper'
require 'date'

def read_adc(adc_pin, clockpin, adc_in, adc_out, cspin)
  cspin.on
  clockpin.off
  cspin.off

  command_out = adc_pin
  command_out |= 0x18
  command_out <<= 3

  (0..4).each do
    adc_in.update_value((command_out & 0x80) > 0)
    command_out <<= 1
    clockpin.on
    clockpin.off
  end
  result = 0

  (0..11).each do
    clockpin.on
    clockpin.off
    result <<= 1
    adc_out.read
    if adc_out.on?
      result |= 0x1
    end
  end

  cspin.on

  result >> 1
end

clock = PiPiper::Pin.new :pin => 11, :direction => :out
adc_out = PiPiper::Pin.new :pin => 9
adc_in = PiPiper::Pin.new :pin => 10, :direction => :out
cs = PiPiper::Pin.new :pin => 5, :direction => :out

adc_pin = 0

level_size = 30
raw_value = read_adc(adc_pin, clock, adc_in, adc_out, cs)
raw_value_volts = ((raw_value * 3.3)/1023)
level = ((level_size * raw_value_volts)/3.3)

puts "The raw value is: #{raw_value}, the mill-volts is #{raw_value_volts} and the level in CM is #{level}"

Technical Details of Etape (The data sheet only shows the standard etape that in not incased already and have 4 pins, I been told by PIHut the etape I have is a resistive divider / analog out) - DataSheet

Dimensions: 305mm x 29mm x 13mm / 12" x 1.14" x 0.51"

Weight: 56g

Sensor Output: 400-2000Ω ±20%

Ref. Resistance: 2000Ω ±20%

Actuation Depth: Nominal 25.4mm / 1"

Resistance Gradient: 60Ω/cm / 150Ω /inch

Power Rating: 0.5 Watts

Temperature Range: 15°F - 150°F / -9°C - 65°C

Answer 1

The data sheet says this is a 4-terminal part, with the inner 2 pins being a variable resistor that varies with water pressure, and the outer 2 being a fixed resistor. Unfortunately your circuit diagram shows this as a 3-terminal device, so it is unclear how you have connected the 4 pins.

If you have access to a multimeter, disconnect the device from your circuit, and measure the resistance between the inner 2 pins, and you should see it vary as the water level changes. Then connect the sensor up as in the first of the 'sample circuits', with a 3.3V supply and the 2 resistors acting as a potential divider, and measure the voltage on the junction of the 2 resistors; you should see that change as the water level changes. Once that is working OK, connect that point to the input of the ADC, and run your code, comparing the values you get with the voltage indicated by the meter.

If you don't have access to a multimeter, then I suggest you start by checking that the ADC is working OK; you can do this by disconnecting your circuit and connecting the ADC input to the 0V rail, and see if it returns a value fluctuating around zero; then connect the ADC input to 3.3V and check that you get value around full-scale (4095 units). However, if you are doing that, make absolutely sure that you don't put more than 3.3V into the ADC, otherwise it will be damaged. If the ADC returns sensible values, connect the sensor to its input as described above, and hopefully it should work - but I would suggest you first use a multimeter, if at all possible.

Answer 2

Question

How to use Rpi Python or Ruby to talk to the eTape liquid level sensor?

Answer

Part 1 - Calibration

1. We need first to use a multimeter to calibrate to make sure the meter is working OK.

1.1 Dip the liquid sensing meter "ruler" to a vessel of water at about 6 inches.

1.2 The multimeter, "meter" should read the ruler's output (inner 2 pins of the 4 pin connector) resistance roughly 1.4 kΩ

1.3 Pour water so that the "ruler" now is about 9 inches.

1.4 The meter should now read 0.9 kΩ (900Ω)

Note 1

For this part of calibration, we don't need any power source such a battery etc.

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References

(1) Continuous Fluid Level Sensor PN-12110215TC-X - Milone Tech (Innovative Fluid Sensing)

(2) pi-piper Project 4: MCP3008 - jwhitehorn 2013jan21

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