***Question***

> Does anyone have any ideas on how I can use RPI with ADC to measure a
> tank level sensor which has a range of only 20 to 200 ohm?

***Answer***

> Some months ago I played with a couple of moisture sensor / level
> detectors.  Below is an example.

    

[![water level sensor][1]][1]
[Water Level Detection Discussion - Rpi StackExchnge][2]

However the above sensor does not answer the OP's question, which asks to detect resistance using ADC, not analog or digital voltage levels.  So I think I need to start from a variable resistance and check out how to use ADC to measure the resistance, within a range of 20 to 200 ohms.

***Long Answer***

*/ long answer to prune later, ...*  

*Introduction and Summary*

*/ to continue, ...*


*Question Analysis*

> 1. I think the preliminary functional spec of the project is very good, because it is concise and precise.
> 
> 2. There are many reasons that the question is so short, but I will not list my wild guesses for now.  Perhaps later.  
> 
> 3. The spec includes the following key words/phrases:
> 
>  "***Rpi, ADC, tank level sensor, 20Ω ~ 200Ω***. 
> 
> 4. The word "tank", is crucial.  She says "tank", not "container", perhaps she wants to indicate that the "tank" is deep , say, not a 5 inches
> deep fish "bowl" or "pond".  
> 
> 5. The phrase "20Ω ~ 200Ω" is also crucial.  There are basically two types of liquid level sensors: ***active and inactive***.  Active sensors
> are usually 5V or 12V powered, and the sensor output is a DC voltage
> level.  Inactive sensors don't have any power.  Usually the sensor is
> just a reed switch and floating magnet, or a long "resistor" whose
> resistance varies as the water level.  Usually the resistor is far,
> perhaps 10ft, and up to 1,000 ft from the measuring instrument.

***References***

[Water Level Detection Discussion - Rpi StackExchnge][2]

[AliExpress Level Sensors][3]

[AliEXpress Non-contact Level Sensors][4] 


[AliExpress10A/250V Water Level Controller Module][5] 



[AliExpress Water level sensor Water drops tester][6]


[AliExpress 5~24V 5mA Non-contact Induction Water Level Sensor][7] 

[Gig (ee, rpi, arduino) projects][8]

*/ to continue, ...*

  [1]: https://i.sstatic.net/GVzFX.png
  [2]: https://raspberrypi.stackexchange.com/questions/98491/water-level-detection-sensor-3-3-volt-vs-5-volt/98497#98497
  [3]: https://www.aliexpress.com/w/wholesale-level-sensor.html
  [4]: https://fr.aliexpress.com/item/Non-contact-Pipe-Level-Sensors/32255031027.html?spm=a2g0w.search0104.3.265.531f76856zZiC8&ws_ab_test=searchweb0_0,searchweb201602_9_10065_10068_319_10059_10884_317_10887_10696_321_322_10084_453_10083_454_10103_10618_10307_537_536,searchweb201603_51,ppcSwitch_0&algo_expid=af3cfe02-0556-4738-a8ae-46353f4edbf3-33&algo_pvid=af3cfe02-0556-4738-a8ae-46353f4edbf3
  [5]: https://fr.aliexpress.com/item/10A-250V-Water-Liquid-Level-Controller-Module-Automatic-Water-Level-Detection-Sensor-Temperature-Sensor-Water-Level/32820105118.html?spm=a2g0w.search0104.3.310.531f76856zZiC8&ws_ab_test=searchweb0_0,searchweb201602_9_10065_10068_319_10059_10884_317_10887_10696_321_322_10084_453_10083_454_10103_10618_10307_537_536,searchweb201603_51,ppcSwitch_0&algo_expid=af3cfe02-0556-4738-a8ae-46353f4edbf3-39&algo_pvid=af3cfe02-0556-4738-a8ae-46353f4edbf3
  [6]: https://fr.aliexpress.com/item/DC-3-5V-High-Sensitivity-Water-Liquid-Level-Sensor-Tester-Water-Level-Monitoring-Measuring-Tool-Water/32814866978.html?spm=a2g0w.search0104.3.216.620d755cWn7ee7&ws_ab_test=searchweb0_0,searchweb201602_9_10065_10068_319_10059_10884_317_10887_10696_321_322_10084_453_10083_454_10103_10618_10307_537_536,searchweb201603_51,ppcSwitch_0&algo_expid=276db9f8-3b68-4295-a2c2-1516543028cd-31&algo_pvid=276db9f8-3b68-4295-a2c2-1516543028cd
  [7]: https://fr.aliexpress.com/item/D-tecteur-de-niveau-de-liquide-5-24-V-5mA-capteur-de-niveau-de-liquide-sans/32971512924.html?spm=a2g0w.search0104.3.269.5eb5755cG2N9tK&ws_ab_test=searchweb0_0%2Csearchweb201602_9_10065_10068_319_10059_10884_317_10887_10696_321_322_10084_453_10083_454_10103_10618_10307_537_536%2Csearchweb201603_51%2CppcSwitch_0&algo_pvid=f09e6f0d-a67d-4c18-bace-f8494991ae4c&algo_expid=f09e6f0d-a67d-4c18-bace-f8494991ae4c-43
  [8]: https://www.eetasia.com/news/article/EE-Pushed-into-the-Gig-Economy