The TL-136 is a hydrostatic pressure sensor with a 4-20-mA current loop – the proven, inexpensive way to measure the fill level of cisterns, rain barrels and IBC containers. The key working principles and application tips at a glance.
At a Glance
- Hydrostatic principle: The pressure at the sensor depends only on the height of the water column (ρ·g·h) – not on the volume of the tank.
- 4-20-mA output: 4 mA = empty, 20 mA = end of range. Robust against interference, transmission over 100 metres or more without any problem.
- Pick the range to fit: As close as possible to the maximum fill height (e.g. 0–1 m). An oversized sensor (0–3 m) noticeably worsens accuracy.
- Robust & simple: Piezoresistive transducer with no moving parts, readable directly via an ADC (e.g. ADS1115).
Technical Details
Measuring principle: Hydrostatic pressure = ρ · g · h. Since density ρ and gravitational acceleration g are constant, the pressure is directly proportional to the fill height h – the signal is independent of tank shape and volume.
Signal conditioning: The TL-136 provides a standard signal (usually 4–20 mA, less often 0–10 V / 0–5 V). A precision resistor converts the current into a voltage, which an ADC (e.g. ADS1115, 16 bit) digitises.
Choosing the measuring range – the most important point:
- 0–1 m sensor: 4 mA at 0 m, 20 mA at 1 m → the full 16 mA span is used.
- 0–3 m sensor on a 1-m tank: only ~9.33 mA at 1 m → just ~33 % of the signal range. The absolute deviation triples (0.5 % FS = ±1.5 cm instead of ±0.5 cm), the ADC loses two thirds of its resolution, and temperature drift hits three times as hard.
Adjustment: The “Zero”/“Span” trimmers set the zero point and end voltage exactly to your range (e.g. 0 V and 3.3 V) – this maximises the resolution at the ADC.
Cross-References
- Full build with ESPHome, ADS1115 & calibration → Measure water level in cisterns
Hi! I'm Markus – the Fricklr. ⚡ Electrical engineer, 🎵 musician (bass, guitar, keys) and professionally 'something with the internet' 🌐 for over 30 years – nowadays with AI too 🤖 – a 55-year-old generalist from Bavaria, Germany.
Your feedback helps me write even more interesting posts. After "Yes" there's also a small option to support my work.