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Uncalibrated bench characterisation of low-cost turbidity, TDS, pH and level sensors on Arduino Uno R4 WiFi

· Nanupatruni Mahesh Kumar

  • Sensing
  • TRL 3
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PDF, 21 pages, 1.5 MB · v1.1

The paper's cover: Uncalibrated bench characterisation of low-cost turbidity, TDS, pH and level sensors on Arduino Uno R4 WiFi

Summary

Four sensor modules for a water-monitoring system (turbidity, TDS, pH, and water level) were each wired to an Arduino Uno R4 WiFi and tested in one bench session in dry air and household liquids, without reference standards. All four produced signals distinct from dry air and moving in the expected direction. Acidic water moved the pH output by +0.920 V and salt water by only +0.044 V. One turbid sample spanned 0.83 V depending on agitation alone. The TDS module ranked RO, drinking and tap water correctly but read distilled water out of order. No reading is calibrated and the ADC reference is unverified, so the work is an experimental proof of concept below TRL 4. The paper turns the observed effects into six rules for the calibration protocol and sets out the path to TRL 4.

Key findings

  • pH. Acidic lemon water moved the output +0.920 V from near-neutral water; salt water moved it only +0.044 V. The probe responds strongly to acidity and only slightly to salinity.
  • Turbidity. One turbid sample read anywhere across a 0.83 V span depending only on how recently it was stirred. Sample agitation must be fixed before calibration.
  • TDS. Two dips into the same distilled water agreed within 1.67 %, but distilled water read 6.4 times higher than RO water, against the expected order.
  • Water level. The switch reported dry and wet correctly with no chatter, over a single transition.
  • Protocol. Six rules for the calibration phase, each traced to a measured effect (Figure 10).
  • Readiness. Experimental proof of concept (TRL 3), with a defined path to TRL 4 (Figure 12). No reading is calibrated yet.

Inside the paper

6 of the paper's 12 figures. Open any of them full size to zoom in.

Figure 1: The four sensors as tested, each in its own session on an Arduino Uno R4 WiFi and read over USB serial.Block diagram of the four set-ups, each tested in its own session. Turbidity: KIE-TS-300B turbidity module, OUT to A0, on an Arduino Uno R4 WiFi with a 14-bit ADC and 200 ms averaging. TDS: two-needle probe and generic TDS module, AOUT to A0, 14-bit ADC, 200 ms average and a filter. pH: glass electrode on BNC and PH-4502C module, PO to A0, 14-bit ADC, 200 ms average. Water level: XKC-Y25-PNP capacitive switch, PNP output to D2, digitalRead with a 50 ms debounce. All four report over USB serial at 115200 baud, one JSON line per second, logged with arduino-cli, with settled readings recorded in each session. Marked as later and not part of this work: integration into one water-monitoring system. AS TESTED: FOUR SENSORS, EACH IN ITS OWN SESSION Turbidity KIE-TS-300B turbidity module OUT → A0 Arduino Uno R4 WiFi 14-bit ADC 200 ms average TDS Two-needle probe + generic TDS module AOUT → A0 Arduino Uno R4 WiFi 14-bit ADC, 200 ms average + filter pH Glass electrode (BNC) + PH-4502C module PO → A0 Arduino Uno R4 WiFi 14-bit ADC 200 ms average Water level XKC-Y25-PNP capacitive switch PNP OUT → D2 Arduino Uno R4 WiFi digitalRead + 50 ms debounce USB serial, 115200 baud: one JSON line per second logged with arduino-cli; settled readings recorded in each session Later, not part of this work: integration into one water-monitoring system
Figure 1 The four sensors as tested, each in its own session on an Arduino Uno R4 WiFi and read over USB serial.
Figure 6: Turbidity module output per condition. Clear liquids read above dry air; the four powder-suspension readings of the same cup spread widely with agitation.Range chart of the KIE-TS-300B's output voltage, uncalibrated, on 2026-10-19, with the dry-air baseline marked. Dry air 3.081 to 3.082 V; distilled water 3.574 to 3.576 V; lemon water 3.48 V; powder suspension, dip 1, 3.00 to 3.13 V; dip 2, more powder and still mixing, 2.41 to 2.66 V; dip 3, unstirred minutes later, 3.12 to 3.24 V; continuously stirred, 2.81 to 2.95 V. TURBIDITY: KIE-TS-300B, 2026-10-19 Dry-air baseline Liquid Settled range as logged; dot = midpoint 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 Output voltage (V), uncalibrated Dry air (baseline) 3.081–3.082 V Distilled water 3.574–3.576 V Lemon water 3.48 V Powder suspension, dip 1 3.00–3.13 V Powder suspension, dip 2 (more powder, still mixing) 2.41–2.66 V Powder suspension, dip 3 (unstirred, minutes later) 3.12–3.24 V Powder suspension, continuously stirred 2.81–2.95 V
Figure 6 Turbidity module output per condition. Clear liquids read above dry air; the four powder-suspension readings of the same cup spread widely with agitation.
Figure 7: TDS module output per condition. RO, canned and tap water rise in the expected order; both distilled-water dips sit out of order between canned and tap water.Range chart of the generic two-needle TDS module's output voltage, uncalibrated, on 2026-10-17, with the dry-air baseline marked. Dry air 0.006 V; RO water (user-described) 0.079 V; canned or bottled drinking water 0.100 to 0.101 V; distilled water, dip 1, 0.507 to 0.508 V; distilled water, dip 2, 0.516 V; fresh tap water 0.721 V. TDS: GENERIC TWO-NEEDLE MODULE, 2026-10-17 Dry-air baseline Liquid Settled range as logged; dot = midpoint 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Output voltage (V), uncalibrated Dry air (baseline) 0.006 V RO water (user-described) 0.079 V Canned/bottled drinking water 0.100–0.101 V Distilled water, dip 1 0.507–0.508 V Distilled water, dip 2 (re-dip) 0.516 V Fresh tap water 0.721 V
Figure 7 TDS module output per condition. RO, canned and tap water rise in the expected order; both distilled-water dips sit out of order between canned and tap water.
Figure 8: pH module output per condition. Tap, distilled and salt water cluster together; acidic lemon water stands well apart. The excluded unplugged-electrode reading is not shown.Range chart of the PH-4502C module's output voltage, uncalibrated, on 2026-10-19, with the dry-air baseline marked. Tap water 1.370 to 1.372 V; distilled water 1.387 to 1.389 V; salt water (table salt in tap water) 1.422 to 1.425 V; dry air 1.486 V; lemon water (diluted, acidic) 2.299 to 2.300 V. PH: PH-4502C, 2026-10-19 Dry-air baseline Liquid Settled range as logged; dot = midpoint 1.2 1.4 1.6 1.8 2 2.2 2.4 Output voltage (V), uncalibrated Dry air (baseline) 1.486 V Tap water 1.370–1.372 V Distilled water 1.387–1.389 V Salt water (table salt in tap water) 1.422–1.425 V Lemon water (diluted, acidic) 2.299–2.300 V
Figure 8 pH module output per condition. Tap, distilled and salt water cluster together; acidic lemon water stands well apart. The excluded unplugged-electrode reading is not shown.
Figure 10: Six rules for the calibration protocol, each traced to an effect observed in this work.Six rows, each pairing what the bench showed with what the calibration protocol must do. Turbidity: a 0.83 V spread in one cup of turbid water from agitation alone, so fix the agitation state, stirring continuously or reading at a fixed time after stirring stops. pH: 25 to 60 s to settle after immersion, so set warm-up from measured settling data; the 500 ms firmware placeholder is far too short. pH: 1.121 V between near-neutral output and the firmware's 2.5 V placeholder, so calibrate with certified buffers on both sides of pH 7 before reporting any pH value. TDS: distilled water read 6.4 times higher than RO water, against the expected order, so take calibration constants only from certified standards, never from an unverified sample. ADC: 19.1 to 19.2 % of full scale with A0 jumpered to 5 V, where near 100 % was expected, so verify the ADC chain with a calibrated multimeter before any calibration run. Records: the raw serial stream, temperatures, sample recipes and containers were not saved, so log the raw stream to CSV and record temperature, sample recipe, container and immersion depth. FROM MEASURED EFFECT TO PROTOCOL RULE What the bench showed (calculated from the logs) What the calibration protocol must do Turbidity 0.83 V spread in one cup of turbid water, from agitation alone Fix the agitation state: stir continuously, or read at a fixed time after stirring stops pH 25–60 s for the pH reading to settle after immersion Set warm-up from measured settling data; the 500 ms firmware placeholder is far too short pH 1.121 V between near-neutral output and the firmware's 2.5 V placeholder Calibrate with certified buffers on both sides of pH 7 before reporting any pH value TDS 6.4× distilled water read higher than RO water, against the expected order Take calibration constants only from certified standards, never from an unverified sample ADC 19.1–19.2 % of full scale with A0 jumpered to 5 V, where near 100 % was expected Verify the ADC chain with a calibrated multimeter before any calibration run Records Not saved raw serial stream, temperatures, sample recipes and containers Log the raw stream to CSV and record temperature, sample recipe, container and immersion depth
Figure 10 Six rules for the calibration protocol, each traced to an effect observed in this work.
Figure 12: Path from this work to TRL 4.Five steps in sequence. 1, set limits first: pass/fail limits for accuracy, repeatability and settling, fixed before testing. 2, verify the chain: a calibrated multimeter on the 5 V rail and a reference voltage on A0. 3, get standards: pH buffers 4, 7 and 10, two conductivity standards and turbidity standards. 4, calibrate and repeat: runs repeated on more than one day, with raw CSV and temperature saved, and both level transitions. 5, validate: check each calibration against a held-out sample. These lead to TRL 4, each sensor validated in the lab against defined criteria. Then: define the relevant environment (TRL 5) and integrate one multi-sensor node (TRL 6). PATH FROM THIS WORK TO TRL 4 1 Set limits first Pass/fail limits for accuracy, repeatability and settling, fixed before testing 2 Verify the chain Calibrated multimeter on the 5 V rail and a reference voltage on A0 3 Get standards pH buffers 4, 7, 10; two conductivity standards; turbidity standards 4 Calibrate, repeat Repeat runs on more than one day; raw CSV and temperature saved; both level transitions 5 Validate Check each calibration against a held-out sample TRL 4: each sensor validated in the lab against defined criteria Then: define the relevant environment (TRL 5) and integrate one multi-sensor node (TRL 6)
Figure 12 Path from this work to TRL 4.

About this paper

Version
v1.1, published 21 October 2026
Written by
Nanupatruni Mahesh Kumar
Reviewed by
Ravi Kumar Chintalapudi
Readiness
TRL 3: experimental proof of concept
Tested on
An Arduino Uno R4 WiFi (Renesas RA4M1, 14-bit ADC) with a KIE-TS-300B turbidity module, a generic two-needle TDS module, a PH-4502C pH module and an XKC-Y25-PNP level switch; one bench session per sensor, in dry air and household liquids, read through the board's uncalibrated ADC
Standards
None followed. ISO 10523 (pH), ISO 7888 (conductivity), ISO 7027-1 and IS 3025 (Part 10):2023 (turbidity) are planned for the calibration phase
The paper
21 pages, 1.5 MB, PDF

The full paper has the method, every table, the defects found along the way and what is still needed for the next readiness level.

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