SNAA420A June   2025  – August 2025 HDC3020 , HDC3020-Q1 , HDC3021 , HDC3021-Q1 , HDC3022 , HDC3022-Q1 , HDC3120 , HDC3120-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 Motivation
    2. 1.2 The Physics of Humidity and Water Ingress
  5. 2Test Methodology
  6. 3Assumptions
  7. 4Proposed Algorithm Using Slew Rate Threshold
  8. 5Test Results
    1. 5.1 Test Results at Indoor Ambient Conditions
    2. 5.2 Test Results at Hot and Cold Temperature Conditions
    3. 5.3 Vent Submersion and Air Exchange Tests
  9. 6Summary
  10. 7References
  11. 8Revision History

4 Proposed Algorithm Using Slew Rate Threshold

The proposed method to detect a water ingress event uses a threshold for the slew rate of the RH measurement. The humidity sensor needs to be set to measure RH at a rate of 1-2 samples per second (1-2Hz). Then, the slew rate needs to be calculated over a 10 second window. To compute the 10-second RH slew rate, first ΔRH is obtained by taking the difference between the current RH measurement (RHN) and the measurement from 10 seconds ago (RHN-10). Then, ΔRH is divided by Δtime and multiplied by 1000 to yield the slew rate in units of m%RH/s (milli-percent RH per second), as shown in Equation 2.

Equation 2. R H   S l e w   R a t e     m % R H s e c = Δ R H Δ t i m e = 1000 × ( R H N - R H N - 10 )   10   s e c

This calculation can be made in software, and has a memory requirement of storing RH measurements from the last 10 seconds within a data buffer, such as a FIFO (first-in first-out) buffer. Figure 4-1 shows the proposed algorithm as a the flow chart. The slew rate threshold method was tested across various temperature and humidity conditions, given a steady-state temperature and pressure (as outlined in the Assumptions section).

Water Ingress Detection Algorithm Flowchart Figure 4-1 Water Ingress Detection Algorithm Flowchart

A slew rate threshold needs to be set in the software to alarm when the measured slew rate exceeds the threshold, indicating a water ingress event has occurred. The designed for slew rate threshold depends on factors such as the enclosure size and the required sensitivity (volume of water that must be detected). Based on the data collected, 10m%RH/s was identified as the designed for threshold to reliably detect an ingress event of ≤0.07mL in the given 1.7L enclosure. Tests across various conditions showed that leak events of 0.07mL or more produced a rapid rise in RH well above the chosen threshold, whereas normal ambient humidity fluctuations stayed below the threshold. Similar test procedures can be followed to characterize an appropriate threshold for different system enclosures.

Figure 4-2 shows how the slew rate threshold can be used to detect a 0.07mL water ingress event using data collected for test #2 (22°C, 45%RH, 0.07mL water). The vertical dark blue line marks the ingress event (when water was introduced into the test system), the horizontal black line marks the proposed 10m%RH/s slew rate threshold, and the vertical light blue line marks the ingress alarm event (when the threshold was exceeded). The next section contains the detailed test results and data analysis across the multiple tested conditions.

Test 2 (22°C, 45%RH, 0.07mL water): RH 10-second Slew Rate vs. Time Figure 4-2 Test 2 (22°C, 45%RH, 0.07mL water): RH 10-second Slew Rate vs. Time