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Texas Instruments Capacitive Sensing: Out-of-Phase Liquid Level Technique Application notes
Application Report
SNOA925 – January 2015
Capacitive Sensing: Out-of-Phase Liquid Level Technique
David Wang
ABSTRACT
Various methods have been used to determine the level height in liquid tanks or containers. Recently,
capacitive sensing has gained popularity due to the accuracy and resolution of the measurements as well
as the simplicity, flexibility, and low cost of the sensor design. The sensors in a capacitive-based sensing
system can be either in direct contact with the container (placed on its outside) or can be remote (a few
cm away). The system design is also independent from environmental conditions such as temperature and
humidity and can be made robust against external interferences. The conventional liquid level technique
does have limitations with robustness since any external interference (for example – a human hand)
causes capacitance drifts. An alternative approach to the conventional liquid level technique provides the
necessary barrier to minimize any interference to maximum the signal-to-noise ratio and overall
robustness of the system. This approach is referred to as the Out-of-Phase (OoP) technique. The OoP
technique relies on a symmetrical sensor layout as well as using the shield drivers in a unique way to
stabilize measurements. This application note discusses the use of TI’s FDC1004 capacitive-to-digital
converter with the OoP technique and how it can be implemented in various liquid level applications to
improve performance versus the conventional approach.
spacer
Contents
Conventional Liquid Level Sensing Approach ........................................................................... 3
1.1
Gain and Offset Compensation ................................................................................... 4
2
Problems with the Conventional Approach ............................................................................... 5
3
The Out-of-Phase Liquid Level Approach ................................................................................ 6
3.1
Theory ................................................................................................................ 6
3.2
Sensor Layout ....................................................................................................... 6
4
Experimental Comparison .................................................................................................. 8
4.1
Test Setup ........................................................................................................... 8
4.2
FDC1004EVM GUI Setup ......................................................................................... 9
4.3
Results ............................................................................................................... 9
5
Conclusion .................................................................................................................. 10
Appendix A
Liquid Level Approach ............................................................................................ 11
1
List of Figures
..........................................................................................
1
Ratiometric Measurement Setup
2
Capacitance Measurements for LEVEL, RL, and RE Electrodes ..................................................... 4
3
Comparison of the Conventional Electrical Model With and Without Human Body Presence..................... 5
4
Comparison of Conventional Approach and OoP Electrical Model
5
6
7
8
9
10
...................................................
OoP Technique Sensor Layout for LEVEL and REF Sections ........................................................
Side View of SHLD Electrode Height Compared to RL Electrode Height ............................................
Test Setup for the OoP Technique ........................................................................................
Sensor Layout and Connections to FDC1004EVM .....................................................................
FDC1004EVM GUI Configuration Settings ...............................................................................
Level Calculation Comparison .............................................................................................
3
6
7
7
8
8
9
9
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List of Tables
1
2
3
2
........................................................
Conventional Approach Experimental Result ...........................................................................
OoP Approach Experimental Results ....................................................................................
OoP and Conventional Liquid Level Technique Comparison
Capacitive Sensing: Out-of-Phase Liquid Level Technique
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11
12
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Conventional Liquid Level Sensing Approach
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1
Conventional Liquid Level Sensing Approach
Liquid level sensing is based on the theory of a ratiometric measurement, using three sensors as shown in
Figure 1:
1. LEVEL – The capacitance of the LEVEL electrode is proportional to the liquid height (hw). It has to be
as high as the maximum (MAX) allowed liquid level.
2. REFERENCE LIQUID (RL) – The REFERENCE liquid electrode accounts for the interval unit
measurements of the LEVEL electrode. The liquid level has to be higher than the RL height in order to
have a liquid and temperature independent measurement system.
3. REFERENCE ENVIRONMENT (RE) – A second (optional) reference electrode accounts for container
properties. It has to be placed above the maximum (MAX) allowed level of liquid to track environmental
factors and not the primary target (the liquid in the container).
A key aspect of this approach is that all three sensors are driven with the same excitation signal.
Changes in the excitation signal due to changing capacitance are measured and used to calculate
the corresponding liquid level.
Ground
MAX
SHIELD
hL
hW
hR
RL
CINx
GND
GND
RE
LEVEL
hR
GND
Z Y
X
Figure 1. Ratiometric Measurement Setup
The working principle of the liquid level sensing involves measuring the fringing capacitance between the
primary LEVEL electrode (CINx) and a ground (GND) electrode in the parallel fingers topology. The
fringing capacitance becomes a function of the dielectric variation in the x-axis direction, and proportional
to the liquid height, as given by Equation 1:
Cmeas µ hw e w + (hL - hw )ea
(1)
Where:
hL = maximum height of the liquid
hw = height of liquid
εw = dielectric of liquid
εa = dielectric of air
To calculate the level of the liquid at any interval height, use Equation 2:
C
- Clevel (0)
Level = hRL level
CRL - CRE
(2)
Where:
hRL = the unit height of the reference liquid sensor (often 1)
Clevel = capacitance of the LEVEL sensor
Clevel(0) = capacitance of the LEVEL sensor when no liquid is present (empty)
CRL = capacitance of the REFERENCE liquid sensor
CRE = capacitance of the reference environmental sensor
NOTE: If RE is not used in the system, replace CRE with CRL(0) in Equation 2.
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Conventional Liquid Level Sensing Approach
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Figure 1 also illustrates the use of a shield behind both electrodes, which focuses the sensing direction
toward the liquid target and provides a barrier from any interference affecting the measurements from the
backside. The FDC1004 features two dedicated shield drivers which can drive up to 400-pF capacitance
each. The shield is driven with the same excitation signal as the other sensors. Because it is charged to
the same potential as the other sensors, there is no electric field on the shield side of the sensors, so the
only active field is in the direction of the liquid. The sensor size of RE should be the same size as RL so
the measurements can be subtracted from one another. If the sensor sizes are not matched, a differential
measurement cannot be performed since fringing capacitance is not linear/proportional to area size (unlike
the parallel plate form).
Figure 2 illustrates an example of the capacitance of the LEVEL and REFERENCE electrodes based on
the liquid level height. The capacitance of the LEVEL electrode (green line) increases linearly as the liquid
level increases. Once the liquid level is above the hR height, the RL capacitance saturates and becomes
constant (blue line). The reference empty (red line) shows the behavior of the RE electrode and any
change from environmental factors with this electrode can be used to eliminate the change seen on the
level and RL electrode.
Figure 2. Capacitance Measurements for LEVEL, RL, and RE Electrodes
1.1
Gain and Offset Compensation
Even though the capacitance measurements are proportional to the liquid level height, the calculated level
compared to the actual liquid level can vary dramatically. This is due to variations in the LEVEL, RL, and
RE electrode capacitances for each liquid level interval. Gain and offset compensation, typical for the
system-under-measure, is necessary in order to match the actual with the measured levels. A first-order
linear correction algorithm (Equation 3) can be applied to compensation for the variations:
Level' = Level × Gain + Offset
(3)
The FDC1004 allows gain and offset compensation-per-measurement and can be changed in real-time to
adjust for system-environment conditions.
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Problems with the Conventional Approach
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2
Problems with the Conventional Approach
The conventional approach of driving all the sensors and shield with the same excitation signal works well
when the system is isolated from any external noise or interference, but any grounded interference or EMI
causes significant deviations in capacitance measurements. These large changes caused by interference
cannot be distinguished from small or large changes in liquid level, ultimately compromising the accuracy
and reliability of the system.
The electrical model of the liquid level system contains the capacitance/resistance of the water and the
capacitance of the container (shown in Figure 3) from the CINx (LEVEL) electrode to the GND electrode.
The measured capacitance as liquid level increases should be linear. Once the human body presence
(human hand) is in close proximity to the liquid source, an additional parasitic capacitance is introduced
into the model and causes the potential difference due to the liquid to change relative to the absence of
the hand. This potential difference corresponds to disturbances (as shown in the graph on the right in
Figure 3) along the linear data plot. An alternative approach to mitigate this additional parasitic
capacitance is the Out-of-Phase (OoP) technique.
Conventional Electrical Model with Human
Body Capacitance Effects (Proximity or Touch)
Conventional Electrical Model
CW
CW: Water Capacitance
RW: Water Resistance
CP: Container Capacitance
CH: Container Capacitance
CW
H2O
RW
CW
RW
RW
CP
CP
CP
CINx
CINx
Expected Cmeas vs Liquid Level Function
Effect of Human Body That Gets Close to the Liquid
1.2
1.2
1
1
0.8
0.8
Cmeas [pF]
Cmeas [pF]
CW
H2O
RW
CP
CH
0.6
0.4
0.2
0.6
0.4
0.2
0
0
0
20
40
60
80
100
0
Liquid Level Container Height [%]
20
40
60
80
100
Liquid Level Container Height [%]
Figure 3. Comparison of the Conventional Electrical Model With and Without Human Body Presence
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The Out-of-Phase Liquid Level Approach
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3
The Out-of-Phase Liquid Level Approach
3.1
Theory
The OoP technique relies on a symmetrical sensor layout as well as using the shield drivers in a unique
way to counteract the effects of the human body capacitance and stabilize measurements. In the
conventional approach, the liquid experiences a voltage potential difference to GND. In the OoP
technique, the liquid potential is kept constant during the excitation/drive phases by using a differential
capacitive measurement, thus eliminating the human body capacitance effects from the measurements.
Figure 4 shows the comparison of the conventional and OoP electrical model. The OoP technique takes
advantage of the unique features of the FDC1004 to drive a CINx electrode and a SHLDy electrode in
differential mode to make the voltage potential at node CH fixed. The SHLDy electrode takes the place of
the GND electrode and is actively driven. Specifically, the FDC1004 is configured for differential mode
(CINx–CINy), for example CIN1–CIN4, in this case. By default, SHLD1 is in-phase with CIN1 and SHLD2
is in-phase with CIN4. Because CIN1 and CIN4 are 180 degrees out of phase with respect to each other,
node CH is maintained at a constant potential. See the FDC1004 datasheet (SNOSCY5) for more
information about differential mode configurations and how the shields are paired with the channels.
Conventional Approach
CP
CINx
RW
CW
Out-of-Phase Counteraction
CP
CINx
RW
C
W
FDC1004
H2O
CH
FDC1004
H2O
CH
CW
RW
CW
RW
SHLDy
GND
CP
CP
Figure 4. Comparison of Conventional Approach and OoP Electrical Model
3.2
Sensor Layout
The OoP technique is effective because the capacitance towards the liquid seen by the in-phase and the
out-of-phase excitation/driver signal is the same. This approach relies heavily on symmetry of the channel
and shield electrodes. If there is any mismatch, the liquid will not be at a constant potential. Symmetry is
the key. Figure 5 shows the sensor layout that incorporates shield barriers on the backside of the
electrodes. OoP works because the FDC1004 can be configured for differential mode. Most other
capacitive-to-digital converters cannot be configured this way.
To implement the OoP method of liquid level sensing using the FDC1004, the following sensor
assignments can be used:
• CIN1 – LEVEL electrode
• CIN2 – REFERNCE LIQUID electrode (RL)
• CIN4 – Floating, no electrode attached
FDC1004 measurements are configured as follows:
• Meas1 = CIN1 (CHA) – CIN4 (CHB). CIN1 is set as the positive input channel, and CIN4 is set as the
negative input channel.
• Meas2 = CIN2 (CHA) – CIN4 (CHB). CIN2 is set as the positive input channel, and CIN4 is set as the
negative input channel.
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With Meas1 and Meas2 in differential mode, CIN1/2 is in-phase with SHLD1 and CIN4 is in-phase with
SHLD2. CIN1/2 and CIN4 are out-of-phase by 180 degrees. The SHLD2 electrode adjacent to CHx needs
to be shielded by another SHLD2 electrode adjacent to SHLD1 to match in-phase and out-of-phase
excitation/drive symmetry, shown in Figure 5.
Liquid
Container Wall
CHx
SHLD2
SHLD1
SHLD2
Figure 5. OoP Technique Sensor Layout for LEVEL and REF Sections
To allow further symmetry, SHLD1 and SHLD2 (furthest away from the liquid) are exactly the same size
as the SHLDs for the LEVEL electrode (shown in Figure 6). SHLD1 and SHLD2 are shared between
LEVEL and RL. Because the FDC1004 samples the capacitance channels sequentially, when it reads the
capacitance for the LEVEL measurement, the RL electrode is floating but the SHLD1 and SHLD2 paired
with the RL section are connected during the LEVEL measurement. Creating symmetry between the
LEVEL and RL sections is as important as symmetry within each measurement section.
SHLD1/2
RL
hSHLDx = hLEVEL
Shield Height for RL Electrode is the
Same Height as LEVEL
Figure 6. Side View of SHLD Electrode Height Compared to RL Electrode Height
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Experimental Comparison
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4
Experimental Comparison
4.1
Test Setup
Two liquid level setups were compared experimentally: the conventional and OoP approach. The main
focus of this experiment was to determine how much the human hand (in proximity or in contact with the
container with liquid) affected capacitance measurements for the two methods. Both sensor layouts were
attached to separate plastic rectangular containers (Figure 7 shows container used for the OoP method).
The FDC1004 measurements were collected using the FDC1004EVM and EVM GUI. At specific liquid
level heights, measurements were taken from the baseline condition (no hand interference). The
measurements were then repeated by placing a hand at fixed distances away from the container,
including touching the container. Both the LEVEL and RL (also referred to as REF) measurements were
collected and the calculated level height and error from baseline conditions were computed (see Appendix
A for measurement analysis). Water was used as the liquid to obtain the measurements.
Figure 7. Test Setup for the OoP Technique
SHLD2
SHLD1
SHLD1
Insulator
SHLD2
SHLD2
Insulator
LEVEL t CIN1
REF t CIN2
SHLD2
GND
SHLD1
CIN1
CIN2
SHLD1
SHLD2
CIN3
GND
CIN4
SHLD2
EVM Connector
Figure 8. Sensor Layout and Connections to FDC1004EVM
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Experimental Comparison
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4.2
FDC1004EVM GUI Setup
The FDC1004EVM was used to obtain the capacitance measurements to calculate the height of the liquid
level. MEAS1 and MEAS2 were setup for differential mode (MEAS1 = CH1 – CH4, MEAS2 = CH2 – CH4).
Figure 9. FDC1004EVM GUI Configuration Settings
NOTE: CDC configuration measurement rate and uC sampling rate can be changed based on the
system requirements.
4.3
4.3.1
Results
Liquid Level Data
From the capacitance measurements seen in Table 2, the level height of the water was calculated using
the level equation in Section 1. Figure 10 illustrates the difference in calculated levels from the actual
levels. As the water level increases, the error in the calculated level gets significantly worse due to
variations in LEVEL and REF capacitance for each level interval. A first-order linear correction algorithm
was applied with a gain and offset setting of 1.23 and –0.6, respectively. The green and blue plot in
Figure 10 illustrates that the corrected levels matches the actual levels fairly well, compared to the original
calculated level.
Figure 10. Level Calculation Comparison
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Conclusion
4.3.2
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Parasitic Capacitance Interference
The conventional method exhibited a maximum error of ≈11% whereas the OoP method showed a
maximum error of only ≈0.5%, which occurred when the water level was maximum. Table 1 shows the
LEVEL measurements for the water level at 10 cm. The parasitic capacitance introduced by the human
hand was reduced significantly.
Table 1. OoP and Conventional Liquid Level Technique Comparison
10-cm
Water Level
4.3.3
Change in Cap From Baseline (fF)
Calculated Level Error (%)
Hand Distance
(cm)
Conventional
OoP
Conventional
OoP
5
157.5
0.7
-2.51
0.03
4
180.3
0.8
-2.75
0.14
3
218.5
1.7
-3.28
0.09
2
275.7
1.3
-4.06
0.15
1
380.0
1.6
-5.13
0.20
0
1000.5
-1.3
10.34
0.50
System Considerations
Since the experimental setup was constructed using manually cut copper tape, it is possible to further
reduce the error using more precise PCB layout rather than copper tape. Another variation from this setup
that caused deviations in calculated level measurements compared to the expected level was matching
the actual liquid level height to the expected level height when pouring the liquid into the container. The
FDC1004 can detect changes in capacitance less than 1mm of height, so any variations above or below a
level will be detected. A syringe can be used to accurately dispense the appropriate volume of water
based on specific height intervals for experimental setup consistency.
NOTE:
5
Water is the liquid used in this experiment. Liquids that cannot be used for liquid-level
sensing include oil-based liquids, soap, or a liquid with a sticky or slick residue.
Conclusion
From the experimental data, the OoP method has a significant robustness advantage compared to the
conventional method. With approximately 0.5% calculated error measurements from the OoP method,
liquid level sensing applications are able to accurately determine the height of the liquid at a high
resolution and confidence. Keeping the potential of the liquid constant allows the capacitance
measurements to be undisturbed from external interference. The conventional approach is still a valid
implementation if the system is isolated from any interference, but for most liquid level sensing
applications, the OoP method is the optimal solution.
10
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Appendix A
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Liquid Level Approach
Table 2. Conventional Approach Experimental Result
Conventional Liquid Level Approach
LEVEL MEAS
Water
Level
(cm)
0
1
3
5
10
Change in
Cap From
Baseline
(fF)
REF MEAS
Error From
Hand Based
Capacitance
on Change
(pF)
in Water
Level (%)
Change in
Cap From
Baseline
(fF)
Error From
Calculated
Hand Based on Level (cm)
Change in Water
Level (%)
Error
(%)
Hand
Distance
Capacitance
(pF)
Baseline
2.1712
5cm
2.1781
6.9
1.0052
3.6
4cm
2.1797
8.5
1.0059
4.3
3cm
2.1807
9.5
1.0068
5.2
2cm
2.1823
11.1
1.0072
5.6
1cm
2.1843
13.1
1.0081
6.5
Touch
2.1898
18.6
1.0098
8.2
Baseline
2.7135
1.3806
0
5cm
2.7424
28.9
5.3292
1.4152
20.8
5.2953
1.381
0.03
4cm
2.7478
34.3
6.3249
1.419
24.6
6.2627
1.3814
0.06
3cm
2.7537
40.2
7.4129
1.4232
28.8
7.332
1.3816
0.08
2cm
2.7624
48.9
9.0171
1.4301
35.7
9.0886
1.3797
-0.07
1.0016
1.3944
1cm
2.7784
64.9
11.9675
1.4421
47.7
12.1436
1.3784
-0.16
Touch
2.8235
110
20.284
1.4777
83.3
21.2067
1.3701
-0.76
Baseline
3.5097
5cm
3.5667
57
4.2585
1.4216
22.9
4cm
3.5768
67.1
5.0131
1.4254
3cm
3.5906
80.9
6.0441
1.4312
2cm
3.6157
106
7.9193
1cm
3.6456
135.9
10.1532
Touch
3.8275
317.8
23.743
Baseline
4.2265
5cm
4.3096
83.1
4.0432
4cm
4.3271
100.6
3cm
4.3544
127.9
2cm
4.3873
1cm
3.3707
0
5.7668
3.3226
–1.43
26.7
6.7237
3.3167
–1.6
32.5
8.1843
3.304
–1.98
1.441
42.3
10.6522
3.2874
–2.47
1.453
54.3
13.6741
3.2663
–3.1
1.5279
129.2
32.5359
3.1471
–6.63
5.3219
0
1.4109
23.1
5.9814
5.2245
–1.83
4.8947
1.4155
27.7
7.1724
5.2087
–2.13
6.2229
1.4233
35.5
9.1921
5.1771
–2.72
160.8
7.8237
1.4326
44.8
11.6002
5.1418
–3.38
4.4574
230.9
11.2344
1.452
64.2
16.6235
5.0759
–4.62
Touch
4.7699
543.4
26.439
1.5381
150.3
38.9177
4.8438
–8.98
Baseline
5.9343
5cm
6.0918
157.5
4.1854
1.4123
26.4
4cm
6.1146
180.3
4.7913
1.4157
3cm
6.1528
218.5
5.8064
1.422
2cm
6.21
275.7
7.3264
1cm
6.3143
380
Touch
6.9348
1000.5
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1.3987
1.3878
1.3859
9.7921
0
6.8696
9.5461
–2.51
29.8
7.7544
9.5228
–2.75
36.1
9.3937
9.471
–3.28
1.4315
45.6
11.8657
9.3947
–4.06
10.0981
1.4476
61.7
16.0552
9.2895
–5.13
26.5871
1.5442
158.3
41.1918
8.7792
–10.34
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Appendix A
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Table 3. OoP Approach Experimental Results
OoP Liquid Level Approach
LEVEL MEAS
Water
Level
(cm)
0
1
3
5
10
12
Change in
Cap From
Baseline
(fF)
REF MEAS
Error From
Hand Based
on Change
in Water
Level (%)
Capacitance
(pF)
Change in
Cap From
Baseline
(fF)
Hand
Distance
Capacitance
(pF)
Baseline
2.1615
5cm
2.1619
0.4
0.6251
0.1
4cm
2.1623
0.8
0.6252
0.2
3cm
2.1627
1.2
0.6256
0.6
2cm
2.1631
1.6
0.6257
0.7
1cm
2.1633
1.8
0.6259
0.9
Touch
2.1639
2.4
0.6263
1.3
Baseline
2.7554
5cm
2.7555
0.1
0.0168
1.1116
4cm
2.7558
0.4
0.0674
3cm
2.7561
0.7
0.1179
2cm
2.7564
1
1cm
2.7564
Error From Hand Calculate
Based on Change d Level
(cm)
in Water Level
(%)
Error
(%)
0.625
1.1113
1.2213
0
0.3
0.0617
1.2207
–0.04
1.1123
1
0.2056
1.2196
–0.14
1.1119
0.6
0.1234
1.2212
–0.01
0.1684
1.1119
0.6
0.1234
1.2218
0.04
1
0.1684
1.1123
1
0.2056
1.2208
–0.04
2.2
0.3704
1.1141
2.8
0.5758
1.2188
–0.2
Touch
2.7576
Baseline
3.9345
5cm
3.9344
-0.1
-0.0056
1.2201
4cm
3.9354
0.9
0.0508
1.2198
3cm
3.9364
1.9
0.1072
1.22
2cm
3.9363
1.8
0.1015
1.2201
1cm
3.9373
2.8
0.1579
Touch
3.9393
4.8
0.2707
Baseline
5.0393
5cm
5.0394
0.1
0.0035
1.2399
-0.1
4cm
5.0403
1
0.0347
1.2397
-0.3
3cm
5.0408
1.5
0.0521
1.2404
2cm
5.0401
0.8
0.0278
1.2403
1cm
5.0409
1.6
0.0556
Touch
5.0417
2.4
0.0834
Baseline
7.7165
5cm
7.7172
0.7
0.0126
1.276
-0.1
4cm
7.7173
0.8
0.0144
1.2753
3cm
7.7182
1.7
0.0306
1.2757
2cm
7.7178
1.3
0.0234
1cm
7.7181
1.6
Touch
7.7152
-1.3
1.2198
2.9808
0
0.0504
2.9792
–0.06
0
0
2.9823
0.05
0.2
0.0336
2.983
0.07
0.3
0.0504
2.9824
0.05
1.2206
0.8
0.1345
2.9815
0.02
1.2208
1
0.1681
2.9839
0.1
0.3
1.24
4.6793
0
-0.0163
4.6803
0.02
-0.0488
4.6833
0.08
0.4
0.065
4.6787
–0.01
0.3
0.0488
4.6784
–0.02
1.2394
-0.6
-0.0976
4.6865
0.15
1.2386
-1.4
-0.2276
4.6939
0.31
1.2761
8.5317
0
-0.0154
8.5341
0.03
-0.8
-0.1229
8.5434
0.14
-0.4
-0.0614
8.5396
0.09
1.2753
-0.8
-0.1229
8.5442
0.15
0.0288
1.275
-1.1
-0.1689
8.5486
0.2
-0.0234
1.2727
-3.4
-0.5222
8.5745
0.5
Capacitive Sensing: Out-of-Phase Liquid Level Technique
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SNOA925 – January 2015
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