Figure 1:
(a) IDC 1-1-1 sensor structure, (b) IDC 1-3-1 sensor structure. IDC, inter-digital capacitive.
Figure 2:
Electric field distribution for (a) 1-1-1 SS, (b) 1-1-1 SSWS, (c) 1-1-1 DS, (d) 1-3-1 SS, (e) 1-3-1 SSWS and (f) 1-3-1 DS IDC sensors.
Figure 3:
Equivalent circuit (a) 1-1-1 IDC structure, (b) 1-3-1 IDC structure. IDC, inter digital capacitive.
Figure 4:
Sensor Fabrication procedure: (a) Dimension of PCB, (b) Print on photopaper, (c) Print on PCB after thermal press.
Figure 5:
Fabricated IDC sensor (a) 1-1-1 (b) 1-3-1. IDC, inter digital capacitive.
Figure 6:
Experimental set up.
Figure 7:
Normalized sensor response for various structures of 1-1-1 with different concentrations (a) 2-FAL (b) Moisture. 2-FAL, 2-furfuraldehyde.
Table 1:
Sensitivity of 1-1-1 configuration
| IDC Sensor | Sensitivity | |
|---|---|---|
| 2-FAL (fF/ppm) | Moisture (fF/ppm) | |
| SS | 40 | 29 |
| SSWS | 72 | 52 |
| DS | 96 | 72 |
[i] DS, double side; 2-FAL, 2-furfuraldehyde; IDC, inter digital capacitive; SS, single side; SSWS, single side with shielding.
Figure 8:
Normalized sensor response for various structures of 1-3-1 with different concentrations (a) 2-FAL (b) Moisture. 2-FAL, 2-furfuraldehyde.
Table 2:
Sensitivity of 1-3-1 configuration
| IDT sensor | Sensitivity | |
|---|---|---|
| 2-FAL (fF/ppm) | Moisture (fF/ppm) | |
| SS | 36 | 28.2 |
| SSWS | 64 | 47.74 |
| DS | 92 | 70.6 |
[i] DS, double side; 2-FAL, 2- furfuraldehyde; IDC, inter digital capacitive; SS, single side; SSWS, single side with shielding.
Table 3:
Sensor calibration for 2-FAL in (1-1-1) structure
| Structure type | Linear fit equation | Coefficient of determination (R2) | Max error (%) |
|---|---|---|---|
| SS | y = 0.0371x + 16.728 | R2 = 0.9606 | 1.1 |
| SSWS | y = 0.073x + 26.066 | R2 = 0.992 | 0.61 |
[i] 2-FAL, 2- furfuraldehyde; SS, single side; SSWS, single side with shielding.
Table 4:
Sensor calibration for 2-FAL in (1-3-1) structure
| Structure type | Linear fit equation | Coefficient of determination (R2) | Max error (%) |
|---|---|---|---|
| SS | y = 0.0379x + 12.664 | R2 = 0.9884 | 0.76 |
| SSWS | y = 0.0667x + 17.747 | R2 = 0.9667 | 1.6 |
| DS | y = 0.0923x + 21.801 | R2 = 0.9288 | 2.2 |
[i] DS, double side; 2-FAL, 2- furfuraldehyde; SS, single side; SSWS, single side with shielding.
Table 5:
Sensor Calibration for moisture in (1-1-1) structure
| Structure type | Linear fit equation | Coefficient of determination (R2) | Max error (%) |
|---|---|---|---|
| SS | y = 0.0269x + 16.545 | R2 = 0.9606 | 0.83 |
| SSWS | y = 0.0527x + 25.817 | R2 = 0.992 | 0.45 |
| DS | y = 0.0736x + 30.293 | R2 = 0.9278 | 1.4 |
[i] DS, double side; SS, single side; SSWS, single side with shielding.
Table 6:
Sensor calibration for moisture in (1-3-1) structure
| Structure type | Linear fit equation | Coefficient of determination (R2) | Max error (%) |
|---|---|---|---|
| SS | y = 0.0297x + 12.45 | R2 = 0.9884 | 0.61 |
| SSWS | y = 0.0497x + 16.811 | R2 = 0.9794 | 1.1 |
| DS | y = 0.0708x + 20.57 | R2 = 0.9288 | 2.1 |
[i] DS, double side; SS, single side; SSWS, single side with shielding.
Figure 9:
Sensor response for 20 ppm moisture for (a) 1-1-1 structure (b) 1-3-1 structure.
Table 7:
Comparison of developed sensor with similar sensors
| Ref. | This work | [6], [7] | [26] | [14] | [5] | [15] | [16] |
|---|---|---|---|---|---|---|---|
| Method | IDC | Cross capacitive | Cross capacitive | Thin film based MIP | Thin film parallel plate | Optical | Optical |
| Remark | Simple and exact | Simple and exact | Simple and exact | Complex | Simple | Complex | Complex |
| Usage | Insulating oil degradation detection | Insulating oil degradation detection | Non-contact Microdroplet detection | 2-FAL detection in transformer oil | Detection of moisture in oil | Detection of moisture in oil | Detection of 2-FAL in transformer oil |
| Range | 0–50 ppm | 0–60 ppm | ……… | 0–20 ppm | 0–90 ppm | 0–5% | 0–2 ppm |
| Relative humidity | |||||||
| Contactless | Yes | Yes | Yes | No | No | No | No |
| Accuracy | High, depends on sensor length | High, depend on single length | High, depend on single length | Moderate | Moderate | Moderate | Moderate |
| Re-useable | Many times | Many times | Many times | Only 1 time | Few time | Few time | Few time |
| Sensitivity | 72 fF/ppm in moisture; 96 fF/ppm in 2-FAL | 0.303fF/ppm in moisture, 0.59 fF/ppm in 2-FAL | ………. | 0.084 pF/ppm | 6 pF/ppm | ………. | ………. |
[i] 2-FAL, 2- furfuraldehyde; IDC, inter digital capacitive; MIP, molecularly imprinted polymer.