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Novel Dew Point Hygrometer Fabricated with Inkjet Printing Technology
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 120 (2015) 1099 – 1102
EUROSENSORS 2015
Novel dew point hygrometer fabricated with inkjet printing technology Grzegorz Tarapataa*, Michał Marzęckia, Rafał Selmaa, Daniel Paczesnya, Ryszard Jachowicza a
Institute of Electronic Systems, Warsaw University of Technology, Nowowiejska 15/19, Warsaw 00-665, Poland
Abstract
This paper reports a novel, first time made Dew Point Hygrometer (DPH) detector, which was fully fabricated with the inkjet printing technology. This type of the sensor operation was already known in DPH which is based on a silicon complex dew detector [1]. However, in our device the silicon substrate has been replaced by a thin, flexible Kapton foil. All components integrated in the detector: the interdigitated capacitive sensor electrodes, the thermoresistor and the heater were made by ink-jet printing technology. All individual elements of the printed dew point hygrometer detector have been characterized, and finally, their measurement features were described. © Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ©2015 2015The TheAuthors. Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROSENSORS 2015. Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
Keywords: dew point, hygrometer, humidity, inkjet printing, sensor
1. Introduction Air humidity describes the amount of water vapour in air. Even the marginal small values of humidity can have significant influence on living organisms or inanimate objects. But the greatest impact it has on technological processes, therefore measuring and regulating it is often of high importance. Air humidity is most commonly described by either of two values – dew point temperature or relative humidity [2, 3]. In a hygrometers the dew point (DP) temperature is the temperature in which the condensation of water vapour begins on a surface of that temperature.
* Corresponding author. Tel.: +48 22-234-7820; fax: +48 22-825-2300. E-mail address: [email protected]
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
doi:10.1016/j.proeng.2015.08.788
1100
Grzegorz Tarapata et al. / Procedia Engineering 120 (2015) 1099 – 1102
Higher DP temperature means that the air is more humid. The DPH must consists of a detector to detect the instant when the water vapor condense on the surface at the dew point temperature as well as a temperature sensor and a heater [4]. They are used to measure the surface temperature of the detector and to heat up the detector for condensed water evaporation. The principle of operation of such a dew point hygrometer is based on gradually cooling the surface of the sensor, while the impedance comb detector must checks if the water condense on the surface. When this state occurs, the measured surface temperature is taken and treated as a gas humidity value. Finally, the condensed water is evaporated by heating up with the heater and the whole process is repeated continuously. It is worth to point out, that the DP temperature is the absolute unit of gas humidity. Moreover, typically the DP hygrometers are much more accurate than RH meters. 2. Sensor design and fabrication The humidity sensor described in this paper was manufactured using Inkjet Printing technology. This technology uses printers similar to office inkjet printers, but they are more versatile, offers more precise printouts which are desirable in printed electronics [5, 6]. This rapidly evolving technology allows production of sensors considerably cheaper than traditional silicon-based ones [1], and is expected to take over areas where big amounts of flexible, cheap and not too complex sensors is required. The hygrometer structure presented it the paper was printed as a double layer structure, where the bottom layer of the film substrate incorporates a heater and a thermoresistor. While on top layer the interdigitated comb electrodes were formed. All components were printed on 12.5μm flexible Kapton foil. The design and printout close up of hygrometer structure are shown in Fig. 1 and 2. Active area of the detector has dimensions of 8 mm per 11.3 mm, where the comb electrodes have a width and space in between as well of 100 μm. The heater and thermoresistor were designed to have 300 μm and 75 μm respectively. The printing process was carried out using the digital material printer Dimatix DMP2831 and with conductive silver ink EDM5603. Fully functioned structure of the dew-point hygrometer detector was achieved by placing printed structure on a Peltier cell (Fig. 3). b)
a)
contact pads
heater
thermoresistor
impedance comb detector
contact pads
Figure 1: The design of a) the bottom side: the heater and the thermoresitor, b) the top side: the impedance comb detector (contacts not to scale)
a)
b)
c)
Figure 2: Close-ups of: a) manufactured dew detector, b) meander-like shape of the thermistor and of the heater, c) printed DPH structure with long contact pads
1101
Grzegorz Tarapata et al. / Procedia Engineering 120 (2015) 1099 – 1102 Top: comb dew detector Bottom: heater and thermoresistor
Kapton substrate
Peltier couple radiator
Figure 3: Cross section of the hygrometer with the inkjet printed dew point detector
3. Tests of the printed hygrometer structure A series of experiments was carried out on the DP sensors in order to characterize all sensor components and to investigate its behaviour under working conditions. DP printed sensor was tested on automated measurement stand, consisting of: a Peltier cell placed on a radiator, a PT100 thermistor placed on said Peltier cell for reference temperature measurement, a power source powering the Peltier cell and the multimeters allowing the resistance reading of both PT100 and printed thermoresistor. A precise RLC meter was used to measure comb detector’s parallel capacitance (Cp) and resistance (Rp). Additionally, a humidity generator was used to generate gas of given humidity and reference dew point meter (Michell Instruments S8000 Integrale) to verify generated gas humidity. All meters and gas generator were controlled via a PC computer. 4. Results and discussion Measured resistance of the heater and thermoresistor was 150 Ω and 740 Ω respectively at 22 °C. The thermoresitor was calibrated with usage of climate chamber, and its characteristic was linear in operational temperature range. The determined sensitivity was 655 mΩ/K and it is good enough to measure temperature of the sensor surface. Since the dew sensor is located on the top side of the foil but the thermoresistor on the bottom side, temperature difference between top and bottom side of the foil could be expected. Because the temperature measurement accuracy is the crucial parameter of the DPH we decided to measure if the temerature difference is big enough to be taken into account additional structure featuring heater and thermistor on both top and bottom side of the foil was manufactured. This structure allowed to measure temperatures gradient through the foil in a simulated hygrometer cycle. Results of such experiment is shown in Fig. 4 and 5. The temperature gradient varies from 0.2 K to 0.6 K. 24
0,7
Top side thermistor
23
0,6
Bottom side thermistor
22
0,5
temperature [°C]
temperature [°C]
21 20 19 18 17
16
0,3 0,2 0,1
15 14 1000
0,4
1005
1010
1015
1020
1025
1030
time [s]
Figure 4. Measured temperature on the top and bottom side of the sensor substrate during cooling and heating cycle
0 1000
1005
1010
1015
1020
1025
1030
time [s]
Figure 5. Temperature gradient between top and bottom side of sensor during cooling and heating cycle
To heat up and evaporate condensed water, the heater is supplied with voltage of 14 V, which for heater resistance of around 150 Ω gives an average dissipation power of 750 mW during one measurement cycle. It allows to increase the sensor temperature of about 12 °C even when the Peltier cell is active. In real DPH operation we increase the detector temperature of some 2 K by heating which means the dissipation power is much lower and temperature gradient between two sides of the foil has also lower difference.
1102
Grzegorz Tarapata et al. / Procedia Engineering 120 (2015) 1099 – 1102
Figure 6 shows an exemplary static characteristic of the detector impedance versus temperature during water condensation for gas humidity of 13 °C of DP temperature. Finally, the impedance threshold for the dew detection in dynamic condition during regular running of hygrometer was arbitrary chosen as 25% of its amplitude. It is a greater value than is consistent with the static characteristics. Result of regular operation of DPH is shown in Fig. 7. Obtained humidity inaccuracy was estimated as ΔT=0,4 K, and measurement time constant of 2s. Applied algorithm running at PC usage of external measurement devices allows to perform humidity readings even every 2 ÷ 3 seconds. 25
3500
23
3000
|Z| ]kΩ]
2500
temperature [°C]
T
2000 1500
Z th
1000
instantaneous sensor surface temperature
21
dew point temperature
19
17 15 13
500
11
0 0
5
10
15
20
25
30
detector temperature [°°C]
Figure 6: The impedance characteristic of the detector during cooling. Zth is the impedance threshold for the dew condensation.
9 0
10
20
30
40
50
60
70
80
time [s]
Figure 7. Detector surface temperature oscillations in regular operation during for gas humidity of 13.0 °C
Summary In conclusion, integrated dew point hygrometer was successfully fabricated fully with the inkjet printing technology. For the mature printing technology such structure can replace classical silicon dew point hygrometer. It's properties are comparable to typical hygrometer, but the technology is significantly cheaper. During operation cycle of DPH a temperature gradient through the substrate up to 0.6 °C was observed. Obviously, the gradient strongly depends on the material type and its thickness. To decrease inaccuracy of temperature measurement a thinner foil or higher thermal conductivity of it is required. From the other side, since the DP temperature is taken at the lowest temperature during cooling process, a constant value of around 0.6 °C could be subtracted. References [1] R.Jachowicz, J.Weremczuk, D.Paczesny, G.Tarapata: "A MEMS-based super fast dew point hygrometer - construction and medical applications", Meas. Science & Technology, IoP124008, vol. 20, No.122009, pp. 1-10 [2] J.Weremczuk, G.Tarapata, R. Jachowicz: "The ink-jet printing humidity sorption sensor - modelling, design, technology and characterization", Measurement Science & Technology, IoP, 014003,vol. 23,No.1, 2012, 10 pages [3] Milind V. Kulkarni, Sanjay K. Apte, et al. " Ink-jet printed conducting polyaniline based flexible humidity sensor", Sensors and Actuators B: Chemical, Volume 178, Issue null, Pages 140-143 [4] Jachowicz, R. S; Dew point hygrometer with heat injection—principle of construction and operation, Sensors and Actuators B: Chemical,7,1,455-459,1992, Elsevier. [5] Andersson, H.; Lidenmark, C.; Ohlund, T.; Ortegren, J.; Manuilskiy, A.; Forsberg, S.; Nilsson, Hans-Erik, "Evaluation of Coatings Applied to Flexible Substrates to Enhance Quality of Ink Jet Printed Silver Nano-Particle Structures," Components, Packaging and Manufacturing Technology, IEEE Transactions on , vol.2, no.2, pp.342-348, Feb. 2012. [6] Wu, S.P.; Yung, K. C.; Xu, L. H.; Ding, X.H., "Fabrication of Polymer Silver Conductor Using Inkjet Printing and Low Temperature Sintering Process," Electronics Packaging Manufacturing, IEEE Transactions on , vol.31, no.4, pp.291-296, Oct. 2008.
ScienceDirect Procedia Engineering 120 (2015) 1099 – 1102
EUROSENSORS 2015
Novel dew point hygrometer fabricated with inkjet printing technology Grzegorz Tarapataa*, Michał Marzęckia, Rafał Selmaa, Daniel Paczesnya, Ryszard Jachowicza a
Institute of Electronic Systems, Warsaw University of Technology, Nowowiejska 15/19, Warsaw 00-665, Poland
Abstract
This paper reports a novel, first time made Dew Point Hygrometer (DPH) detector, which was fully fabricated with the inkjet printing technology. This type of the sensor operation was already known in DPH which is based on a silicon complex dew detector [1]. However, in our device the silicon substrate has been replaced by a thin, flexible Kapton foil. All components integrated in the detector: the interdigitated capacitive sensor electrodes, the thermoresistor and the heater were made by ink-jet printing technology. All individual elements of the printed dew point hygrometer detector have been characterized, and finally, their measurement features were described. © Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ©2015 2015The TheAuthors. Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROSENSORS 2015. Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
Keywords: dew point, hygrometer, humidity, inkjet printing, sensor
1. Introduction Air humidity describes the amount of water vapour in air. Even the marginal small values of humidity can have significant influence on living organisms or inanimate objects. But the greatest impact it has on technological processes, therefore measuring and regulating it is often of high importance. Air humidity is most commonly described by either of two values – dew point temperature or relative humidity [2, 3]. In a hygrometers the dew point (DP) temperature is the temperature in which the condensation of water vapour begins on a surface of that temperature.
* Corresponding author. Tel.: +48 22-234-7820; fax: +48 22-825-2300. E-mail address: [email protected]
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
doi:10.1016/j.proeng.2015.08.788
1100
Grzegorz Tarapata et al. / Procedia Engineering 120 (2015) 1099 – 1102
Higher DP temperature means that the air is more humid. The DPH must consists of a detector to detect the instant when the water vapor condense on the surface at the dew point temperature as well as a temperature sensor and a heater [4]. They are used to measure the surface temperature of the detector and to heat up the detector for condensed water evaporation. The principle of operation of such a dew point hygrometer is based on gradually cooling the surface of the sensor, while the impedance comb detector must checks if the water condense on the surface. When this state occurs, the measured surface temperature is taken and treated as a gas humidity value. Finally, the condensed water is evaporated by heating up with the heater and the whole process is repeated continuously. It is worth to point out, that the DP temperature is the absolute unit of gas humidity. Moreover, typically the DP hygrometers are much more accurate than RH meters. 2. Sensor design and fabrication The humidity sensor described in this paper was manufactured using Inkjet Printing technology. This technology uses printers similar to office inkjet printers, but they are more versatile, offers more precise printouts which are desirable in printed electronics [5, 6]. This rapidly evolving technology allows production of sensors considerably cheaper than traditional silicon-based ones [1], and is expected to take over areas where big amounts of flexible, cheap and not too complex sensors is required. The hygrometer structure presented it the paper was printed as a double layer structure, where the bottom layer of the film substrate incorporates a heater and a thermoresistor. While on top layer the interdigitated comb electrodes were formed. All components were printed on 12.5μm flexible Kapton foil. The design and printout close up of hygrometer structure are shown in Fig. 1 and 2. Active area of the detector has dimensions of 8 mm per 11.3 mm, where the comb electrodes have a width and space in between as well of 100 μm. The heater and thermoresistor were designed to have 300 μm and 75 μm respectively. The printing process was carried out using the digital material printer Dimatix DMP2831 and with conductive silver ink EDM5603. Fully functioned structure of the dew-point hygrometer detector was achieved by placing printed structure on a Peltier cell (Fig. 3). b)
a)
contact pads
heater
thermoresistor
impedance comb detector
contact pads
Figure 1: The design of a) the bottom side: the heater and the thermoresitor, b) the top side: the impedance comb detector (contacts not to scale)
a)
b)
c)
Figure 2: Close-ups of: a) manufactured dew detector, b) meander-like shape of the thermistor and of the heater, c) printed DPH structure with long contact pads
1101
Grzegorz Tarapata et al. / Procedia Engineering 120 (2015) 1099 – 1102 Top: comb dew detector Bottom: heater and thermoresistor
Kapton substrate
Peltier couple radiator
Figure 3: Cross section of the hygrometer with the inkjet printed dew point detector
3. Tests of the printed hygrometer structure A series of experiments was carried out on the DP sensors in order to characterize all sensor components and to investigate its behaviour under working conditions. DP printed sensor was tested on automated measurement stand, consisting of: a Peltier cell placed on a radiator, a PT100 thermistor placed on said Peltier cell for reference temperature measurement, a power source powering the Peltier cell and the multimeters allowing the resistance reading of both PT100 and printed thermoresistor. A precise RLC meter was used to measure comb detector’s parallel capacitance (Cp) and resistance (Rp). Additionally, a humidity generator was used to generate gas of given humidity and reference dew point meter (Michell Instruments S8000 Integrale) to verify generated gas humidity. All meters and gas generator were controlled via a PC computer. 4. Results and discussion Measured resistance of the heater and thermoresistor was 150 Ω and 740 Ω respectively at 22 °C. The thermoresitor was calibrated with usage of climate chamber, and its characteristic was linear in operational temperature range. The determined sensitivity was 655 mΩ/K and it is good enough to measure temperature of the sensor surface. Since the dew sensor is located on the top side of the foil but the thermoresistor on the bottom side, temperature difference between top and bottom side of the foil could be expected. Because the temperature measurement accuracy is the crucial parameter of the DPH we decided to measure if the temerature difference is big enough to be taken into account additional structure featuring heater and thermistor on both top and bottom side of the foil was manufactured. This structure allowed to measure temperatures gradient through the foil in a simulated hygrometer cycle. Results of such experiment is shown in Fig. 4 and 5. The temperature gradient varies from 0.2 K to 0.6 K. 24
0,7
Top side thermistor
23
0,6
Bottom side thermistor
22
0,5
temperature [°C]
temperature [°C]
21 20 19 18 17
16
0,3 0,2 0,1
15 14 1000
0,4
1005
1010
1015
1020
1025
1030
time [s]
Figure 4. Measured temperature on the top and bottom side of the sensor substrate during cooling and heating cycle
0 1000
1005
1010
1015
1020
1025
1030
time [s]
Figure 5. Temperature gradient between top and bottom side of sensor during cooling and heating cycle
To heat up and evaporate condensed water, the heater is supplied with voltage of 14 V, which for heater resistance of around 150 Ω gives an average dissipation power of 750 mW during one measurement cycle. It allows to increase the sensor temperature of about 12 °C even when the Peltier cell is active. In real DPH operation we increase the detector temperature of some 2 K by heating which means the dissipation power is much lower and temperature gradient between two sides of the foil has also lower difference.
1102
Grzegorz Tarapata et al. / Procedia Engineering 120 (2015) 1099 – 1102
Figure 6 shows an exemplary static characteristic of the detector impedance versus temperature during water condensation for gas humidity of 13 °C of DP temperature. Finally, the impedance threshold for the dew detection in dynamic condition during regular running of hygrometer was arbitrary chosen as 25% of its amplitude. It is a greater value than is consistent with the static characteristics. Result of regular operation of DPH is shown in Fig. 7. Obtained humidity inaccuracy was estimated as ΔT=0,4 K, and measurement time constant of 2s. Applied algorithm running at PC usage of external measurement devices allows to perform humidity readings even every 2 ÷ 3 seconds. 25
3500
23
3000
|Z| ]kΩ]
2500
temperature [°C]
T
2000 1500
Z th
1000
instantaneous sensor surface temperature
21
dew point temperature
19
17 15 13
500
11
0 0
5
10
15
20
25
30
detector temperature [°°C]
Figure 6: The impedance characteristic of the detector during cooling. Zth is the impedance threshold for the dew condensation.
9 0
10
20
30
40
50
60
70
80
time [s]
Figure 7. Detector surface temperature oscillations in regular operation during for gas humidity of 13.0 °C
Summary In conclusion, integrated dew point hygrometer was successfully fabricated fully with the inkjet printing technology. For the mature printing technology such structure can replace classical silicon dew point hygrometer. It's properties are comparable to typical hygrometer, but the technology is significantly cheaper. During operation cycle of DPH a temperature gradient through the substrate up to 0.6 °C was observed. Obviously, the gradient strongly depends on the material type and its thickness. To decrease inaccuracy of temperature measurement a thinner foil or higher thermal conductivity of it is required. From the other side, since the DP temperature is taken at the lowest temperature during cooling process, a constant value of around 0.6 °C could be subtracted. References [1] R.Jachowicz, J.Weremczuk, D.Paczesny, G.Tarapata: "A MEMS-based super fast dew point hygrometer - construction and medical applications", Meas. Science & Technology, IoP124008, vol. 20, No.122009, pp. 1-10 [2] J.Weremczuk, G.Tarapata, R. Jachowicz: "The ink-jet printing humidity sorption sensor - modelling, design, technology and characterization", Measurement Science & Technology, IoP, 014003,vol. 23,No.1, 2012, 10 pages [3] Milind V. Kulkarni, Sanjay K. Apte, et al. " Ink-jet printed conducting polyaniline based flexible humidity sensor", Sensors and Actuators B: Chemical, Volume 178, Issue null, Pages 140-143 [4] Jachowicz, R. S; Dew point hygrometer with heat injection—principle of construction and operation, Sensors and Actuators B: Chemical,7,1,455-459,1992, Elsevier. [5] Andersson, H.; Lidenmark, C.; Ohlund, T.; Ortegren, J.; Manuilskiy, A.; Forsberg, S.; Nilsson, Hans-Erik, "Evaluation of Coatings Applied to Flexible Substrates to Enhance Quality of Ink Jet Printed Silver Nano-Particle Structures," Components, Packaging and Manufacturing Technology, IEEE Transactions on , vol.2, no.2, pp.342-348, Feb. 2012. [6] Wu, S.P.; Yung, K. C.; Xu, L. H.; Ding, X.H., "Fabrication of Polymer Silver Conductor Using Inkjet Printing and Low Temperature Sintering Process," Electronics Packaging Manufacturing, IEEE Transactions on , vol.31, no.4, pp.291-296, Oct. 2008.