Chemical Wireless Sensor Network for pH Remote Monitoring
Claudia Manjarrés*, David Garizado*, Maria Calle*, Cecilia Jiménez**
*Department of Electrical and Electronics Engineering, Universidad del Norte, Barranquilla, Atlántico, Colombia **Instituto de Microelectrónica de Barcelona (IMB-CNM), CSIC, Campus UAB, 08193
Bellaterra, Barcelona, Spain
claudian, dgarizado, mcalle (@uninorte.edu.co), [email protected] Abstract
The use of microsensors for in-field monitoring of environmental parameters is gaining interest due to their advantages over conventional sensors. Among them, microsensors and specifically Ion Selective Field Effect Transistors (ISFETs) based on semiconductor technology offer additional advantages such as small size, robustness, low output impedance and rapid response. ISFETs sensors can be integrated into a wireless network in order to monitor pH from different locations and transmit information to a central point. The paper proposes a Chemical Wireless Sensor Network (CWSN) for pH monitoring through long distances, showing the general system and preliminary results.
Keywords: pH, ISFET, Chemical Wireless Sensor Network, Long Range Introduction
Environmental monitoring often requires measuring a variable in different points simultaneously, since concentration of these variables often change with time or location.
Examples include pollutants in a city or pH in a water source. One possible solution is to employ Wireless Sensor Network concepts to transmit chemical variables, defined as CWSN [1]
combined with ISFET based sensors that have demonstrated to be a good alternative for environmental monitoring [2]. The proposed system includes hardware and software for a three-node network, scalable up to 65535 nodes. Hardware section allows for signal adaptation, analog to digital conversion, calibration and radio transmission. The software receives information from the network and shows an alert when pH value is out of range.
Results show pH maximum error of 2.7% when compared to a WTW Multi3420 pHmeter.
Related Work
Although there is some literature concerning CWSN, such as [1], [3] and [4], these studies include short distance communication.
Additionally, fewer studies include quantitative data on system accuracy.
System Description
Figure 1 presents a block diagram of the system. Each node includes a data acquisition board, Arduino Uno module for processing and one Xtend modem (by manufacturer Digi) as wireless Interface.
Processing Board Data
Aqcuisition Board
Wireless
Interface Monitoring Software
Figure 1. Block Diagram of CWSN. The prototype includes three nodes
Data acquisition board includes a circuit for providing the ISFET with a constant current of 100 µAmperes similar to [5]. ISFET output voltage may be negative and it depends upon pH values, thus the signal requires further conditioning to be processed. After conditioning, maximum output voltage is 1 V, connected to processing board’s 10-bit Analog to Digital Converter (CAD), embedded in Arduino microcontroller. The device computes pH measurements and transmits data using a serial port to Xtend modem, whose transmission power ranges from 1 mW to 1 W. Figure 2 shows a schematic diagram of the data acquisition board.
Ibersensor 2012-October 16-19, Puerto Rico IB-58
Figure 2. Detail on data acquisition board. The first op-amp provides a constant current to the ISFET; others create an inverter-adder to convert output voltage to a 0-1V signal
Chemical sensors require calibration that is measurement of signal/pH ratio with at least two standard solutions. The processing board implements a manual two-point calibration routine, to maintain accurate readings. Arduino also implements wireless communication protocols for CWSN. MAC layer uses a TDMA scheme to avoid collisions. CWSN uses a star topology, where all nodes can transmit directly to the final destination. Receiver modem is connected to a PC where resident Java software gathers information from the network and shows historical values. If pH values fall out of range, software will generate an alarm.
pH Measuring Results
The system was tested in laboratory for three solutions with different pH values: 4, 7 and 9. Each node uses one specific ISFET, and it is measured at least three times during 30 minutes. Table 1 shows average values obtained in each node, compared to pH values measured with a WTW Multi3420 pHmeter.
Table 1. Measurements taken by each node compared to readings with WTW Multi3420
Node WTW pH Calibrated pH %Error
1 4,13 4,20 1,64
1 7,13 6,95 2,57
1 8,89 9,01 1,30
2 4,13 4,06 1,72
2 7,13 7,32 2,70
2 8,89 8,77 1,37
3 4,13 4,15 0,38
3 7,13 7,09 0,60
3 8,89 8,92 0,30
Table 1 show measurements agree, with maximum error 2.7 percent. Node 3 shows the best results, even though nodes are implemented with the same design described in Figure 2.
Figure 3 shows calibration curves for the three nodes employed.
0,0 100,0 200,0 300,0 400,0 500,0 600,0
0 2 4 6 8 10
Vout (mV)
pH
Node 1 Node 2 Node 3
Figure 3. Calibration curves for the three nodes in CWSN
According to Figure 3, slopes for different nodes are: node 1 = 54 mV/pH, node 2 = 44 mV/pH and node 3 = 49 mV/pH. Values obtained for the three nodes are acceptable for silicon nitride ISFETs
Conclusions
A three node CWSN prototype was presented. The reproducibility of measurements between the three nodes is acceptable, although future work may improve on this metric.
Wireless communication has been tested in a point-to-point Line of Sight setup, finding adequate transmission up to 4 kilometers. The next step includes integration of these wireless links to each node and measuring network performance.
References
[1] J. Hayes, S. Beirne, K. Lau and D. Diamond, “Evaluation of a low cost Wireless Chemical Sensor Network for Environmental Monitoring”, IEEE Sensors 2008 Conference, Lecce, Italy, October 26-29, pp 530-533, 2008.
[2] C. Jimenez-Jorquera, J. Orozco and A. Baldi, “ISFET Based Microsensors for Environm ental Monitoring”, Sensors 2010, 10, 61-83
[3] S. De Vito et al, “Wireless Sensor Networks for Distributed Chemical Sensing: Addressing Power Consumption Limits With On-Board Intelligence”, IEEE Sensors Journal, Vol. 11, No. 4, pp. 947-955, April, 2011.
[4] V. Devan, “pH Wireless Sensor Network for the meat tenderizing process”, Master Thesis, Auckland University of Technology, Auckland New Zealand, 2010.
[5] F. Valdés-Perezgasga, “Intramyocardial pH Measurem ents using Ion-Sensitive Field-Effect Transistors”, Ph. D. Thesis, University of Newcastle upon Tyne, 1990.