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Development of ion sensors for hydroponics
Copyright © IF AC Mathematical and Control Applications in Agriculture and Horticulture, Matsuyama, Japan 1991
DEVELOPMENT OF ION SENSORS FOR HYDROPONICS S. Yamashita, K. Baba, S. Ito and Y Asano DKK Corporation, 4-13-14 Kichijojikitamachi, Musasinoshi, Tokyo 180, Japan
Abstract. The ion sensors for hydroponics have been developed. The sensor is the combination type. It consists of the sensing tip, sensor protection and reference electrode. The basic performance of the sensor for N03"/K+/ Ca2+ is described in detail. As a example, nitrate ion sensor provides the linear response in the range from 6 to 6,000 mg/1 of nitrate ion with the slope of 55 mV/decade. The results obtained with this sensor gave reasonably agreement with those by conventional method. Keywords. process control; monitoring; ion; sensors; hydroponics. Since we are in the high technology age, measurement of the object component is realized with a rather high accuracy. But the cost reduction to a level on which farmers can make measurement easily. Actually, the following steps are thought, and each step is modified in its own way.
INTRODUCTION The instrumentation technology in vegetable production was introduced fairly early days after the world war II at agricultural experiment stations and other laboratories. Studies in these facilities were accumulated to develop instrumentation technology attracting public attention these days. However,the practical application of this technology is just started; i.e., there is a great gap between laboratory level and field level. There are many hurdles to be overcome.
Stepl: Experimental plant such as agricultural station and university. Step2: Experimental plant for near actual application. Step3: Actual plant managed by farmers or businesses.
We are going to explain the direction of practical application and some topics of the component sensor for vegetable plant. Moisture of hydroponics is separately discussed in an independent section. Since instrumentation technology is directly related with control and information acquisition, it is mainly discussed from the view point of the on line sensors.
When step 3 is reached, the sensor must be refined so that requirements (1) through (4) are satisfied. Two types of vegetable plant available; soil-based plant (advanced house culture) and hydroponics-based plant. Component sensors will be gradually separated into two categories. That is, however, a future problem.
APPLICATION CONDITIONS OF COMPONENT SENSOR IN VEGETABLE PLANT
MAJOR COMPONENT SENSORS USED IN VEGETABLE PLANT Of course, necessary sensors in this field are roughly classified into those for controlling meteorological environment and those for controlling culture medium. Major items and their measurement principles are as tabulated in Table 1.
Compared with ordinary sensors of temperature, pressure and flow rate, the component sensor has generally a complicated and hard-to-handle constructions. Therefore, to raise its applicability to a practicable level in a vegetable plant, the following requirements should be considered.
Many of these are, except some part, experienced in process industry. These sensors are now under modification for agricultural purpose. But, as described before, agriculture has its own conditions, and more contrivances must be accumulated to assort a complete series.
(1)Sturdy design durable for the use by non-skilled person in vegetable plant. (2)Long term accuracy with maintenance to some extent. (3)Sensitivity enough for sensing biological information. (4)Low expense. (5)Capability for small scale production of many types till the use of the sensor spreads.
Viewing from general tendency, each component sensor for vegetable plant must be furnished with the following functions, and the present task is to realize the
419
Table 1 Major Component Sensors for Vegetable Plant and their Working Principles
Item
Use Culture liquid/ soil
Meteorology
Principle
Measurement range 0.5^-100 mS/cm
EC (Conductivity) pH
Glass electrode method
5^9 pH
DO(Dissolved Oxygen)
Galvanic/ Polarographic method
5-^20 ppm
ORP (Oxidation/ Reduction Potential)
Platinum electrode method
±500 mV
Various ions
Ion Sensor/ Colorimetry
Ethylene
Infrared absorption method
C02
Gas chromatography
0^5000 ppm
past 25 years. Sensors reached to 31 species for selective measurement of various ions have been developed. They are used, like pH measurement with glass electrode, in combination with a reference electrode. Sensors for agriculture use are as shown in Table 2.
requirement. (l)The sensing tip of the component sensor is desirably disposable as a rule for the convenience of user. (2)If possible, it is preferable that no reagent supplying and no troublesome calibration are required. (3)When the sensor is inserted into the culture liquid or the soil, some contrivance is required to prevent contamination of sensor surface. (4)The signal transmitter should be of compact construction, and resistant to high temperature and high humidity environment. (5)It is recommended that a simple indication related with measured value be provided as the rough standard of the field work.
These are mostly called liquid membrane type sensors. An organic compound selectively reacts with ions is dissolved in a water-insoluble solvent and mixed with PVC, then shaped in a plate to produce the sensor. When this sensor is immersed in the sample water, the organic compound is dissolved by a little amount to react with specific ions causing a potential proportional to the concentration. The weak point of the ion sensor is the interfering components as shown in the Table 2. The presence of such a interfering component causes an error which seems to exert little effect on actual hydroponic liquid control. Since lead ion sensor is sensitive to P043", this ion sensor has a possibility to be used as the P043" sensor. Also since the cation sensor is sensitive to Mg2+ and Ca2+, Mg2+ion concentration is obtained by subtracting the separately measured Ca2+ from Mg2+ and Ca2+ concentration. Fig. l shows the typical combination type N03" sensor for hydroponics. And Fig. 2 shows the comparison of the data measured by sensor method and by conventional method.
ION SENSORS AND ION MONITOR IN HYDROPONICS In hydroponics culture, particularly when the hydroponic liquid is circulated, concentration control of the liquid by the component sensor is essential. Among general items of hydroponic liquid control, basic ones are pH, EC, N03", K+, and Ca2+. And dissolved oxygen (DO), P043", Mg2+, etc. may be added. In any case, the ion sensor is the main stream among component sensors used in hydroponics. The following is the brief description of the sensors.
HYDROPONIC LIQUID MONITOR IN HYDROPONICS
Ion Sensor in Hydroponics The ion sensor using an electrode have been put into practice by sensor makers over the
The noteworthy point of the component
420
Table 2 Typical Ion Sensors used in Hydroponics
Measurement range
Object
Measured pH
60,000^6
NO3
K+
4,000-
0.4
4,000-
0.4
mg/1
Ca 2+
mg/1
sensor body
lead
Hg 2+
7
mg/1
Bi", NO2 , C N -
Cs+
6
Zn2+, Pb 2 + ,Sr 2 +
6
liquid junction
/
Interfering component
sensor tip
sensor protection
:/n
Fig. 1 The typical combination type N0 3 " sensor for hydroponics.
73
o
300 E (ppm) Y-1.07X+1.26 b r-0.979 (0
c
(0
200
Ό
Φ v_ 3 CO (0
100H
E
100
200
300 (ppm)
NO3" measured by conventional method
Fig. 2 Comparison of ion sensor method and conventional method
Fig. 3 A example of hydroponic liquid monitor
421
sensor for hydroponics is the trouble due to microbes adhered on the sensor surface. The trouble causes delayed response and errors. Though the cleaner was used on trial bases, it was expensive. Thus occasional cleaning will be the most practicable. Fig. 3 shows the hydroponic monitor which collects component sensors in a compartment for the convenience of maintenance. Each electrode can be removed for cleaning in a single action, and several flow paths can be switched for controlling several points with one set of the monitor. OTHER COMPONENT SENSORS In recent vegetable plant, kerosine is burnt for temperature keeping, and for promoting the crop growth under C02 rich atmosphere, and inexpensive infrared absorption type C02 analyzer came into the scene. It has been found that ethylene gas is related with the life of vegetable. The ethylene sensor utilizing gas Chromatograph and chemiluminescence will be developed soon. Further, if physiological measurement and growth measurement of vegetable become possible, growth conditions of vegetable can be adequately performed. These are important future tasks. We hope the sensor development goes to a sensor that detects the disease factor, the most hard problem in vegetable plant. SUMMARY In the near future, more ion sensors for agriculture will be developed and it is expected for them to prompt efficiency in hydroponic culture.
422
DEVELOPMENT OF ION SENSORS FOR HYDROPONICS S. Yamashita, K. Baba, S. Ito and Y Asano DKK Corporation, 4-13-14 Kichijojikitamachi, Musasinoshi, Tokyo 180, Japan
Abstract. The ion sensors for hydroponics have been developed. The sensor is the combination type. It consists of the sensing tip, sensor protection and reference electrode. The basic performance of the sensor for N03"/K+/ Ca2+ is described in detail. As a example, nitrate ion sensor provides the linear response in the range from 6 to 6,000 mg/1 of nitrate ion with the slope of 55 mV/decade. The results obtained with this sensor gave reasonably agreement with those by conventional method. Keywords. process control; monitoring; ion; sensors; hydroponics. Since we are in the high technology age, measurement of the object component is realized with a rather high accuracy. But the cost reduction to a level on which farmers can make measurement easily. Actually, the following steps are thought, and each step is modified in its own way.
INTRODUCTION The instrumentation technology in vegetable production was introduced fairly early days after the world war II at agricultural experiment stations and other laboratories. Studies in these facilities were accumulated to develop instrumentation technology attracting public attention these days. However,the practical application of this technology is just started; i.e., there is a great gap between laboratory level and field level. There are many hurdles to be overcome.
Stepl: Experimental plant such as agricultural station and university. Step2: Experimental plant for near actual application. Step3: Actual plant managed by farmers or businesses.
We are going to explain the direction of practical application and some topics of the component sensor for vegetable plant. Moisture of hydroponics is separately discussed in an independent section. Since instrumentation technology is directly related with control and information acquisition, it is mainly discussed from the view point of the on line sensors.
When step 3 is reached, the sensor must be refined so that requirements (1) through (4) are satisfied. Two types of vegetable plant available; soil-based plant (advanced house culture) and hydroponics-based plant. Component sensors will be gradually separated into two categories. That is, however, a future problem.
APPLICATION CONDITIONS OF COMPONENT SENSOR IN VEGETABLE PLANT
MAJOR COMPONENT SENSORS USED IN VEGETABLE PLANT Of course, necessary sensors in this field are roughly classified into those for controlling meteorological environment and those for controlling culture medium. Major items and their measurement principles are as tabulated in Table 1.
Compared with ordinary sensors of temperature, pressure and flow rate, the component sensor has generally a complicated and hard-to-handle constructions. Therefore, to raise its applicability to a practicable level in a vegetable plant, the following requirements should be considered.
Many of these are, except some part, experienced in process industry. These sensors are now under modification for agricultural purpose. But, as described before, agriculture has its own conditions, and more contrivances must be accumulated to assort a complete series.
(1)Sturdy design durable for the use by non-skilled person in vegetable plant. (2)Long term accuracy with maintenance to some extent. (3)Sensitivity enough for sensing biological information. (4)Low expense. (5)Capability for small scale production of many types till the use of the sensor spreads.
Viewing from general tendency, each component sensor for vegetable plant must be furnished with the following functions, and the present task is to realize the
419
Table 1 Major Component Sensors for Vegetable Plant and their Working Principles
Item
Use Culture liquid/ soil
Meteorology
Principle
Measurement range 0.5^-100 mS/cm
EC (Conductivity) pH
Glass electrode method
5^9 pH
DO(Dissolved Oxygen)
Galvanic/ Polarographic method
5-^20 ppm
ORP (Oxidation/ Reduction Potential)
Platinum electrode method
±500 mV
Various ions
Ion Sensor/ Colorimetry
Ethylene
Infrared absorption method
C02
Gas chromatography
0^5000 ppm
past 25 years. Sensors reached to 31 species for selective measurement of various ions have been developed. They are used, like pH measurement with glass electrode, in combination with a reference electrode. Sensors for agriculture use are as shown in Table 2.
requirement. (l)The sensing tip of the component sensor is desirably disposable as a rule for the convenience of user. (2)If possible, it is preferable that no reagent supplying and no troublesome calibration are required. (3)When the sensor is inserted into the culture liquid or the soil, some contrivance is required to prevent contamination of sensor surface. (4)The signal transmitter should be of compact construction, and resistant to high temperature and high humidity environment. (5)It is recommended that a simple indication related with measured value be provided as the rough standard of the field work.
These are mostly called liquid membrane type sensors. An organic compound selectively reacts with ions is dissolved in a water-insoluble solvent and mixed with PVC, then shaped in a plate to produce the sensor. When this sensor is immersed in the sample water, the organic compound is dissolved by a little amount to react with specific ions causing a potential proportional to the concentration. The weak point of the ion sensor is the interfering components as shown in the Table 2. The presence of such a interfering component causes an error which seems to exert little effect on actual hydroponic liquid control. Since lead ion sensor is sensitive to P043", this ion sensor has a possibility to be used as the P043" sensor. Also since the cation sensor is sensitive to Mg2+ and Ca2+, Mg2+ion concentration is obtained by subtracting the separately measured Ca2+ from Mg2+ and Ca2+ concentration. Fig. l shows the typical combination type N03" sensor for hydroponics. And Fig. 2 shows the comparison of the data measured by sensor method and by conventional method.
ION SENSORS AND ION MONITOR IN HYDROPONICS In hydroponics culture, particularly when the hydroponic liquid is circulated, concentration control of the liquid by the component sensor is essential. Among general items of hydroponic liquid control, basic ones are pH, EC, N03", K+, and Ca2+. And dissolved oxygen (DO), P043", Mg2+, etc. may be added. In any case, the ion sensor is the main stream among component sensors used in hydroponics. The following is the brief description of the sensors.
HYDROPONIC LIQUID MONITOR IN HYDROPONICS
Ion Sensor in Hydroponics The ion sensor using an electrode have been put into practice by sensor makers over the
The noteworthy point of the component
420
Table 2 Typical Ion Sensors used in Hydroponics
Measurement range
Object
Measured pH
60,000^6
NO3
K+
4,000-
0.4
4,000-
0.4
mg/1
Ca 2+
mg/1
sensor body
lead
Hg 2+
7
mg/1
Bi", NO2 , C N -
Cs+
6
Zn2+, Pb 2 + ,Sr 2 +
6
liquid junction
/
Interfering component
sensor tip
sensor protection
:/n
Fig. 1 The typical combination type N0 3 " sensor for hydroponics.
73
o
300 E (ppm) Y-1.07X+1.26 b r-0.979 (0
c
(0
200
Ό
Φ v_ 3 CO (0
100H
E
100
200
300 (ppm)
NO3" measured by conventional method
Fig. 2 Comparison of ion sensor method and conventional method
Fig. 3 A example of hydroponic liquid monitor
421
sensor for hydroponics is the trouble due to microbes adhered on the sensor surface. The trouble causes delayed response and errors. Though the cleaner was used on trial bases, it was expensive. Thus occasional cleaning will be the most practicable. Fig. 3 shows the hydroponic monitor which collects component sensors in a compartment for the convenience of maintenance. Each electrode can be removed for cleaning in a single action, and several flow paths can be switched for controlling several points with one set of the monitor. OTHER COMPONENT SENSORS In recent vegetable plant, kerosine is burnt for temperature keeping, and for promoting the crop growth under C02 rich atmosphere, and inexpensive infrared absorption type C02 analyzer came into the scene. It has been found that ethylene gas is related with the life of vegetable. The ethylene sensor utilizing gas Chromatograph and chemiluminescence will be developed soon. Further, if physiological measurement and growth measurement of vegetable become possible, growth conditions of vegetable can be adequately performed. These are important future tasks. We hope the sensor development goes to a sensor that detects the disease factor, the most hard problem in vegetable plant. SUMMARY In the near future, more ion sensors for agriculture will be developed and it is expected for them to prompt efficiency in hydroponic culture.
422