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1

Technical Reports

＜Feature＞ The new U-20 Series Multi-parameter Water Quality Monitoring System

Feature Article

Takeshi KOBAYASHI

Abstract

The new U-20 Series Multi-parameter Water Quality Monitoring System can automatically measure up to 13

items including pH, conductivity, temperature, dissolved oxygen and turbidity. This system has following features:

1)Measurement up to a depth of 100 meters, 2)Measurement up to one month, 3)Process functions for depth

analysis and GPS mapping can be added with a combination of system units, 4)Versatility enables the system to

be used for on-site research of water quality or continuous monitoring of factory effluent in rivers and wells.

The new U-20 Series Multi-parameter Water Quality

Monitoring System

Additional adapter

Printer Personal computer Global positioning system

Measurements・calibration results

・calibration history

Positional information・water depth value

・measurements

Three-dimensional measurement

Using of latitude and longitude

Body

Display

CPU• Operation of measuring function

• Operation of adjusting function

• Operation of calibration function

• Operation of set up function

Communication

ADC

Memory2880points

Sensor holder

Preamplifier

* Option includes fluoride, potassium and anmmonia ions

Nitrate ion sensor*

Chloride ino sensor*

Calcium ion sensor*

pH sensor

ORP sensor

DO sensor

Conductivity sensor

Turbidity sensor

Temperature sensor

Water depth sensor

CPU Communication

2 No.18 March 1999

Fig. 2 Configuration of the U-23 sensor probe

Chloride ino sensor

Sensor probe connector

Cover for memorybackup battery

Sensor probe

Water depthmeasuring sensor

Cover for protectCalcium ion sensor

Nitrate ion sensor

DO sensor

pH/OPR sensor

TEMP/COND/TURB sensor

1. Introduction

As worldwide environmental issues become more critical,water quality analysis is gaining its importance. Inparticular, the demand for a small accurate water qualitymonitoring system has been increasing. Horiba answeringthe need and using their advance technology developed theU-20 Series Multi-Parameter Water Quality MonitoringSystem.

2. The Configuration of U-20 Series

The device consists of the meter, which includes a displayand keypad, and the sensor probe unit. The cable, whichconnects the body and the sensor probe, can be orderedand supplied up to a length of 100 meters. There are twotypes of the sensor probe: Single Probe (U-21/22), whichmeasures up to 10 items except for ion sensor, and DoubleProbe (U-23), which includes the ion sensor. Also, incombination of the system units, you can add a variety offunctions such as GPS (Global Positioning System), printeroutput, RS-232C, and AC adapter. The U-20 Series systemconfiguration is shown in Figure 1, and so is measurementitem in Table 1.

Type U-21 U-22 U-23

Measurement items

pH　　　　　　　　　　　　　　　 ○　　　　○　　　○ DO ○　　　　○　　　○ Conductivity ○　　　　○　　　○ Salinity (in conductivity equivalent) ○　　　　○　　　○ Total dissolved solid (in conductivity ○　　　　○　　　○ equivalent)

Specific gravity of seawater ○　　　　○　　　○ (in conductivity equivalent)

Temperature ○　　　　○　　　○ Turbidity —　　　　○　　　○ Water depth —　　　　○　　　○ Oxidation reduction potential —　　　　○　　　○ Chloride ino — 　　　—　　　○ Nitrate ion — 　　　—　　　○ Calcium ion — 　　　—　　　○ Fluoride ion — 　　　—　　 (○) Potassium ion — 　　　—　　 (○) Anmmonia ion — 　　　—　　 (○)

Table 1 Measurement items of the U-20 series

○ .......Correspondence

(○)......Correspondence in case of optional sensor

Fig. 1 System construction of the U-20 series

U-20 Body

Carring case including

extended function

Pressure sensor

Use only U-23

U-20/22 sensor U-23 sensor

IONsensor

pH/ORPsensor

DOsensor

COND/TEMPTURB sensor

Zoomup

3

Technical Reports

＜Feature＞ The new U-20 Series Multi-parameter Water Quality Monitoring System

3. The Point of Sensor Development

The U-20 Series works as a portable system that can beeasily used in the field as either a portable unit or stationedat a site as a fixed system for continuous measurement forup to one month. These features were achieved by providinga memory function to the sensor probe and improving andincluding various features in the sensor.

3.1 New Developments:(1) pH/ORP sensor In the past, because of its composition, the pH/ORPsensor was susceptive to the influence of pressure, and attimes the sample sometimes entered the reference electrodeunder high pressure and caused minor errors. For thisreason, the U-20 includes a pressure compensationmembrane, providing the capability of conducting highlyaccurate measurement at a depth of 100 meters.(2) Dissolved Oxygen Sensor The U-20 Series uses the cartridge-type galvanic cellmethod, which is superior in long-term stability as adissolved oxygen sensor. In the past, it was necessary tostir the sample for an accurate reading. This sensor however,can measure accurately without the need of any stirring.This was made possible by reducing the size of the electrodeand the amount of oxygen needed. Therefore it no longrequires oxygen to be stirred and sent from the sample toinside of the sensor. To cope with water pressure, weincreased the pressure of the membrane and decreased theexposed area and flexability of the membrane to reducedthe influence of the pressure.(3) Conductivity SensorThe new conductivity sensor incorporates an electrode madeof a nickel & cobalt compound which is anticorrosive. Inaddition, by using a new AC four-electrode method ofmeasurement, you can easily and accurately measure high-density solutions like seawater.(4) Salt, Specific Gravity of Seawater, TDS The U-20 Series calculates and displays the salt level,specific gravity of seawater, and TDS(Total Dissolved Solid)from conductivity and temperature. The specific gravity ofseawater is often used to indicate the condition of seawater,and since the actual density of seawater is somewherebetween 1.0 and 1.031 it can be determined by the formula:σ =(ρ -1)1000. When the atmospheric pressure is p=0,σ is shown as σ t. In Japan, σ 15 at the temperature of15℃ is often called the standard specific gravity. In othercountries, the valueσ0 and 0 ℃is often called the standardspecific gravity. Therefore, the U-20 Series allows you tochose the display from σ t, σ 15 and σ 0. TDS means the amount of total dissolved solids in thesolution, and it is shown as the value that raises conductivityto the power of the material-specific coefficient. To correctdisplay TDS, TDS coefficient must be accurately setup.However, the U-20 series are setup with equivalents of KCland CaCO3 solution as the default.(5) Turbidity Sensor U-22 and 23 use light scattering transmission to measure

turbidity. The principle of this method is to apply light tothe sample, calculate the strength ratio of penetrating lightand scattering light that is scattered by turbidity ingredients,and display the turbidity determined by the relation of thestrength ratio and the turbidity. With the U-20, because ofthe strength of scattered light, optical limitations, anddownsizing of the sensor, it is detected by a photo-detectorlocated at 60 degrees in front. As the standard solution, hormazine or kaolin standardsolutions are used because they have stable turbidity. Theinterrelation of hormazine and kaolin standard solution isshown in Figure 3. Because both are in straight-linerelationship shown as follows, it is possible to use the sameweigh line: Tk=0.744 ×TfNow, Tk means the turbidity(mg/l)) of kaolin standardsolution and Tf means the turbidity (NTU) of hormazinestandard solution.

The function for exchanging the unit system was addedto the U-20 Series, to allow the user to make correctionsfrom both standard solutions. However, because the speedof sedimentation is so high and the error margin of themeasurement gets greater when kaolin standard solution isover 400 mg/l, it is not used as the standard solution(6) Ion Sensor The ion electrode method is used to measure ion (U-23).The possible measurement water depth depends on thecharacteristics of each ion response membrane. Chlorideion, nitrate and fluoride ion can all be measured up to adepth of 100 meters. The calcium ion, (due to its solid thinresponse membrane) has a 15 meter limit, the potassiumion is 3 meters, and the cartridge-type ammonium electrodeis up to 15 meters. Additionally, single compound standardsolution is available for the ion combinations of nitric acid,chloride and calcium to make your correction work easier.(7) Water Depth Measurement U-22 and 23 use a pressure sensor to measure the depth.A pressure sensor performs zero adjustment upon automaticcorrection before the measurement. The use of the waterdepth value and GPS positional information enable a three-dimensional measurement of water quality.

Fig. 3 Turbidity relation between formazine standard solution and kaolin standard solution

Kao

lin s

tand

ard

(mg/

L)

Formazine standard (NTU)

4 No.18 March 1999

Fig. 5 Field test point at Lake Biwa

3.2 Pressure Resistant As the overall quality of our water resources become amore critical issue, there are growing needs to have accurateknowledge of the water qualities of dams, lakes, marshesand oceans. The U-20 Series can accurately performmeasurement at a depth of up to 100 meters by employing avariety of features mentioned above. As shown in Figure 4,it is confirmed that the influence of pressure is very littleand it is superior in pressure resistant.

4. Data Recording and Process Functions

The U-20 Series can record up to 2880 sets of measurementsand call them up at any time. In addition, a periodic recordingallows long-term monitoring. If the interval is set at 15 minutes,it can keep recording for up to a month. By connecting anadditional adapter to the instrument, recorded data can beprocessed on the spot through a PC, or loaded and usedafter you get back to the lab.

5. An Example of Actual Measurement in Lake Biwa Japan

On September 29, 1998, we selected 8 sites (St1～St8;see Figure 5) from the vicinity of Nagahama-toride of LakeBiwa to the Azumi River, and took measurements at a depthof one meter at each site. An example of the measurementsin Figure 6 shows the distribution of water temperature,pH, dissolved oxygen, turbidity, and nitric acid ion in adepth-by-depth fashion at every site. There results give youa wide variety of information regarding the water qualityof Lake Biwa. For instance, it shows that the water temperature plungesbelow 20 meters, and it is less than 10℃ at below 30 meterswhich is 15℃ difference comparing to the surface area.Also, from the result of pH measurement givesyou ideassuch as that at a deeper depth, the water is a little acidic.

d) Conductivity sensor e) Turbidity sensor

Fig.4 Effects of pressure on the Model U-20 series sensor

a) pH sensor b) ORP sensor c) Dissolved oxygen sensor

Wat

er d

epth

(m

)

ph DRP (mV) DO (mg/L)

Conductivity (S/m) Turbidity (NTU)

Wat

er d

epth

(m

)

Wat

er d

epth

(m

)

Wat

er d

epth

(m

)

Wat

er d

epth

(m

)

5

Technical Reports

＜Feature＞ The new U-20 Series Multi-parameter Water Quality Monitoring System

Takeshi KOBAYASHI

Scientific Instruments R&D Dept.

pH Team

6. Conclusion

We have described the main functions and actualexamples of measurement of the U-20 series Multi-parameter Water Quality Monitoring System and theanalysis of the data including pH, conductivity, temperature,dissolved oxygen, and turbidity. We hope that by usinginstruments such as our U-20 series, a better globalenvironment is realized through the prevention of waterpollution and the improvement of the environment. Finally,we would like to extend our deep gratitude to Dr.YousuiYamashiki of International Lake Environment CommitteeFoundation, who helped us in conducting measurement inLake Biwa.

Reference1) Japan Ocean Society “Instruction for Ocean”

Fig. 6 Field test data at Lake Biwa

a) Turbidity distribution b) pH distribution c) Dissolved oxygen d) Turbidity e) Nitrate ion