Membrane covered optical CO 2 sensor

Transcription

1 Membrane covered optical CO 2 sensor - Manual - 30 mg/l CO 2, 4-20 ma output Exclusively Distributed & Supported By: info@edaphic.com.au Ph:

2 Warning! Do never try to unscrew the protection cage! This leads to the damage of the sensor and this is not covered by the guarantee! Therefore the thread of the protection cage is marked with a seal! 2

3 Contents 1 Preface 2 Technical data 3 Structure of the CO 2 sensor 4 Putting into operation 5 Measurement and calibration 5.1 CO 2 concentration inside of the sensor (=first step for calculation in water) 5.2 Calculation of the CO 2 concentration in water 5.3 Calculation of the total inorganic carbon (TIC) 5.4 Calibration 6 Maintenance 3

4 1 Preface The measurement of the dissolved carbon dioxide gas (excluding HCO 3 - and CO 3 2- ) in the water is one of the most important parameters. Until now the so called m-value method has been used, which has some disadvantages. Apart from the very time-consuming manual analysis, the m-value method is not only influenced by CO 2, but also by all the other compounds with influence to the ph-value (e.g.phosphates, silicates, hydrogen carbonate, carbonate). Additionally, the m-value method requires sampling, which may lead to errors too. On the other hand, a direct reading sensor is required to allow the control of the conditions in the water. We have developed a membrane covered optical sensor for the determination of carbon dioxide in water for depths of up to 1,000 m, resp. for 1,000 dbar pressure. The principle of this sensor could be explained as follows: The inner sensor volume is separated from the sample by means of a gas permeable silicone membrane. Liquids and solids are not able to pass the membrane. If the sensor is dived into a sample, a CO 2 partial pressure equalisation is achieved between the inner sensor volume and the sample. Inside the sensor there is an optical sensor mounted. Its working principle is based on Single-Beam Dual Wavelength NDIR. This measurement of the CO 2 partial pressure has to be accompanied by the measurement of the temperature in the water for calculating the carbon dioxide concentration in water. The perfect functioning and operational safety of the sensor can only be ensured if the user observes the safety precautions as well as the specific safety guidelines stated in the present manual. 4

5 2 Technical Data of the CO 2 sensor *) measuring principle: membrane-covered optical sensor based on Single-Beam Dual Wavelength NDIR power supply: VDC output: 4-20 ma, maximum load: 500 Ω dimensions: maximum diameter: 36 mm total length: 225 mm connector: Subconn MCBH4M (titanium) housing: Titanium concentration range: 0-30 mg/l CO 2 accuracy (25 C): mg/l CO 2 : ± 0.06 mg/l mg/l CO 2 : ± 2 % of the measuring value mg/l CO 2 : ± 3.5 % of the measuring value pressure range: up to 1,000 dbar temperature range: C (for measuring and storage) response time: t 90% : approx. 10 minutes (depends on concentration steps) long time stability: < ± 1% full scale per year power consumption: < 0.4 W warm up time: < 2 minutes *) Changes for technical improvement are reserved. Pin definitions of the sensors plug: Pin 1: Pin 2: Pin 3: Pin 4: power ground signal ground signal ma power supply V DC 5

6 3 Structure of the CO 2 sensor 6

7 4 Putting into operation For putting into operation please act as follows: 1 Compare the plug connections at your probe system with the plug connections of the sensor as described in chapter 2. Make sure that the pin assignments are correct. 2 Grease the bulk head connector of the sensor with silicone grease and link it with your probe system or interfacing cable. 3 Put the sensor into your sample. 4 Switch on the power supply. Wait up to two minutes for warm up to reach the full accuracy. Now the sensor is ready for CO 2 measurements. 7

8 5 Measurement and calibration The carbon dioxide sensor is suitable both for liquids and for gases. It is very important to consider, that the sensor detects only the dissolved carbon dioxide gas. The sensor is not sensitive to carbonate and hydrogen carbonate. The mathematical formulas for the calculation of the concentrations are explained step by step in the following. Please be careful when using the units. Use always the units mentioned below CO 2 concentration inside of the sensor (=first step for calculation in water) The sensor delivers the CO 2 volume concentration C V (with internal temperature compensation) with the unit volume per cent (% Vol ): C V [% Vol ] = (I-4 ma) x 2%/16 ma C V [% Vol ] CO 2 volume concentration [% Vol ] I current output signal [ma] 5. 2 Calculation of the CO 2 concentration in water C water Now you have to determine the partial pressure of carbon dioxide p using the CO 2 volume concentration C V : p = C V x 1.013/100% p [bar] partial pressure of CO 2 Henry coefficients at different temperatures (Sontheimer et al. 1980): K H = Σ (a i x t i ) i = a 0 = 76, a 1 = -3, a 2 = 0, a 3 = -3, x 10-3 a 4 = 3, x 10-5 K H [mmol/lbar] Henry coefficient t [ C] temperature Calculation of the CO 2 concentration in water by means of the Henry's Law: C f = K H x p C f [mmol/l] CO 2 concentration in water p [bar] partial pressure of CO 2 If you prefer the unit mg/l please calculate: C Water [mg/l] = C f x 44 8

9 Example for calculating C water : I = 4.8 ma C V = 0.1% t = 20 C K H = mmol/l bar p = x 10-3 bar C f = mmol/l = mg/l C water 5. 3 Calculation of the total inorganic carbon (TIC) If you have calculated the CO 2 concentration in water (C water ) using the equations in chapter 5.1 and 5.2, you can also calculate the concentration of the total inorganic carbon (C TIC ), if you know or if you have measured the ph-value. It has to be pointed out, that C TIC is the sum of the carbon concentration C in CO 2, HCO 3 - und CO Please use the following equations for the calculation of C TIC : c H 10 ph C TIC K 1 = 10-6,35 K 2 = 10-10,33 C TIC C c Water H K 1, K 2 C Water 2 2 c c K K K / c 0, H H H - 2- sum of the carbon concentration C in CO 2, HCO 3 and CO 3 in mg/l CO 2 concentration in water in mg/l concentration of the H + -ions, calculated using the ph-value equilibrium coefficients Remark: The factor 0, is due to the percental mass of the carbon C in carbon dioxide and is calculated by dividing the relative atomic weight of carbon and carbon dioxide (12/44= ). 5.4 Calibration The carbon dioxide sensor is delivered calibrated. If the mathematical equations described in chapter 5.1 and 5.2 are used, the factory calibration is used automatically. The drift of the sensor is less than ± 1% full scale in one year. For re-calibration please return the sensor to AMT Analysenmesstechnik GmbH. 9

10 6 Maintenance The carbon dioxide sensor is nearly maintenance free, except any re-calibration at any time. But we recommend to rinse the sensor carefully with water after using it in liquid samples if you plan a longer break and if you want to store the sensor. The Subconn underwater connector is actually maintenance-free. However it has proved itself to be recommended to lubricate the sealing surfaces of the pins with silicone grease. This reduces wear whilst plugging and unplugging. 10