USER MANUAL & CHARACTERISATION DATA
Contents
1. NOTES FOR CIRCUIT DESIGN e In this arrangement the output voltage Gas Concentration ppm x Cell Output A x RG Q e So if RG is 100k cell output is 500nA ppm and gas concentration is 10ppm then VOUT 10 x 500x10 x 100x10 0 5V e RL is the cell load resistor typically 5 500 Speed of response can be increased by reducing the value of RL but signal noise may be increased as a consequence The recommended values are shown on sensor datasheets e Amplifiers are precision low input offset types eg OP90 or similar e Some sensors require temperature compensation circuitry A simple compensation network can be incorporated into this circuit by replacing RG with a thermistor Typically this is a NTC thermistor that has 3435K of B constant adjusting the output accuracy to within 10 in the range of 10 C 50 C Any thermistor with a B constant around 3500K and resistance value R25 of 10 KQ can be used Alternatively temperature compensation may be undertaken using software lookup tables e For many applications it is necessary to automatically identify critical sensor failure and in particular open and short circuit conditions Nemoto has developed a patented electrical method suitable for battery and mains voltage operation If this is required please contact Nemoto and we will be happy to provide further details subject to a non disclosure agreement NT H2S LI CD doc issue 1 August 2008 page 12 of 14 NOTE2. N L gt Nano Environmental Technology G csa aZ The Reliable Alternative Samm Ea USER MANUAL amp CHARACTERISATION DATA NT H2S LI Electrochemical Hydrogen Sulphide Gas Sensor N E T S r l 20010 Cornaredo MI ITALY Via Legnano 2 Tel 39 02 93544190 Fax 39 02 93540347 C F e P IVA VAT IT 03231490966 CAPITAL EURO 36 000 00 Socio Unico Website www nenvitech com E mail info nenvitech com NT H2S LI CD doc issue 1 August 2008 INTRODUCTION Nemoto amp Co Ltd was established in 1941 and continues to develop unique technologies for the Safety Security and Health markets Using our unique experience of fine chemical preparation and printing we were able to enter the gas sensor market in 1979 with a range of high quality hot wire type sensors pellistors Nemoto is now one of the World s leading manufacturers of chemical sensors and has so far delivered over 30 million devices to the market As a result of three years development at our Tokyo R amp D centre we released our first electrochemical gas sensors in 2000 The NT H2S LI Gas Sensor is a new 3 Electrode Electrochemical cell designed for the detection and measurement of hydrogen sulphide in the range 0 100ppm By adhering to industry standards for size and connection orientation the NT H2S LI can be retrofitted easily to existing product designs By using our experience of design for manufacture and our high volume production facil
3. The reference electrode maintains the healthy operation of the cell It is surrounded by electrolyte sees no gas and no current is allowed to be drawn from it Its electrochemical potential hence always remains constant at a level known as the rest air potential and this is used to regulate the potential of the working electrode regardless of the current it is generating during operation The use of a reference electrode in this way i e three electrode operation helps to extend the working range of the sensor improves linearity and results in a number of performance benefits compared with similar sensors working with 2 electrodes only NT H2S LI CD doc issue 1 August 2008 page 4 of 14 FEATURES Electrochemical gas sensors have the following superiority to conventional semiconductor type and hot wire type gas sensors Linear output in proportion to gas concentration High reproducibility Highly gas specific Unaffected by humidity Stable output for long periods Low power consumption because no heater is used can be battery operated Small and lightweight can be used in portable devices No mechanical structure so highly resistant to shocks and vibrations NT H2S LI has been developed from our accumulation of technologies in production of hot wire type gas sensors long research experience into catalysts fine printing and assembling of sensors The NT H2S LI is small and less expensive but has high sensitivi
4. 5 Calibration and gas testing Calibration of detectors or densitometers should be done after the output value has been stabilized in clean air Evaluation of gas sensitivity should be made in clean humidified air When a test gas is blown directly to the gas intake area higher gas sensitivity may be observed It is therefore best to test and calibrate gas detection instruments and sensors in diffusion mode This can be achieved by using a suitable test housing where a low flow rate is used lt 1I min and where the air is agitated to ensure equal gas diffusion throughout 6 Other Unless otherwise advised by Nemoto voltage should not be supplied directly to the electrode pins Do not bend the pins Do not apply more than 5 kgs cm to the sensor Take care not to block the gas intake area as it may prevent gas entering the sensor Never put foreign material in the gas intake area as it may cause the electrolyte leakage Do not expose the sensor to excess vibration or shock If the sensor housing is damaged do not use the sensor After the sensor is exposed to high concentration gas for long periods the output signal may require time to recover to normal operation Do not blow organic solvents paints chemical agents oils or high concentration gases directly onto sensors Do not disassemble the sensor as this may cause electrolyte leakage NT H2S LI CD doc issue 1 August 2008 page 14 of 14
5. S 1 Long term drift of gas sensitivity All electrochemical gas sensors lose sensitivity over time due to small changes on the surface of the working electrode reducing its oxidation capability To reduce this the NT H2S LI uses a newly developed electrode catalyst that will not deteriorate by more than 5 year Typically these changes are limited to less than 5 but we recommend that this deterioration should be taken into account when designing application circuits 2 Environmental affects on gas sensitivity Due to the hygroscopic nature of the electrolyte used in electrochemical sensors moisture is absorbed from or released to the surrounding atmosphere In high humidity moisture is absorbed causing an increased sensitivity In low humidity moisture is released back to the atmosphere and the sensitivity decreases Conventional electrochemical gas sensors show annual variation of gas sensitivity as much as 10 20 The NT H2S LI utilises an advanced electrolyte management design and combined with the unique electrode catalyst structure these changes can be greatly reduced Under normal operating conditions gas sensitivity should change by no more than 5 of the output value The NT H2S LI is designed so that all changes due to moisture uptake release are completely reversible If the gas intake area of the sensor is blocked with water drops or other liquid gas cannot enter the sensor The NT H2S LI is fitted with an integral h
6. g oxidation reaction H2S 3H 0 SO 8H 8e The SO generated either vents away from the cell via the capillary or dissolves in water to form Sulphuric acid electrolyte whilst the hydrogen ions H migrate into the electrolyte within the cell The electrons e generated at the working electrode are collected by the external circuit via a metal strip in contact with it in the form of a small nA electric current The reaction at the working electrode is balanced by a reciprocal reduction reaction at the counter electrode using Oxygen from the surrounding atmosphere The electrons consumed in this reaction are supplied by the external circuit via a metal strip in contact with the counter electrode NT H2S LI CD doc issue 1 August 2008 page 3 of 14 Working Counter Electrode Electrode BHt Current Electrolyte _ _ 0 Reference Electrode Output Potentiostat Thus water is consumed whilst Hydrogen ions are generated at the working electrode whilst the water is re created and hydrogen ions are consumed at the counter electrode At the same time the reaction at the working electrode generates electrons whilst the reaction at the counter electrode consumes electrons By connecting the working and counter electrodes together via a special circuit flow of electrons between the two electrodes is measured as a nA level current signal proportional to the carbon monoxide concentration
7. ities in Japan and China we have successfully reduced the cost of the NT H2S LI whilst being able to maintain the highest performance quality This characterisation document does not constitute a specification but is intended as a guide informing the instrument designer of the performance characteristics of the sensor which were observed by Nemoto Environmental Technology s Engineers It should be read in conjunction with Technical Information Sheet DS N NT H2S LI which includes the full technical specification for the NT H2S LI Gas Sensor NT H2S LI CD doc issue 1 August 2008 page 2 of 14 PRINCIPLES OF OPERATION I Capillary Charcoal Filter Working Electrode _ Glass Fiber R C Electrode Electrolyte Body The NT H2S LI consists of 3 porous noble metal electrodes separated by an acidic aqueous electrolyte housed within a plastic PPO enclosure Gas enters the cell via a gas phase diffusion barrier capillary and a charcoal based filter that removes unwanted gases which might interfere with the sensors performance or give a false signal An electrolyte reservoir ensures an excess of electrolyte is available at all times and the sensor is vented to ensure that the internal and external pressure of the sensor is in equilibrium In operation gas enters the cell via the capillary and filter and comes into contact with the working electrode Any hydrogen sulphide present undergoes the followin
8. se line shift should be taken into account to optimise overall accuracy In this manual the output values are calculated to be equivalent to H2S gas concentrations Gas sensitivity Output signals Using the NT H2S LI 500 100nA is generated at 1ppm of H2S gas For instance the generated current value will be about 5uA at 10ppm of H2S gas 10ppm x 500nA This generated current is generally recorded as a voltage produced by a Current Voltage converting circuit In our recommended circuit as the conversion is done through a resistor of 10 KQ Response time tg9 This is the time taken to reach to 90 of the maximum output value in clean air Repeatability This is the maximum variation of output signals when tests are repeated under the same measuring conditions temperature humidity gas concentration etc The repeatability of NT H2S LI is 2 and this means that all of the test results would fall in the range of 98 102 Temperature dependence All electrochemical sensors are affected by changes in the ambient temperature and the output increases as the ambient temperature rises This is caused by the rate of oxidation reaction on the surface of the catalyst the dispersibility of the gas in the capillary and the thermal affects on the mobility of ions in the electrolyte This temperature dependency can be compensated relatively easily by using a NTC thermistor NT H2S LI CD doc issue 1 August 2008 page 11 of 14
9. ties The following table gives cross sensitivity information for a variety of commonly encountered gases Test Gas NT H2S LI CD doc issue 1 August 2008 Tested H2S equivalent cross sensitivity Concentration reading ppm ppm 200 lt 2 lt 1 10 10 100 1000 lt 15 lt 1 5 5000 0 0 5000 0 0 30 lt 4 lt 14 10 0 2 to 1 lt 10 10 1 4 14 100 0 0 100 0 2 0 02 1000 lt 1 2 lt 0 15 10 lt 0 5 lt 5 page 8 of 14 4 Response Characteristics The following are plots of responses and recovery times for exposures to carbon monoxide at various concentrations NT HS Response Time 1ppm amp 5ppm Output ppm equivalent AS 120 Time secs NT HS Response Time 10ppm amp 20ppm Output ppm equivalent NT H2S LI CD doc issue 1 August 2008 Time secs page 9 of 14 5 Long Term Drift Characteristics NT HS Long Term Stability 10ppm Original Output 100 Time days NT H2S LI CD doc issue 1 August 2008 page 10 of 14 DEFINITIONS Base line Base line shift Base line means the output level in clean air The output current value at 20 C would be less than 600nA but this tends to increase as the ambient temperature rises more than 30 C The base line shift means this variation of the output level i e a maximum of 600nA would be put out at 50 C This ba
10. ty long life and leak free performance even under severe operating conditions Air vent The electrolyte used for chemical sensors is very hygroscopic i e it has affinity for water and its volume varies depending on ambient temperature and humidity This variation causes pressure inside the sensor to rise and fall In the worst case the electrolyte may leak out of the sensor and damage the circuitry around it To prevent this the NT H2S LI utilizes an air vent capability This maintains equilibrium between internal and external pressures and allows the sensor to be used in any orientation and under high temperature and humidity conditions NT H2S LI CD doc issue 1 August 2008 page 5 of 14 PERFORMANCE DATA 1 Linearity A Q O L S a 2 2 5 gt o E a 2 5 a 2 5 e N Q L NT H2S LI CD doc issue 1 August 2008 NT HS Linearity 300 Gas Concentration ppm page 6 of 14 2 Uncompensated Temperature Dependence The following graph illustrates the typical effect of temperature on the output signal of the NT H2S LI for a sensor calibrated at 20 C Temperature dependence relative sensitivity Relative value 100 at 20 degree C 10 20 Temperature C Temperature dependence zero point Output ppm equivalent NT H2S LI CD doc issue 1 August 2008 10 20 Temperature C page 7 of 14 3 Cross Sensitivi
11. ydrophobic barrier to prevent this but we recommend the use of additional membrane barriers if the sensor in highly condensing RH conditions H2S gas is almost the same weight as air but as it is a combustion by product it will usually be heated and therefore rise H2S detecting devices should be installed at least 5ft from the floor and preferable on the upper part of walls or ceilings If the sensor is to be used in more irregular atmospheres please contact us for assistance 3 Storage of sensors Electrochemical sensors should be stored in a clean air under room temperature preferably 0 C 20 C and in non condensing RH conditions The maximum storage period would be 6 months after delivery For sensors stored for more than 6 months the performance guaranteed period will be shortened by the excess storage period Unlike semiconductor type or hot wire type gas sensors the gas sensitivity of electrochemical gas sensors will change as time passes regardless of whether sensor has been used or not NT H2S LI CD doc issue 1 August 2008 page 13 of 14 4 Mounting sensors Electrode pins must be connected correctly to ensure operation A thermistor for temperature compensation must be located near the sensor and away from heat sources such as transformer NT H2S LI can be mounted in any orientation NT H2S LI pins cannot be soldered as excess heating may cause the deformation of the housing and eventually leakage of electrolyte
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