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1. 20 mm long Pillar 7 Calibration The 4 20 mA transmitter is shipped pre calibrated to the range shown in Table 1 Periodic re calibration is required for all gas sensors especially in safety critical applications To Calibrate e Connect communications link to PC USB port and run configuration software Connect the Equipment Under Test EUT to the communications link Up Load all present sensor parameters e Ensure that a high quality zero gas source is available e g cylinder of zero air or cleaned and scrubbed compressed air and a bottle of calibration gas with validated accuracy see Table 4 below e Apply zero gas for 10 minutes at the flow rate shown in Table 2 Read the sensor output current on the screen ensure it is in the correct range Click the Zero button in the sensor calibration frame Follow instruction to enter gas sample in ppm Normally 0 ppm e Then press the CAL button The transmitter is now calibrated against the sensor zero e Apply test gas for ten minutes the recommended test gas concentration for calibration is shown below in Table 4 Read the sensor output current on the screen ensure it is in the correct range Click the Span button in the sensor calibration frame Follow instruction to enter gas sample in ppm Then press the CAL button The transmitter is now calibrated against the sensor Span e Although it should not be necessary it is good practice to recheck the zero after
2. setting the span Check the gas ppm reading is within ppm in clean air zero gas Allow at least 10 minutes for full recovery to zero after the calibration gas has been removed e At this stage the transmitter is correctly re calibrated If required the output range can be re configured by changing the output low and high range values ppm Alphasense Limited Page 8 of 10 Toxic 4 20mA Table 4 4 20 mA Transmitter full span 8 Communication Link The communications link uses interface hardware connected to the USB port of a PC The USB port is used to Provide power to the transmitter during configuration and calibration Two way Digital communication is performed down the 4 to 20 mA loop using current pulses The communication rate is 1200 baud To use the link connect interface transmitter to a PC as shown in Figure 8 then run the configuration software and follow instructions Figure 8 Communications Method amp LOOP TRANSMITTER INTERFACE COMPUTER SOFTWARE Alphasense Limited Page 9 of 10 Toxic 4 20mA The communication link provides the user with access to the following list of process variables configuration and calibration settings Type Units Description Sensor Current uA True sensor current Raw Sensor output ppm Value without temperature compensation Sensor output ppm Temperature compensated value Ambient t
3. ALPHASENSE ALPHASENSE USER MANUAL 4 20mA Digital Transmitter for Toxic Sensors 1 INTRODUCTION Figure 1 Block Diagram 4 20 mA Transmitter POTENTIOSTAT Q GAS SENSOR BIAS K gt BUFFER 4 20 mA LOOP MICRO PLUS DIGITAL M D COMMUNICATION Y ADC Kk PINK TEMPERATURE SENSOR K gt The Transmitter PCB includes circuitry for a 3 electrode toxic sensor to convert the pA output signal from the sensor to a 2 wire 4 20mA signal The Circuitry is based around a low power Micro controller which provides digital communication to a PC via the 4 20 mA loop This communication link allows the user to monitor process variables perform active calibration of the sensor and configure the transmitter parameters A special interface is required for communication the interface connects directly into the USB port of a PC During configuration the transmitter is powered direct from the interface Figure 2 Function of Micro controller INPUT CONVERT p nee ya M TEMPERATURE pmp CONVERT p READ ADC RAW ADC TO TO ben VALUE COMPENSATE ppm to mA DAC RAW COUNTS TRUE uA VALUE PP ppm OUTPUT OUTPUT FACTORY SET SENSOR OUTPUT FACTORY SET CALIBRATION CALIBRATION SOMFENSATION RANGE CALIBRATION DATA DATA DATA TEMPERATURE a DIGITAL COMMUNICATION LINK TO PC VIA LOOP AND USB INT
4. ERFACE PROVIDES USER WITH ACCESS TO CALIBRATION CONFIGURATION AND PROCESS VARIABLE DATA The transmitter board includes four mounting pillars and hardware that may be used if required Alphasense Limited Page 1 of 10 Toxic 4 20mA Alphasense 4 20 mA transmitters offer convenience and easy maintenance for toxic sensors e Transmitters are shipped pre calibrated for immediate use e Small circuit board size allows smaller sensor housing e Conformally coated circuit board gives environmental protection of circuit e Low power circuitry with excellent performance means no degradation of sensor performance due to electronics e Close PCB tracking of sensor transducer reduces noise pick up and RFI EMI susceptibility e Temperature compensation is provided together with switched bias e Configurable range allows user to select any range within the sensors operating range Besides periodic sensor re calibration the transmitter electronics require no maintenance throughout the lifetime of the sensor The sensor can be replaced at the end of the sensor working life Re calibration is required when the sensor is changed Please read these instructions to ensure correct installation use and calibration of your gas sensor transmitter 2 Transmitter Specification 20 C Input Type Range Minimum Span Sample Rate Thermal Drift Protection Resolution Bias Temperature Compe
5. allowing backward compatibility with the earlier PCB design Figure 3 Dimensions 19 5 19 5 32 39 32 39 Alphasense Limited Page 5 of 10 Toxic 4 20mA The pillars are tapped to accept M3 screws We recommend a screw length of at least 8mm to ensure rigid fixing It is good practice to hold the pillar when screwing into the pillars to stop the pillar from rotating on the PCB It may be easier to remove the sensor whilst screwing the circuit board pillars to your housing If you remove the pillars ensure that if mounting to the lid of your housing that you include the washer between the pillar and PCB to ensure correct height of the pillar assembly See Figures 4 and 6 Connector Screw Figure 4 Mounting pillar configuration for Figure 5 Mounting pillar configuration for attaching to attaching to the lid of an enclosure the base of an enclosure Long pillars required Part No 034 0005 00 rc 165 MG Fa i iae chassis Pillars mounting pillars aP MUST be fitted both sides Figure 6 Side view of mounting to lid of an enclosure Figure 7 Side view of mounting to the base of an enclosure Allow 20 minutes after plugging the sensor back into the board for the output to stabilise Ensure that the sensor is sealed securely to the top face of your housing The O ring supplied with your transmitter sensor should be used to ensure an airtight seal avoiding any ac
6. cess of toxic or corrosive gases to the circuit board and the housing interior Alphasense Limited Page 6 of 10 Toxic 4 20mA 4 2 Connection and Wiring Power to the transmitter board is via a Molex 2 pin mini plug Type 22 27 2021 supplied with the transmitter Connect using a screened 2 core cable to the wires black is ground red is positive by either soldering or using a screw terminal block These leads can be shortened or extended as needed 4 3 Power Supply Your power supply must be between 10 and 30 VDC with less than 1 V RMS ripple Warning Do not supply Mains AC power to this unit this will destroy the transmitter and void the warranty The transmitter is protected against incorrect polarity and over voltage but will not function if you have reversed the power supply wires by connecting the Molex plug incorrectly to the transmitter board socket When selecting the power supply voltage you must not exceed the maximum total loop resistance which includes your measuring resistor used to change the 4 20 mA current into a measured voltage The transmitter requires a minimum of 10 volts to operate therefore the maximum potential drop allowed across your sensing resistor and cable is Power supply voltage 10 V Assuming full scale deflection at 20 mA use Ohm s law to calculate the maximum loop cable plus sensing resistor allowed resistance To maintain CE requirements it is recommended that eithe
7. emperature C Sensor temperature used for compensation Output Current mA The output current being transmitted CONFIGURATION O O S O Type Units Description Sensor Type Sensor Part Number Bias mV Select Bias off 200mV 300mV Ta Compensation Temperature compensation off on Low Range ppm Sensor ppm 4mA output High range ppm Sensor ppm 20 mA output Sensor Calibration zero ppm Gas Sample Normally 0ppm uA Sensor current zero gas sample Sensor Span Calibration ppm Gas sample span uA Sensor current span gas sample Additional data available with the communications link but intended for factory use only e Temperature Compensation characteristics e Current input calibration Factory set e Current output calibration Factory set 9 Warranty Transmitters are warranted for two years Sensors are warranted separately If you have any difficulties or problems then contact Customer Support Alphasense Limited Sensor Technology House 300 Avenue West Skyline 120 Great Notley Essex United Kingdom CM77 7AA Tel 44 0 1376 556 700 Fax 44 0 1376 335 899 Email sensors alphasense com Web Site www alphasense com Alphasense Limited Page 10 of 10 Toxic 4 20mA
8. es Although the sensor and transmitter are pre calibrated and the ranges are preset it is possible to change range by using the communication link to re configuring the output range Both Low 4 ma and High 20mA range can be set anywhere within the sensors working range this makes it possible for example to have a 20 to 4 mA output for say a 0 to 100 ppm input Different sensors can be fitted to a transmitter board the communications link allows the operator to select the correct sensor type and then perform active sensor calibration Alphasense Limited Page 4 of 10 Toxic 4 20mA 4 Set up 4 1 Mechanical Mounting Transmitters should be mounted to your housing using the pillars provided These pillars should first be attached to the PCB using the supplied screws and washers Important note Ensure that a washer is fitted on BOTH sides of the PCB see Figure 3 Two sets of mounting holes are provided so that the assembly can be fixed to either the housing top using the locating holes in the corner of the PCB or to the base of the housing using either set of locating holes Note Long pillars will be required for this configuration Alphasense Part No 034 0005 00 Figure 3 below shows mounting hole locations dimensions are in mm Figure 3 diagrams the locating holes normally the outer holes are used for mounting while the inner holes are in the same location as the earlier issue of this PCB
9. nsation Connection 3 wire gas sensor Series A or Series B 4 or 7 Series 50 uA to 100 uA 3uA 500 ms per sample 0 05 uA C Power off shorting FET 4 nA resolution Selectable off 200 mV 300 mV 10 mV tolerance On board temperature sensor range 30 C to 60 C 0 5 C used for selectable Software correction zero and span Plug in sockets Alphasense Limited Page 2 of 10 Toxic 4 20mA Output Output Maximum Output Range Operating Voltage Accuracy Sensor out of range Supply sensitivity Loop ripple effect Thermal drift Loop resistance Resolution Protection Connection General Warm up Time Environment Ambient Temperature Ambient Storage Ambient Humidity Coating CE approval 4 to 20 mA 2 wire loop powered 3 8 to 21 mA 10 to 30 VDC 5uA Up Scale gt 21 mA lt 0 03 10 to 30 VDC 2 uA measure 1 volt RMS 50 Hz supply ripple 0 2uA C 700R 24 VDC 0 75 uA Reverse connection protected and over voltage 2 Pin MOLEX plug Ref 22 27 2021 2 Minutes to full accuracy 30 C to 60 C 40 C to 70 C 0 to 95 Continuous Non Condensing Conformally spray coated BS EN 61326 Industrial Alphasense Limited Page 3 of 10 Toxic 4 20mA 3 Range Options Sensor and transmitter boards are shipped from Alphasense pre calibrated Standard available ranges are listed below Table 1 Transmitter rang
10. r twisted pair or screened cable are used for cable lengths greater than 30 metres It is important to ensure the 4 TO 20 mA loop is grounded at one point ideally at the power source 5 Correct Usage and Maintenance Ensure that there is a good gas seal between the sensor and the housing also if the sample is pumped then ensure that the flow rate is sufficient Alternatively the sample gas can be allowed to diffuse to the front face of the sensor The table below shows the recommended gas flow rate in standard cubic centimetres per minute sccm Higher flow rates may be used but beware that pulsing flow and higher pressure drops may lead to erratic readings Table 2 Pumped gas recommended flow rates 300 to 500 400 to 700 400 to 700 Clz NO3 400 to 700 Alphasense Limited Page 7 of 10 Toxic 4 20mA The only required maintenance is changing of the O ring if it has been exposed to extreme environments for long periods this O ring should last the lifetime of the sensor in normal conditions In addition if the top dust oil filter has become badly contaminated then contact Alphasense for replacement dust filter Section 5 6 Reordering Part Numbers Replacement sensor O rings and dust oil filters can be ordered by quoting the part numbers below Table 3 Replacement Part Numbering 039 0002 00 024 0011 00 010 0178 00 034 0005 00 Replacement O ring Self adhesive dust oil filter Cable assembly 150 mm

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