XDT Manual - Pioneer Air Systems Engineering

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XDT User’s Manual AppendicesAppendix I: XDT Grounding ConsiderationsPluggable Block TerminalsPower SupplyModule0.0047uf0.1uf4.3mH0.0047ufRFI filter boardLogic ground and ACPower ground areconnected for safetyand electromagneticinterferenceconsiderations1M OhmLOALARMSHI0.5A 250V0.1uf275VACInstrument Circuit boardSensorMeasuringCircuitry33V 400W1 AC LIVE2 AC NEUTRAL3 Not Connected4 AC GROUND5 SIG-IN white6 SIG-RETblack7 A-OUT8 A-RET Analog Output 4/20mA9 TxD10RxD11 RET12PS+13AC GROUND14Not Connected15NC16NO17C18Not Connected19NC20NO21CChart recorder, A/D converter etc.RS 232 I/OPC, Data Logger etc.SensorSensor casing connects to pipingconnection throughEarthSafetyGroundFacilityAC PowerPossible Earth Ground at Analog Output monitoring equipmentconnectionPossible Earth Ground at terminating equipmentstructural members pipesloopgroundcauseNOTetc. doesthrough equipment ground COULDconnection through equipment ground COULDcause aground loopcause aground loop1. The sensor ground is isolated from the signal and frame grounds, however it is shunted toframe ground with a 33V Transient Voltage Suppressor,1M Ohm resistor and 0.1uF capacitorto prevent electrostatic buildup and noise pickup. When the sensor is connected to equipmentwhich is grounded to earth ground (because of pipes etc.), there is no ground loop because thesensor is not connected to the earth ground on the circuit board.2. The optional Analog Output and RS-232 Output are both referenced to the circuit board logicground, which is jumpered at a single point to the AC power ground. Therefore if these outputsare connected to equipment which connects the signal grounds to earth ground (this is acommon occurrence in PC's), then a ground loop may be formed. This can be best avoided byremoving the signal to earth ground connection at the termination equipment; if that is notpossible then the AC power ground connection to XDT logic ground may be removed but thismay cause a EMI problem. Please consult with your representative for information onoptional isolated analog and RS-232 outputs. Keep in mind that in general, a ground loop on adigital line such as RS-232 will not cause problems.3. The Earth Safety Ground (AC GROUND) at pin 4, should not be omitted since the RFI filtercapacitors will cause half the line voltage to develop at the instrument’s ground. This mayhave safety and EMI implications.4. When installing DC powered instruments (15 to 30VDC at pins #12 and #11), similar conditionsmay exist, if the DC power source is earth grounded.49
XDT User’s Manual AppendicesAppendix J: Analog Output vs. DewpointThe current output of the XDT, varies with the measured dewpoint. To use the current to calculatethe value of the dewpoint measurement, one must know the settings of the low and high endsof the analog output range, then:D=( I–Z) × ( H–L)---------------------------------------- + LSwhere: I = current supplied by XDT analog output in mA.Z = value of zero output of current loop in mA: for 4/20 = 4, for 0/24 = 0.H = value of High end of Analog Output range, converted to selected engineering unitsL = value of Low end of Analog Output range, converted to selected engineering unitsD = dewpoint measured by instrument in selected engineering units.S = span in mA of current loop output: for 4/20 = 16, for 0/24 = 24.Consult section 3.4.5.7&8 and/or appendix B, to check and set the Analog Output low and highranges; the factory default settings are -100°C and +20°C respectively.For example a unit with factory default settings, supplying 12mA is computed to be measuring adewpoint of -40°C:( 12 – 4) × ( 20 – (–100))----------------------------------------------------------- + (–100)= – 4016Note that the computation is such that the current is linear to the selected engineering units.Hence, selecting ppmV or LBS or G/M3 units, will cause the analog output to be linearly proportionalto those units (approximately logarithmically proportional to dewpoint), refer to the graphthat follows. Naturally selecting °C or °F will cause the analog output to be linearly proportionalto dewpoint.When monitoring in ppmV or LBS or G/M3, the analog output low & high ranges may have to beadjusted to provide a useful output. Consider an example where the area of interest to be monitoredis 10 to 100 ppmV, and the analog output is set up with the factory defaults of -100°C to+20°C (which is 0.014 to 23612 ppmV); then the current loop output will vary only from ~4.1 to~4.2 mA in the area of interest (consult with the graph on the following page). In most instancesthis would be an unacceptable output for proper monitoring of the measurement. In this examplethe user should adjust the analog output low & high ranges such that the output range is bettersuited to the measurement of 10 to 100 ppmV. It may be useful to select the low and high rangesto be 5 and 150 ppmV respectively, thus out of range conditions will be detected properly. Thenthe low range will be set to 5ppmV which is -65.5°C dewpoint, and the high range will be set to150ppmV which is -38.5°C dewpoint. Now the current loop output will be 4.55 to 14.48 mA inthe range of 10 to 100 ppmV, the ~10mA variation is more than sufficient for a good measurementby the user’s equipment. One may carry out similar calculations for LBS or G/M3 and choose theappropriate settings.While making these computations it may be useful to obtain a copy of Xentaur’sdewpoint calculator, this is a Microsoft Windows program which simplifies the processof converting dewpoint measurement units. It is available at www.xentaur.com. If you are not certainhow to carry out such calculations, send by e-mail to xentaur@xentaur.com or by fax to (631)345-5349, your system specifics, and some one will get back to you with appropriate analog outputsettings.In general, if the dewpoint is monitored in °C or °F, there is no need to change the factory default50
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