Epson XDT manual Appendix J Analog Output vs. Dewpoint

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XDT User’s Manual Appendices

Appendix J: Analog Output vs. Dewpoint

The current output of the XDT, varies with the measured dewpoint. To use the current to calcu- late the value of the dewpoint measurement, one must know the settings of the low and high ends of the analog output range, then:

(I Z) ⋅ (H L)

D = ---------------------------------------- + L

S

where: 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 units L = value of Low end of Analog Output range, converted to selected engineering units D = 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 high ranges; 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 a dewpoint of -40°C:

(12 – 4) ⋅ (20 – (–100)) ( )

----------------------------------------------------------- + –100 = –40

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Note 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 propor- tional to those units (approximately logarithmically proportional to dewpoint), refer to the graph that follows. Naturally selecting °C or °F will cause the analog output to be linearly proportional to dewpoint.

When monitoring in ppmV or LBS or G/M3, the analog output low & high ranges may have to be adjusted to provide a useful output. Consider an example where the area of interest to be moni- tored is 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 instances this would be an unacceptable output for proper monitoring of the measurement. In this example the user should adjust the analog output low & high ranges such that the output range is better suited to the measurement of 10 to 100 ppmV. It may be useful to select the low and high ranges to be 5 and 150 ppmV respectively, thus out of range conditions will be detected properly. Then the low range will be set to 5ppmV which is -65.5°C dewpoint, and the high range will be set to 150ppmV which is -38.5°C dewpoint. Now the current loop output will be 4.55 to 14.48 mA in the range of 10 to 100 ppmV, the ~10mA variation is more than sufficient for a good measurement by the user’s equipment. One may carry out similar calculations for LBS or G/M3 and choose the appropriate settings.While making these computations it may be useful to obtain a copy of Xen- taur’s dewpoint calculator, this is a Microsoft Windows™ program which simplifies the process of converting dewpoint measurement units. It is available at www.xentaur.com. If you are not cer- tain how 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 out- put settings.

In general, if the dewpoint is monitored in °C or °F, there is no need to change the factory default

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Contents Yyyyyyy y Sensor Serial No Instrument Serial No XDT User’s Manual Iii XDT User’s Manual Table of Contents Index Overview of the XDT XDT User’s Manual Sensor and Sampling Techniques Precautions using the sensor Sensor Installation & Sampling Techniques Sensor Technical SpecificationsNot Recommended In-situ InstallationExtractive Installation Mechanical Installation SAT Troubleshooting unexpected readingsElectromagnetic Compatibility Considerations Instrument Precautions using the XDTIsolation Instrument Technical SpecificationsInstrument Mechanical Installation InstallationXDT-PM DIN43700 Enclosure Installation XDT-NEMA IP65 Enclosure InstallationElectrical Connections Connecting PowerLow Voltage DC Powered Option Electrical Power Connection AC Mains Electrical Power ConnectionWiring the Alarm Contacts Sensor ConnectionInterfacing to the Analog Output Operating the Instrument Interfacing to the RS-232 optionStarting up Display Conventions 12 3 4 5 6 7 8Push Buttons Operating State Viewing Dewpoint ModeXDT User’s Manual Alarms Start Calibration HI **** ˚CSpanCheck Mode Single Point Self Calibration, manual or scheduled Instrument XDT User’s Manual SetUp State Viewing Serial Number ModeXDT User’s Manual Secured Password Protection Resetable Audio-Visual Alarm Option Nfpa compliant Troubleshooting the Instrument Display Explanation Required Action Maintenance XDT User’s Manual Glossary DB9 Glossary XDT User’s Manual Sign on message Calibration without key press Appendix a Flow Diagram of Operating State User InterfaceHere 16.24˚C˚F Appendix B Flow Diagram of Set-Up State User InterfaceSensor Mechanical AppendixAppendix D Optional Sensor Fittings Side View Appendix E XDT Circuit Board DimensionsAppendix E XDT-PM Enclosure Mounting Dimensions Appendix E XDT-NEMA Enclosure Mounting Dimensions Appendix F Sensor/SpanCheck Theory of Operation Appendix G Dewpoint Response time Analysis XDT User’s Manual Appendices Tested Instrument ˚C Appendix H Sample Gas Filter ConsiderationsSensor Appendix I XDT Grounding ConsiderationsAppendix J Analog Output vs. Dewpoint Instrument reading in ppmV Help command Receive Help Data Who Reset Login Data commandAppendix K RS-232C Interface Protocol K n m Error SensOpen or Error SensShort or Error SensSatSerial Number Command ? commandWho Command Login CommandState User Interface Appendix L Procedure for Exchanging XDT SensorsAppendix M Uncertainty in LBS & ppmV calculations To obtain a RAN fill out this form and fax to 631 Xentaur Return Authorization Number Request FormPage Index Enclosure dimensions Pounds of H2O per million standard cubic Voltage, mains supply Iii Warranty