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Delta-T SPN1 Technical Note

Use of the dni function
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Use of the DNI Function
Version 2 Jul 2016
Contents
Introduction .............................................................................................................................................. 1
About DNI: What is it and why should I care? ........................................................................................ 2
DNI: What is it? .................................................................................................................................... 2
DNI: Why needed? ............................................................................................................................... 2
How do I calculate DNI if my SPN1 points directly at the sun? .......................................................... 2
How do I calculate DNI if my SPN1 is horizontal?.............................................................................. 3
Location & Time ...................................................................................................................................... 3
Time Zone ............................................................................................................................................ 3
Location ................................................................................................................................................ 3
GPS time and position .......................................................................................................................... 4
SPN1 electrical connections ..................................................................................................................... 5
RS232 pin connections ......................................................................................................................... 5
Electrical limits ..................................................................................................................................... 5
Baud rates ............................................................................................................................................. 5
Power supply ........................................................................................................................................ 5
RS232 Commands for obtaining DNI ...................................................................................................... 6
SPN1 DNI Command usage ..................................................................................................................... 7
General command flow ........................................................................................................................ 7
Strategies for use of these commands ................................................................................................... 7
Conversion of GPS sentences for use with SPN1 ..................................................................................... 8

Introduction

SPN1 Firmware version (1.09 and later), which runs on the microprocessor in the SPN1, is also able to
calculate DNI, the Direct Normal Irradiance.
This information is supplied via the RS232 serial port upon sending the SPN1 the necessary
information and the appropriate command. For the SPN1 to calculate DNI it needs to be supplied with
the Latitude, Longitude, and the correct date, the local time and the time zone offset from GMT.
The information is only available on the serial output and is obtained by sending RS232 commands to
the SPN1. So to get the DNI value you need a program able to send RS232 commands and receive
RS232 text strings in reply.
This feature is provided mainly for those with a programming background or for engineers who are
familiar with the use of PC programs such as HyperTerminal or building management software, or the
programming language of their chosen data logger.
To use this feature you need one of the following
1 of 8
SPN1 DNI Firmware- Use of the DNI function
in SPN1
Technical Note
5/4/2016
www.delta-t.co.uk

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  • Page 1: Table Of Contents

    The information is only available on the serial output and is obtained by sending RS232 commands to the SPN1. So to get the DNI value you need a program able to send RS232 commands and receive RS232 text strings in reply.

  • Page 2: About Dni: What Is It And Why Should I Care

    Another use of DNI is its use as an intermediate value when calculating how much of the suns’ radiation is scattered or adsorbed by the atmosphere. The suitability of the SPN1 for this is being investigated. For further details see “The SPN1 and Atmospheric Light Scattering” at http://www.delta-...

  • Page 3: How Do I Calculate Dni If My Spn1 Is Horizontal

    GMT. Time Zone You need to know the local time at the SPN1 and how many hours you are ahead or behind GMT (Greenwich Mean Time) or UTC (Universal Time). For our purposes GMT and UTC are equivalent.

  • Page 4: Gps Time And Position

    GPS time and position For simplicity, we recommend that you take the latitude, longitude and GPS time from a GPS receiver, and set the SPN1 time zone to 00:00. These will then be correct wherever in the world you are. 4 of 8 5/4/2016 www.delta-t.co.uk...

  • Page 5: Spn1 Electrical Connections

    ±3V to ±15V. This is suitable for connection to a PC serial port. Only the Tx, Rx and Gnd wires are used for communication. Baud rates The SPN1 communicates at a fixed baud rate of 9600 baud, 8 data bits, no parity, 1 stop bit. Your serial port must be set to match this. Power supply If there is sufficient power available from the DTR signal (an output from the PC serial port), this will power the SPN1 sufficiently to take readings.

  • Page 6: Rs232 Commands For Obtaining Dni

    RS232 Commands for obtaining DNI These commands added to the SPN1 firmware in version 1.09 to enable the calculation of DNI are part of a larger set. For a description of the other RS232 commands and their use please see the SPN1 Technical Manual.

  • Page 7: Spn1 Dni Command Usage

    SPN1 DNI Firmware- Use of the DNI function The SPN1 clock and any input times to these commands are assumed to be in the local time represented by the time zone set using the ‘L’ command. The SPN1 then converts this local time into universal time (UTC or GMT) for use in the solar position calculations.

  • Page 8: Conversion Of Gps Sentences For Use With Spn1

    The SPN1 clock will keep good time for many weeks or months, but the SPN1 has no internal power source, so it will reset on a loss of power. We recommend that you check and set the SPN1 clock on power up, and thereafter once a day, if you are using it to provide your local time.

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