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-------------------------Monitor------------------------
The Newsletter for PC-Based Data Acquisition and Control
Issue 50          www.windmill.co.uk      September 2002
--------------------ISSN 1472-0221----------------------

Welcome to the 50th issue of Monitor. This month sees the 
return of the Excel Corner plus notes on the accuracy of 
temperature measurements. 

We only send this newsletter to people who have 
subscribed - should you wish to cancel your free 
subscription please do so at 
http://www.windmill.co.uk/newsletter.html

CONTENTS
========
* Windmill News: Windmill Manuals in PDF Format
* How Accurate Are Your Temperature Measurements?
* Excel Corner: Saving Data only from the Last Hour
________________________________________________________
________________________________________________________

Windmill News: Windmill Manuals to be Available in pdf 
Format
________________________________________________________

You will soon be able to download all the Windmill 
Manuals in pdf format. The first available are the 
Windmill Installation and Configuration Manual and the 
IML Tools Manual. You can retrieve them now from
http://www.windmill.co.uk/help.html

The pdf format allows documents to be transferred 
electronically and viewed on many platforms. An Adobe 
Acrobat Reader is required to utilise these files. A free 
copy of this is available from Adobe's site at
http://www.adobe.com/acrobat/readstep.html
________________________________________________________
________________________________________________________

How Accurate Are Your Temperature Measurements?
________________________________________________________

Thermocouples are commonly used to monitor temperature, 
but how accurate are the results

There are many factors that limit the accuracy of 
the final figure for the temperature. ABSOLUTE 
ACCURACY relates to how closely the thermocouple 
derived temperature mimics the actual temperature. 
The major errors usually arise from the 
thermocouple materials themselves. There are 
international standards that specify the amount by 
which sensors may deviate from ideal behaviour. 
These figures are reproduced at 
http://www.microlink.co.uk/tctable.html

Typically thermocouples are accurate to 1-2 degrees 
Celsius. As thermocouples age their chemical and 
electro properties change, reducing their accuracy. 
This effect is quicker in harsh environments and 
when you regularly heat and cool the thermocouple 
to the limits of its range.

RELATIVE ACCURACY relates to measurement of changes 
in temperature, which is often the real purpose of 
thermocouples. Here the effect of noise can be very 
important. This happens when the thermocouple leads 
act as aerials picking up environmental electrical 
activity. A lot of this is common to both signal 
wires and a differential amplifier will remove a 
lot of common mode voltage. Differences between the 
signal wires (for example if they are separated 
rather than twisted together) will lead to residual 
voltages being added to the signal, increasing 
noise. Keeping the signal wires as short as 
possible, and as far away from electrical machinery 
as possible, will help. It's good practice to place 
the data acquisition units as close to the 
thermocouples as possible. Distributed systems 
using, for example, RS485, Ethernet or Modbus 
networks allow you to do this.

Further Reading
===============
For more on making temperature measurements with 
thermocouples (and RTDs), see Monitor Issue 5
http://www.windmill.co.uk/thermocouple.html

Monitor Issue 11 discusses differential connections
http://www.windmill.co.uk/differential.html

________________________________________________________
________________________________________________________

Excel Corner
________________________________________________________

The Excel Corner has been on its summer break. However, 
now it's back to tell you how to display data in real-
time in a fixed number of rows of the spreadsheet. When 
the rows are full, the oldest data will be overwritten. 
This is useful when you are only interested in, say, data 
from the last hour. To do this you need to write a macro 
such as the one given below. Each row in Excel will 
contain the time and date of the sample, and readings 
from all connected channels (temperature, pressure, 
latitude, longitude, gas concentration, etc).

                           *

Sub SampleData()

'Asks for the number of rows of data samples to display 
'and the sample interval.
NoOfSamples = Val(InputBox("Please enter no of rows of 
samples to display", "No of Samples"))
SamplePeriod = InputBox("Please enter sample interval in 
seconds", "Sample Interval")

'Coverts interval to fraction of 24 hours
'(Excel expects times in this format).
SamplePeriod = (Val(SamplePeriod)) / 86400

'Initiates conversation with Windmill
ddeChan = DDEInitiate("Windmill", "Data")

'When the required rows have been displayed,
'data in the first row will be overwritten
Restart:

'Ensures only required number of rows shown
'If NoOfRows = 1, the first data will be placed in row 1.
For NoOfRows = 1 To NoOfSamples

'Requests data from all channels and stores it in
'memory in an array called mydata.
mydata = DDERequest(ddeChan, "AllChannels")

'Selects first sheet in default workbook.
Sheets("Sheet1").Select

'Finds the lower & upper boundaries of array, to 
'determine the
'number of columns needed to store the data.
Lower = LBound(mydata, 1)
Upper = UBound(mydata, 1)

'Inserts data from the array into a row of cells.
For Column = Lower To Upper
Cells(NoOfRows, Column).Value = mydata(Column)
Next Column

'Waits for the specified sample interval.
Application.Wait (Now + SamplePeriod)

'Increments number of rows so next set of samples 
'is inserted in the next row down.
Next NoOfRows

'When maximum number of rows reached, specifies to 
'restart in row 1
NoOfRows = 1
GoTo Restart

End Sub

                           *

To use this macro simply 
- in Excel create an empty macro
- copy everything between, and including, "Start Sub" and 
  "End Sub" above
- paste into the macro you have created

If you want to save all data for later analysis, just 
use Windmill Logger to collect it for you. You can 
later open the data file in Excel.

For more on using Excel with Windmill see
http://www.windmill.co.uk/excel.html
http://www.windmill.co.uk/xlchart.html

________________________________________________________

* Copyright Windmill Software Ltd
* Reprinting permitted with this notice included
* For more articles see http://www.windmill.co.uk

We are happy for you to copy and distribute this 
newsletter, and use extracts from it on your own web site 
or publication, providing the above notice is 
included and a link back to our website is in place.


An archive of previous issues is at 
http://www.windmill.co.uk/newsletter.html

Windmill Software Ltd, PO Box 58, North District Office,
Manchester, M8 8QR, UK
Telephone: +44 (0)161 833 2782
Facsimile: +44 (0)161 833 2190
E-mail: monitor@windmill.co.uk
http://www.windmill.co.uk/
http://www.windmillsoft.com/

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