A series of helpful hints to ensure that your sensor or system is being used at its best...
If you wish to contribute to this section, please sent your entry to the Editor at editor@sensorland,com
1). A sensor is normally purchased to measure one particular parameter,
pressure, force, position, vibration, etc. But all sensors are affected by their
operating conditions and part of its output signal will be due to these external
influences. It may be varying temperature, acceleration forces, cable pickup,
extraneous loads....a whole host of factors which will corrupt your data and
give large inaccuracies. Ensure that the sensor you buy is best suited to all
the conditions it will experience and also make sure that all external conditions
that will effect the final output are kept to a minimum.
Sensors Web Portal is one of the biggest frequently up-dated Internet
recourse for design engineers devoted to smart sensors, transducers
and sensor instrumentation.
Click image to visit...
ONLINE CALCULATORS
Useful tools for Engineers from FUTEK - Advanced Sensors Technology, Inc.
Bolt Torque - Shunt Calibration - Span Adjustment - Unit Conversion - Zero Balance
FREE Vibration Calculator
No Batteries Required...
The Endevco Vibration Calculator implements
the equations of dynamic motion relating
frequency, displacement, velocity and acceleration.
Frequencies from 0.01 Hz to 10 kHz, acceleration
in g's, velocity in ins/sec and mm/sec peak,
displacement in mm and inches pk -pk. Acceleration AdB & velocity VdB scales.For your FREE calculator...Email: info@endevco.com
Web: www.endevco.comTo visit this informative website, click image...
3). A Tech Note - Strain Gauge Selection
Designed to guide the reader through the various stages of
successful strain gauge installation. Clear concise text,
selection tables, reference charts and gauge pattern illustrations
combine to ensure that this authorative work is essential reference
material for all those involved with strain gauges.
For your FREE copy, contact Measurements Group UK Ltd. at
+44(0) 1256-462131
FREE Absolute/Relative Humidity Calculator
An extremely useful calculator for the conversion
of Absolute and Relative Humidity, available free
from Michell Instruments Ltd.
For your FREE copy... Email: info@michell-instruments.com
Web: www.michell-instruments.com
FREE Dynamic Measurement Handbook
The Endevco Dynamic Measurement Handbook is a
legend in the industry. Available in a handy pocket-size
(9x16cm) the 60+ page compendium contains formulae,
tables, conversion factors and other technical data used
in dynamic testing.
For your FREE copy... Email: info@endevco.com
Web: www.endevco.com
Visit our ON-LINE EXHIBITION, click here...
4). When fitting an accelerometer, ensure that its mounting face is
completely flat and not on a curved or irregular surface. This allows the
accelerometer to become part of the structure under surveillance and gives
the optimum transfer of vibration levels into the sensor.Accelerometer Mounting Considerations
A collection of four pages of PDF files for you to download - Click here(PDF files require Adobe Acrobat for display...this software is available
FREE from Adobe, if Acrobat is installed on your system, the program
will automatically run on file selection.)
click image to download your FREE copy of Adobe Acrobat
5). Tips and Techniques for successful Condition Monitoring.
A recently released booklet written by Roger Dunn.
...a practical guide to Vibration Monitoring in predictive
maintenance.
Ideal for both newcomers and experienced engineers to this
scientific approach to protecting important factory machinery.For your FREE copy, email your address by clicking here.
6). Accelerometers or vibration sensors can be mounted with a variety
of methods to ensure that the vibrations are transferred as faithfully as
possible into the sensor.
Methods used include fixing with beeswax, hard adhesives (dental type glue),
magnetic mounts and thread fixing (male or female).
Click for conversions...
7). Pressure transducers can be easily overloaded by surge pressures
and transients of which the user is completely unaware. These surge
pressures can be many times greater than the static overload performance
of the sensor. Ensure that the sensor is protected from such transients such
as by fitting snubbers.
8). Mounting a load cell correctly is important to ensure that its
performance is accurate and repeatable. Many load designs exist, with
some models accepting a certain degree of extraneous loading, but it
recommended that its mechanical fitting is flat and secure with the load
being applied vertically through the cell.
9). Understanding linearity. There are several ways to interpret and
represent linearity of a transducer. Those most commonly used are...
End point linearity, Best fit straight line (BFSL), and Least squares BFSL
linearity.End Point Linearity
From the graph plotted of Voltage Output against Increase in Measurand
which usually appears as a curve, a straight line is drawn from the zero
point to the full scale output point. Usually the point which deviates most
from the simple straight line will be used to specify the 'linearity' of the
transducer. This is quoted as a percentage of the normal full scale output
of the transducer.Best fit straight line method of definition
In practice, the relationship between the measurand and the output of most
transducers is not perfectly linear and it is therefore necessary to find a way
of using these devices to achieve the most accurate overall results. This can
be achieved by constructing what is known as the "Best fit straight line" (BFSL)
through the calibration points in such a way that the maximum deviation of the
curve from the line is minimised as indicated in the figure below.This diagram simply illustrates how, instead of
drawing a straight line between the origin and
the end point at "B", we can construct a line
which effectively halves the maximum deviation
that would appear at point "A" and shares it with
the end point at "B". Thus the deviation from the
BFSL, and hence the linearity error, is effectivelyhalved.
However, it should be appreciated that this is simply a means of interpretation
of the results and relies entirely on the system, including the measuring
equipment, being set up to take advantage of this technique.To take advantage of this method in the case illustrated, it Is necessary to set
up the system so that when the transducer is at full scale 'y' at point 'B' the
indicated output would be set to a value of 'x'. This then effectively halves the
error indicated at 'A' which would now deviate from the BFSL by the amount
rather than if we had simply constructed an 'end point straight line'.It can be clearly seen that this enables us to effectively halve the errors in the
system and thus record better results over the whole measuring range. Note
however that although it is quite easy to construct a BFSL on a graph of
transducer output against measurand, it is not so easy to do when faced with
a simple set of figures. In practice we need a better, quicker and more
consistent means of determining the BFSL which can also take into account
transducers with bi-polar outputs such as LVDTs and tension/compression
load cells.The answer to this problem is the 'Least squares method' of determining the
BFSL. This is a statistical method, which enables the BFSL to be determined
mathematically over any chosen working range, and is the most suitable
method for use in computerised calibration systems.Least Squares Best Fit Straight Line Method
In the previous issues of Transducer Action, we have discussed 'End point linearity'
and 'Best fit straight line' methods of definition. Our concluding article deals with the
'Least Squares Best Fit Straight Line' method, preferred by most transducer
manufacturers because it provides the closest possible best fit to all data points
on the curve, and can be most readily adapted to the computerised calibration
systems in common use.The Least Squares Best Fit Straight Line is a statistical method and as such may
not be a 'purist' approach but provided the characteristics of the transducers are
correctly optimised at the design and development stage and are represented by
a continuous smooth curve, the assessment is meaningful and accurate.In practice, up to 20 calibration points will be taken over the whole working range
of the transducer and the measured input and output values at each point used to
provide the data for calculation of the slope of the 'Least Squares Best Fit Straight
Line' using the following equation:
Where: Xd = known input data points
Yd = actual sensor output at each Xd data point
n = number of data points
Having mathematically determined the slope of the best fit straight line it is then
possible to determine the maximum deviation of any point from this line using the
equation:
where fr = full working range
As with all other methods the maximum deviation value would be expressed as
a percentage of the total linear range of the device. It should be noted that since
this evaluation is done over the total range of the transducer the best fit straight
line may not pass through the zero point of a bipolar device such as an LVDT,
Universal load cell, differential pressure transducer etc. However, this does not
have any great practical significance since the zero output point is usually
adjustable electronically anyway.In general it can be readily agreed that this method is by far the most efficient one,
but it must be used with care and understanding. Clearly because of its statistical
nature, the number of data points taken will have a direct bearing on the ultimate
validity of the assessment. In practice, the more uneven the characteristic curve,
the more data points must be used in order to take the incremental non-linearity
into account.
10). NEW Handbook on Environmental Noise from Bruel & Kjaer.
A NEW 56-page Handbook aimed at anyone involved
in measuring and assessing environmental noise.
An introduction to environmental noise with sections
on definition of sound, environmental noise propagation
and indentification of noise sources.
For your copy, contact : Bruel & Kjaer at
Fax: +44(0) 1438 739099 or Email: info@bkgb.co.uk
11). Improve the accuracy of relative humidity (RH) sensors, which
can be affected by both soluble and insoluble contaminants, by ensuring
that periodic checks are made of response, accuracy and calibration.
12). Many process applications for pressure sensors demand that
accurate results must be achieved either under extreme temperatures or
under conditions where temperature fluctuates dramatically.Different types of sensor are affected by temperature in different ways.
With thermal hysteresis (often defined as thermal stability) being a particular
problem. For example, silicon devices can be susceptible to differential
expansion between the various materials used in their construction, while
bonded gauge devices cannot be used in areas of high or low temperatures,
as the bonding materials will either soften or become brittle.Other, perhaps more subtle effects resulting from changes in temperature
include non-linear errors in output measurements. By comparison, thin
film sensors generally exhibit high levels of long term accuracy, with
linearity and hysteresis remaining constant as temperature changes.
13). Transducers are affected by moisture.This can be caused by a
high level of vapour in the surrounding atmosphere, by the need to immerse
the device in water or simply because the end cap has been removed on site.
In each case, it is important to ensure that the trasnducer and connectors are
sealed to IP or similar standards, and that both the electronics and the gauge
structure itself have been selectively coated with a moisture resistant material
such as silicon dioxide.14). FREE Pressure Conversion Slide-Chart
For your FREE slide-chart, email your address details to sales@variohm.com
15). Useful book - Radar level measurement - the user's guide
by Peter Devine
The guide is a full and comprehensive publication in full colour hardback
A5 format covering the following:Part I
1. History of radar
2. Physics of radar
3. Types of radar.
Part II
4. Radar level measurement
5. Radar antennas
6. Radar level installations
a. Mechanical installation
b. Electrical installation
Part III
7. Other level techniques:
Amongst others, DP transmitters, Capactive transmitters,
Electromechanical transmitters and Ultrasonic.8. Applications:
Covering applications in the following industries: Brewing &
Distilling, Cement, Chemical, Food, Metals, Offshore,
Pharmaceutical, Power and Water & Waste water industries.
Part IV9. VEGA radar
Two wire loop powered radar
Vegapuls 50 - technical specification
Vegapuls 40 - technical specification
Vegapuls 40 & 50 options
Echofox software
The book also has additional appendices, which cover the
following: Glossary of terms, Radar power & radar power density,
Dielectric constants Symbols, Photograph acknowledgementsTel: Vega Controls - +44(0) 1444 870055
16). To find out more about the phenomenon of Liquid (Water) Hammer
in pressure measurement, click here for an informative article by
Danfoss Limited.
17). Calibrating a smart HART device.
This step-by-step guide to calibrating a 'smart' pressure transmitter
assumes that it is isolated from the process, is not electrically connected
to a loop power supply, and is configured for psi units.1. Power on the DPC and display the basic HART information for the
transmitter.2. Configure the DPC by selecting MEAS mA, SOURCE psi to measure the
analog mA output and the pressure being applied simultaneously to both the
transmitter input and pressure module.3. Vent the pressure line and clear the pressure module to zero. Select the
instrument for a linear transmitter calibration and fill in the appropnate test
tolerance.4. Apply the input pressures as instructed. When the test is complete, an
error summary table is displayed. Test errors exceeding the tolerance
will be highlighted.5. If the As Found test failed; i.e., there were highlighted errors in the error
summary table, adjustment is necessary.6. Zero the pressure module (vent to atmosphere). First test the Lower
Trim value.
For best results, apply the LRV pressure and retrieve the value being
measured by the pressure module. Then move to the Upper Trim.
As before, apply the URV pressure.7. Next perform the Output Trim. The value of the primary variable
(PVAO) is normally a 4 mA signal. Load the measured mA value.
Send the value to the transmitter to trim the output section for the
4 mA value. Repeat for the 20 mA trim.
After completing Output Trim, carry out the As Left verification test.
Apply the requested pressures. Upon completion an error summary
table is displayed.
If none of the errors are highlighted, the transmitter has passed the
As Left test. If errors are highlighted, the test has failed and further
adjustment is required.
Return to step 5.
FREE download of software for inputting, logging and chart plotting
sensors data through the games port of a PC.
To get your FREE copy, click here...FREE Strain Gauge Selection WallChart from TML...
For your copy, email your address details to...
14). FREE Vibration Conversion Slide-Chart
For your FREE slide-chart, email your address details to trohrer@ctconline.com
For Glossary of Terms page, click here ...
Click image to find out more about CALISO CALIBRATION TOOLBOX,
a Windows based Calibration Management Program for Scientists and
Engineers
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