Feature Transducers, transmitters & sensors


he calibration accuracy of many sensors is fundamentally depen- dent upon the force of gravity at the site of operation. Because of the principles on which they work, the sensitivity of accelerometers, incli- nometers, force transducers and load cells is fundamentally proportional to the force of gravity where they are being used; their absolute sensitivity may well differ when in situ from that of their place of manufacture. The acceler- ation due to gravity varies across the Earth’s surface due to a number of cir- cumstances and, in the extreme, may well translate to a variation of up to 0.5% depending on where in the world it is measured.


For example, electronic weigh scales that use load cells as weight sensors effectively measure the force of gravity acting upon a mass. If on-site gravity compensation is not taken into consideration, the scales will have an error proportional to the difference between the acceleration due to grav-


The importance of latitude and altitude T


Mike Baker, director at Sherborne Sensors, details the science behind the influence of location on the accuracy of inertial sensors, and offers a more simplistic explanation of how gravity can affect sensor calibration accuracy


ity between the installation and origi- nal calibration sites. Sir Isaac Newton’s Law of Universal Gravitation states that: “Every point mass attracts every other point mass by a force pointing along the line intersect- ing both points. The force is propor- tional to the product of the two masses and inversely proportional to the square of the distance between them.” Mathematically, the force due to gravity is expressed by the formula: F= Gm1


. (Where F is the force between masses, G is the Gravitational Constant, m1


m2 /r2 and m2 are the first and second


masses and r is the distance between the centres of the masses). Although Einstein’s Theory of General Relativity has since superseded this law, it continues to be applied, unless there is a need for extreme preci- sion or when dealing with gravitation for massive and dense objects. The Gravitational Constant G is actu- ally very difficult to measure but, in 2010, CODATA, (Committee on Data for


The LSW Series inclinometers are suited to demand- ing and rugged industrial applica- tions


There are several convenient sources of gravitational data available for reference. One of the most useful can be found on the PTB website: www.ptb.de/carto web3/SISproject. php


Figure 1: (far left) Acceleration due to gravity by location


Figure 2: (left) Acceleration due to gravity as a func- tion of altitude


nductive sensors are often, by necessity, placed directly in harm’s way when used in error-proofing and part-present applications. Impact, abrasion, and particulate erosion can destroy conventional plastic-faced sensors, and replacing these damaged sensors is a costly and time consuming process. In order to reduce replacement costs and the associated production downtime, Balluff offers the Steelface sensors which are rugged in construction. The sensors are built to survive longer in the arduous applications. The sensor is machined from a single piece of solid stainless steel, and has a 1mm thick robust steel face that is impact, chemical, cutting fluid and abrasion resistant. Available in M8, M12, M18 and M30 sizes (shielded and unshielded), the sen- sors offer the same sensing range as Balluff’s standard proximity sensors. Additionally, a series of 2x or 3x extended sensing range allows the user to mount the sensor further away from the target, minimising the risk of impact damage. Balluff also offers a range of stainless steel sensors which meet all the specifi- cations and directives in the pharmaceutical and food industry. The sensors are resistant to aggressive cleaning agents and provide IP67, IP68 and IP69K sealing. The range includes mini sensors to M30 inductive sensors, sensors with elevat-


Sensors survive arduous applications I


ed protection for the highest pressures and temperatures. Balluff


T: 0161 282 4700 14 www.balluff.co.uk Enter 219


, with an uncertainty of 1 part in 8,300. Thus, with knowledge of the mass of the Earth and its radius, the force due to gravity can be ascertained. It should be noted that the Law of Universal Gravitation defines a mass as a point mass; the Earth is neither of uni- form shape nor even mass distribution and cannot be treated as such unless to a first approximation. Gravity therefore varies in proportion to latitude, (Figure 1). Also, the height above sea level of land masses varies, so acceleration is proportional to altitude too, (Figure 2). Thirdly, the Earth is spinning on its axis and consequently the force of grav- ity at the equator is reduced by the cen- tripetal force, the effect diminishing to zero at the poles.


Science and Technology) recom- mended the value of G = 6.67384 x 10-11 m3


kg-1 s-2


What it all means Typically a weighing instrument is cali- brated using test masses. But as has been shown, the forces produced by test masses will vary according to loca- tion. Most accurate weighing systems will have a means of adjustment built- in to allow for multi-location deploy- ment throughout the world.


Inertial inclinometers and accelerometers also use gravity as their fundamental calibration reference, but calibration will only be truly valid when the sensors are used at the origi- nal calibration site.


When making accurate measure- ments and unless the sensors can be calibrated locally, it is essential to con- sider the latitude and the altitude at which the sensors will be installed and adjust the sensitivity of the sensors accordingly if the manufacturer’s cali- bration data is to be the sole source of reference.


Sherborne Sensors T: 0870 444 0728


www.sherbornesensors.com SEPTEMBER 2012 Process & Control Enter 220


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