Load cells

Fig.l shows the layout of the SPATE 9000 system. It basically consists of a scan unit connected to a signal amplifier. The signals are then correlated with a reference signal derived from a load transducer (e.g. strain gauge, load cell, accelerometer, or function generator). 

Strain-Gauge Load Cells. The majority of industrial scales today use strain-gauge load cells as the weighing element. The strain-gauge load cell is a device which, when a force is appHed to it, gives an electrical output proportional to the appHed load. 

Fig. 6. Load cell spring element. C, gauges in compression T, gauges in tension.

The advantages of scales based on strain gauge load cells are as follows ... 

A large selection of load cells is available in a variety of configurations and materials with capacities from 0.5 kg to 100 t or more Ease of installation load cells can be used individually or cormected in parallel for multiple load-ceU appHcations... 

A very low output voltage from the load cell, commonly 1 ]lV per displayed division, makes the signal susceptible to noise and degradation (particularly over long distances)... 

Strain-gauge load cells are sensitive to temperature gradients induced by, for example, radiant heat from the sun or resulting from high temperature wash down. Load cells should be shielded from such effects or given time to stabilize before use. 

Crane scales are designed to be inserted between the hook of a crane and the load being lifted. The upper end of the scale has an eye which attaches to the crane s hook, whereas the lower end has a hook that picks up the load. A tension-type load cell connects the eye and hook. The display may be either integral with the load cell or remotely mounted. Capacities range up to 50 t. 

Precision filoor scales have platform sizes of 1.5 m square or larger, and typically use a lever system to transmit the load to a single EMFC load cell. Capacities range up to 6 t with a minimum of 30,000 displayed divisions. 

As a vessel is loaded, it moves downward because of deflection of the load cells and support stmcture. Pipes rigidly attached to a vessel restrict its free movement and assume some portion of the load that cannot be measured by the load cells. This is very detrimental to scale accuracy. Deflection of the load cell is unavoidable deflection of the vessel support stmcture should be minimized. Anything which increases vessel deflection, eg, mbber pads used for shock protection, must be avoided. The total number of pipes should be minimized and be of the smallest diameter, thinnest wall possible. Pipe mns to weigh vessels must be horizontal and the first pipe support should be as far as possible from the vessel. Alternatively, a section of mbber hose or flexible bellows should be used to make the final connection to the vessel. The scale should be caUbrated using weights, not by means of an electrical simulation method, which cannot account for the effects of the piping or test the correct functioning of the scale. 

Cahbration can also be accompHshed usiag material weighed on another scale. The accuracy of this method depends on the accuracy of the other scale, and care must be taken not to lose any of the weighed material. Scales can also be caUbrated electrically usiag a load cell simulator if the load cells rated outputs are known accurately. This method does aot test the mechanical fiinctioning of the scale and is not very accurate, particularly if it has attached piping that restricts its vertical movement. 

Ga.in-in- Weight Feeders. These types of feeders are used only for batching appHcations. The receiving container rests on a scale or on load cells and the system controls the discharge from the filling bin, which can use a volumetric feeder to control rate. A batch accuracy of 1/4% at two standard deviations is not unusual. 

The Rheo-Tex rheometer is an inexpensive, automated instmment using load cell technology to measure indentation and creep. Available software calculates hardness/softness, brittleness, plasticity, and tensile strength. This instmment is particularly valuable for measurements on foods and personal care products. 

Load cells are capable of resolutions of 1 part in 100,000. A/D converters for analog inputs cannot even approach such resolutions. 

One advantage of serial interfaces is that two-way communication is possible. For example, a tare command can be issued to a load cell. 

Of special interest in scale-control systems is the type in which the motion of the scale beam is sensed by a differential transformer or a group of load cells. The output of such devices is proportional to the displacement of the scale beam, which in turn is proportional to the amount of material in the weigh bucket. Many designs use loadsensing devices such as strain gauges or transducers. These eliminate the need for a scale-beam mechanism. The weigh vessel is mounted directly on the load-sensing devices. This provides many benefits in... 

The casing-internal instrument thermocouples are checked to ensure they are reading properly. RTDs and/or load cells are also checked during the test. The steam buffer seal controls are installed and functionally checked during the test. 

There are two types of devices subject to calibration those that are adjustable and those that are not. An adjustable device is one where the scale or the mechanism is capable of adjustment (e.g. micrometer, voltmeter, load cell). For non-adjustable devices a record of the errors observed against a known standard can be produced which can be taken into account when using the device (e.g. slip gage, plug gage, surface table, thermometer). 

Normally, a hoisting system has an even number of working lines between the traveling block and the crown block. The fast line is spooled onto the drawworks hoisting drum. The dead line is anchored to the rig floor across from the drawworks. The weight indicator is a load cell incorporated in the dead line anchor. 

On units equipped with load cell for weight indicator, lubricate pivot bearing. 

Data are generated by placing a test specimen between the two flat, parallel faces of a testing machine and then moving these faces together at a specified rate (ASTM D 695). A displacement transducer may be used to measure the compression of the specimen, while a load cell measures the compressive force exerted by the specimen on the testing machine. Stress and strain are computed from the measured compression load, and these are plotted as a compressive stress-versus-strain curve for the material at the temperature and strain rate employed for the test. 

We can operate at the required liquid volume—say, by putting the reactor on load cells—but the gas-phase volume and thus the total volume may change upon scaleup. Correlations are needed for the gas-phase holdup and for kiAj. A typical correlation for kiAj is that by Middleton ... 

Automated flexure tests are similar. The robot moves the bottom bar from the magazine to the measuring device where its width and thickness are determined, then it places the bar on the flexure test fixture. The PDP-11/44 begins the test by putting the crosshead in motion. Data collection begins when the first load is detected, and the test continues until the specimen bar breaks, the load cell maximum force is reached, or a specified maximum strain value is reached. Then the crosshead is stopped, the specimen is ejected from the fixture, and the crosshead is returned to its initial position. This process is repeated until the test series is complete. 

Richter, G.W. (1978). The iron-loaded cell - the cytopathology of iron storage. A review. Am. J. Pathol. 91, 363—404. 

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