FAQs for Load Measuring

What is a load cell?

A precision machined, alloy steel billet, with strain gauges bonded at defined points which accurately measure the deflection in the steel billet when a force or load is applied.

There exist many types of load cell with different measurement characteristics and performance in respect to accuracy and repeatability. Typical designs include:  Compression, Column, Shear Beam, Double Shear Beam, Shear Web, Tension, Tension & Compression, Single Point, Load Pin, etc...

The mounting and loading, of all load cells, is critical to its performance.

Type selection is down to application and performance required. 

What is a strain gauge?

A strain gauge is a resistive network device which is powered by a DC voltage, generally termed "Excitation Voltage", where the resistance changes if its dimensions are altered by the material it is bonded to. Typical excitation levels are 10, 15 and 20 volts DC with outputs from the strain gauges in milli-volts, where a milli-volt is equal to one thousandth of one volt.    

How does a load cell work?

When a force or load is applied to the machined billet it changes its shape. This shape change causes deflection in strain gauges, bonded on to the billet, changing the resistance of the strain gauge which in turn changes the milli-volt output from the strain gauges.

Provided the load cell billet is not subjected to a force or load that exceed its designed operating capacity, the billet and the strain gauges will return to their original form/shape when the force or load is removed. This characteristic enables the load cell to operate within a predefined accuracy and repeatability tolerance without fatigue.    

What is the difference between analog and digital signals?

An analog signal is one the changes over a variable range such as: 0 to 10 volts or 4 to 20 mA. The milli-volt output from a load cell is an analog signal as is the excitation voltage.

A digital signal is one that changes its state over a fixed range similar to a switch where it is either open or closed. Computers operate on digital signals using the on/off principle or a string of characters consisting of a 0 and 1 where 0 is low and 1 is high (off/on).  

What is parts of line?

Parts of line is defined as number of ropes falls used in a crane rigging configuration to lift and lower the cranes rated capacity. If we ignore friction losses in the crane sheaves and bearings, the load seen at the hook is shared equally between the number of rope falls between the crane winch and the crane hook. Example:

Crane capacity = 100 ton

Parts of Line (number of falls) = 10

Load seen by each part of line = 10 ton   

What is mV/v output?

The term mV/v or milli-volts per volt, represents the output of the strain gauges for a fixed excitation over the working range of the load cell. Example

Load cell capacity = 100 ton

Excitation voltage = 10 volts DC

Load cell output = 2 mV/v

When a force or load of 100 ton is applied to the load cell, the output of the strain gauges will be 20 mV and at 50 ton the output will be 10 mV. 

Can load cells be interfaced to PCs?

Generally standard analog load cells cannot be interfaced directly with a PC without some form of interface instrument that will convert the load cell mV output into a digital signal. The interfacing instrument will supply the excitation voltage to the load cell, intake the milli-volt output from the load cell, convert the milli-volt signal into real engineering units (lbs, tons, tonnes, or kg) then output the engineered values as a serial digital string such as; RS232 and RS485.

Can load cells be interfaced to a PLC?

Generally standard analog load cells can be interfaced directly with a PLC, provided the PLC is fitted with the correct interface cards.

How many sensors can you use with a single readout instrument?

It is assumed that the question is referring to the number of load cells that can be connected to a signal readout instrument?

The Mk2 Series 1010 will power up to four 350 ohm strain gauged load cells, as a standard configuration. The load cell inputs are summated together and displayed as one value in the selected engineering unit and not individual values.

What is meant by overload protection?

Overload protection is a mechanical protection either built into the load cell or the mounting unit in which the load cell is placed.

Does cable length affect load cell output?

Yes it can, if the resistance of the cable causes voltage loss in the load cell signal output.

The voltage loss can be reduced by the use of the correctly sized cable conductors, and the loss in signal can be accounted for at the instrument if a six wire (conductor) connection is used. The two additional sense wires measure the excitation value at the load cell and return this value to the instrument (less the losses caused by the cable resistance) and adjusts for these losses within the instrument.

What does linearity mean?

The linearity of load cells determines its performance against a straight line running from zero load to design capacity. When a load cell is tested, force is applied at a prescribed number of points between zero and full capacity. At each of these points the output of the load cell is compared to the applied force to determine how far it is away from the linear line. 

What does uncertainty mean on load cells?

The uncertainty in load cells is determined by a number of factors so it is best described as the unknown factors which cannot be calculated during calibration of the load cell.

All testing equipment operates with some level of uncertainty, something which cannot be realistically measured. 

What is hysteresis?

Hysteresis the difference in the load cell output between applying a load or force and removing the load or force. It is caused by number of factors including the material type and its condition used to manufacture the load cell.

Many devices operate with hysteresis, a room heating thermostat being a good example. When the preset temperature is reached, the thermostat switches the heating off but does not switch it back on again until the temperature drops below a level to overcome the hysteresis within the thermostat.

What is an NIST traceable calibration?

The National Institute of Standards and Technology (NIST) is a federal governing body that defines standards for calibration.

The term traceable calibration to NIST means that the process adopted and the equipment used to carry out the calibration conforms and is traceable to a NIST standard. To define whether or not a supplier of calibration services conforms to NIST, it is necessary to define the NIST standard to which the equipment has to be calibrated. 

Why should I calibrate?

The requirement to calibrate a portable load cell is subject to regulatory requirements and internal processes of the user. The frequency of calibration is really dependent upon these factors as well as the usage and possible abuse which the equipment receives.

The requirement to calibrate a load measurement system fitted to a crane is different to a portable load measurement device. When the equipment is installed into the crane, it is first time that it is operating within the crane and actually measuring the load applied at the hook. If it is a proportional load measurement system, the load cell or load pin used to measure the force, seen at the point where it is installed, can differ significantly to the load calculated during the design.  

What is meant by environmental protection ratings?

In most case the environmental protection ratings are defined by the NEMA standards, in the US and IP standards, in Europe. Please go to

 http://www.siemon.com/us/standards/nema_comparison.asp

Which NEMA/IP rating is used depends on the application and the environment in which the equipment has to operate and survive.

What's the difference between submersible and water proof?

Generally the major difference between the terms arises when an operating depth is applied to the term submersible.

IP68 is a water proof protection, however, if this level of protection is to be provided at a depth of 3000 yards, then the manner by which the protection is achieved changes.

How do I know what accuracy class to use for my load cell?

The accuracy class required is either driven by regulatory and/or internal requirements for the application.

What is a proportional load measurement system?

The term proportional load measurement system is used to define a system where the load cell is only seeing a proportion of the total load or force being applied.

When a load pin is placed in the compensation sheave of a "mutli-parts of line" crane it is only seeing a proportion of the total load seen at the hook.

What is a signal converter used for?

The signal convertor is used to provide the excitation voltage to the load cell and receive the milli-volt output from it. It will permit this output to be calibrated and converted into real engineering units as well as outputting this modified data to other devices such as a data logger

What is a load cell amplifier?

Can be the same as the signal converter or a simple device to power the strain gauge, calibrate the load cell input against a known load and output is a different format such as; 4 to 20 mA or a digital string.

What is a data logger?

A data logger receives data generally in a digital format and logs it for other uses.

What is peak hold?

The term peak hold, when used with load measurement systems, means that the last highest load or force is held until a higher load or force is detected. Frequently peak hold information is transferred to a data logger for future reference.

What is the standard warranty on your load cells?

12 months from dispatch based on return, and is subject to our findings after review of the returned item.

What do I do when my system requires repair or service?

Call your local service office and book a slot or return the equipment to them for examination and estimate costs for repair.

Is it possible to send a component of my system for service instead of the whole system?

Yes it possible for certain items to be returned for service or repair, but this really depends upon the item, the information collected during installation & calibration and the application.