Ask most engineers to draw a load cell and you get a Wheatstone bridge: four strain gauges in a diamond, four wires out, excitation across one diagonal and signal across the other. It is a good picture and it is incomplete, and this page happens to be the one that proves it.
Look at the two resistance figures in the specification table. Input resistance — measured across the excitation pair — is around four hundred ohms. Output resistance, across the signal pair, is three hundred and fifty. If the bridge were nothing but four gauges, those two numbers would be the same. They are not, and the difference is the most informative thing on the page.
What sits in the gap is compensation. Published guidance on load cell resistance puts it plainly: input resistance on most load cells is higher than output resistance because of the temperature compensation circuitry added to the excitation path. Small resistors, chosen for how their own resistance behaves with temperature, are built into that path at manufacture and trimmed for the individual sensor.
They are there to cancel the metal's own behaviour. Aluminium becomes very slightly less stiff as it warms, so an identical load bends the beam a fraction further and the output would creep upward with temperature. A compensation resistor whose value rises with heat drops a little more of the supply before it reaches the bridge, reducing the excitation by just enough to cancel that gain. A second network brings the unloaded output close to balance. The result is a sensor whose reading barely moves across the compensated band — behaviour a bare beam and four gauges could not produce.
The output side is standardised on purpose. Whatever is going on inside, the signal pair is trimmed to present three hundred and fifty ohms to a tight tolerance, so the sensor looks like a conventional bridge to any instrument connected to it. That is why you can treat it as a standard load cell electrically while it is quietly doing something more sophisticated internally.
Three things follow for anyone using one. The specifications you are buying belong as much to that network as to the beam, which is why an economical aluminium sensor can hold two hundredths of a percent. The compensation is trimmed over a stated temperature band, which is exactly why the compensated range and the safe range are two different numbers rather than one. And the network is not user-serviceable — opening, re-trimming or splicing into a compensated cell throws away the very thing you paid for.
Sensors of this kind are built into bench, platform and floor scales, packaging machinery including fillers, checkweighers and bagging equipment, retail and postal weighing, in-motion conveyor weighing, food portioning, pharmaceutical manufacture and OEM machinery that needs a weight as part of a larger process.
The measuring principle is standard. What is worth working through on this series is what the numbers let you do — including one thing no other sensor in our range makes possible.
Signal, and a tolerance that has consequences. Excitation is 10 VDC and rated output is 1.6 mV/V, so full scale is a little over sixteen millivolts. That figure carries a ten percent tolerance, which means two sensors can differ in sensitivity by a fifth of full scale between them. A tare removes an offset; only a span calibration against a known mass corrects a sensitivity difference, so any product that displays a weight a customer acts on needs one at end of line.
Those two resistance figures are a diagnostic, and a good one. Because both are published, you can check a suspect sensor with an ordinary meter before it goes anywhere near a calibration laboratory. Published guidance on load cell resistance gives healthy bands for a 350 ohm cell of roughly 350 to 410 ohms across the excitation pair and a much tighter 348 to 352 ohms across the signal pair — which is precisely the shape of the figures printed here. An input reading below three hundred or above a thousand ohms is a red flag, and a difference of more than three to five ohms when measuring individual bridge arms points to a damaged gauge. Insulation resistance is the third check: healthy cells read in the gigohms, and a figure that has fallen to the low hundreds of megohms indicates moisture reaching the gauges.
Zero drifts twice as fast as span, and that is normal. The temperature effect on output is two thousandths of a percent per degree Fahrenheit; on zero it is four thousandths. The asymmetry is not an oversight — span compensation acts on a single well-understood mechanism, while zero is affected by everything from differential expansion between the beam and the gauge backing to the fixture the sensor is bolted to. It is easier to compensate what a load does than what nothing doing anything does. Practically, it means zero is the number to re-check when a reading looks wrong after a temperature change.
The platform limit is the largest we publish. There is a distance from centre past which the machined body simply cannot cancel any more moment, and once an object goes beyond it the reading begins tracking position as well as weight. Here that boundary is thirty-five inches square, which is floor-scale territory and roughly three times the area of our mid-range single point cells. It is a hard limit, not a guideline.
Aluminium, and what the sealant does and does not claim. The body is aluminium throughout, which is the right choice at these capacities — light, dimensionally stable, and economical to machine to the shape corner compensation requires. A moisture-proof sealant protects the gauge area and suits damp surroundings such as food production areas and washdown-adjacent equipment. It is not a hermetic specification, so direct high-pressure cleaning, condensing environments and corrosive atmospheres are worth describing to us before ordering.
At these capacities the sensor is usually the least of the engineering — the platform, the frame and the handling around it decide whether the finished scale works. Our application engineers would rather look at the whole arrangement than quote a part number from a capacity alone.
Establish the weighing surface first. The published platform maximum is the one specification that cannot be traded against anything else, so measure your intended surface and check it before considering capacity. A design that needs a larger surface than a single sensor supports is a different architecture — typically several cells sharing the load — and that is a decision to take at concept stage rather than after tooling.
Weigh your own structure before you weigh anything else. At floor and platform sizes a steel deck, a frame, guards and side rails add up to a serious permanent load that is sitting on the sensor whether or not anybody is using the scale — and it eats into the working range and the overload margin alike. Total that, add the heaviest item you expect, and leave room on top for how the equipment will really be treated.
Be realistic about how things arrive. Objects at this scale are lowered by hand, dropped from a few inches, slid on from a roller, or set down by a forklift operator in a hurry. A momentary force well above the static weight is routine rather than exceptional, and safe overload is a survival figure rather than a working allowance. Where rough handling is a certainty, a mechanical stop under the platform is cheap engineering.
Plan the span calibration into production. With a ten percent tolerance on rated output, a nominal scale factor will not give you a defensible weight. Calibrating each assembled unit also captures your own platform, frame and electronics, which is what your customer actually experiences, and it costs a fraction of the time spent explaining discrepancies afterwards.
Confirm the environment. Damp production areas are within scope; sustained wash-down, condensation and chemical exposure are a separate conversation. It is a two-minute question at the enquiry stage and an expensive discovery later.
Then decide what reads it. A sixteen millivolt full-scale signal from a standard 350 ohm output suits either route: straight into a converter on your own board, or through an amplifier signal conditioner module if you would rather buy a voltage or current output ready made. A digital display with setpoints earns its place wherever an operator needs the number or a limit has to trigger something. With Cal-Teds plug and play the sensor carries an IEEE 1451.4 memory chip, so a smart instrument picks up its calibration data on connection and nobody has to key anything in.
Give us the deck dimensions, what your own frame weighs, the heaviest thing that will go on it and an honest word about how roughly it gets used, and we will tell you which capacity fits. The range is stocked, and academic and research purchasers should ask about discounted pricing.
SPL Series Load Cell Applications.
The Transducer Techniques SPL Series single point load cells simplify scale designs, packaging machinery and other industrial weighing equipment, and serve a wide range of industrial weighing applications.
- Industrial Scales: SPL Series load cells are used in industrial scales including bench, platform and floor models.
- Packaging Machinery: These load cells are used in packaging machinery such as fillers, checkweighers and bagging equipment.
- Retail and Commercial Scales: SPL Series load cells are integrated into point of sale, deli and postal scales.
- Conveyor Systems: These load cells are used in conveyor systems for in-motion weighing.
- Food Processing Equipment: SPL Series load cells are used in food processing equipment including portioning machines.
- Automated Packaging Lines: These load cells are integrated into automated packaging lines.
- Pharmaceutical Manufacturing: SPL Series load cells are used in pharmaceutical manufacturing.
- Quality Control: These load cells are used in quality control verification processes.
- Custom OEM Machinery: SPL Series load cells are integrated into custom OEM machinery.
- Multi-Point Weighing Systems: These load cells are used in multi-point weighing systems.
The SPL Series aluminum construction, moisture-proof sealant, and ability to simplify design make it valuable for accurate, cost-effective force measurement across diverse industries.
Frequently Asked Questions
What capacities does the SPL Series cover?
Six, specified in kilograms: SPL-65 at 65 kg (142 lb), SPL-100 at 100 kg (220 lb), SPL-150 at 150 kg (330 lb), SPL-200 at 200 kg (440 lb), SPL-300 at 300 kg (660 lb) and SPL-500 at 500 kg (1,100 lb). That range takes single point weighing well past bench scale sizes and into platform and floor scale territory, where the structure you build around the sensor becomes a significant part of the design.
What is the maximum platform size?
35 inches square — the largest weighing surface any single point cell in our catalogue supports, and roughly three times the area of our mid-range models. That figure is the boundary of the corner compensation rather than a suggestion: an object placed beyond the distance from centre the geometry was shaped to handle applies more moment than the body can cancel, and position sensitivity returns. It cannot be solved by choosing a higher capacity, so check your intended surface against it before anything else.
Why does the page list two different resistance figures?
Because a commercial load cell is more than four strain gauges. Input resistance, measured across the excitation pair, is about 400 ohms; output resistance, across the signal pair, is 350 ohms to a much tighter tolerance. If the bridge were only four gauges those numbers would match. The difference is temperature compensation circuitry built into the excitation path at manufacture, which is what allows an economical aluminium sensor to hold the accuracy and temperature figures published here. The signal side is trimmed to 350 ohms so the sensor still presents itself as a conventional bridge to any instrument.
How accurate is the SPL Series?
Nonlinearity, hysteresis and nonrepeatability are each 0.02% of rated output, with zero balance within 5% of rated output. Those accuracy figures match the best we publish anywhere in the catalogue, which is a striking result on an economical aluminium part at these capacities. They come from the combination of the machined single point geometry and the internal compensation network rather than from the beam alone.
What is the rated output, and why does the tolerance matter?
1.6 mV/V nominal, with a tolerance of plus or minus ten percent — so at the standard 10 VDC excitation, full scale is a little over sixteen millivolts, and two sensors can differ from each other in sensitivity by a fifth of full scale. The consequence for a product designer is concrete: a tare removes an offset but does nothing about sensitivity, so any product reporting a weight a user acts on needs a span calibration against a known mass on each assembled unit.
Why is the zero temperature coefficient twice the output figure?
Output drift is 0.002% of load per °F and zero drift is 0.004% of rated output per °F, and the asymmetry is normal rather than a shortcoming. Span compensation targets one well-understood effect — the metal getting slightly less stiff as it warms — and can be trimmed against it precisely. Zero is pushed around by more things at once, including differential expansion between the beam and the gauge backing and the behaviour of whatever the sensor is bolted to. In practice it means zero is the first thing to re-check when a reading looks wrong after the temperature has moved.
Can I check an SPL with a multimeter?
Yes, and this series is better suited to it than most because both resistance figures are published. With the sensor disconnected, measure across the excitation pair and across the signal pair and compare each against its specification. Published guidance puts the healthy band for a 350 ohm cell at roughly 350 to 410 ohms on the excitation side and a tight 348 to 352 ohms on the signal side, with anything below 300 or above 1,000 ohms treated as a clear fault. Add an insulation resistance check between the circuit and the body, where healthy readings run into the gigohms.
What is the SPL made from, and how well is it protected?
Aluminium throughout, which is the correct choice at these capacities — light, dimensionally stable and economical to machine into the shape corner compensation needs. A moisture-proof sealant protects the gauge area and makes the sensor suitable for damp environments, which covers a great deal of food production and packaging equipment. It is not a hermetic specification, so direct high-pressure washdown, condensing conditions and corrosive atmospheres should be described to us rather than assumed to fall inside what this family covers.
How is the SPL wired?
Four conductors, with an internal temperature compensation and balance network behind them. That network is why the specification table lists two different resistance values and why the temperature figures are as tight as they are. It is also why a compensated load cell should never be opened, re-trimmed or spliced into: the components inside were adjusted for that individual sensor, and disturbing them discards the performance you bought.
Can I use several SPLs together in one weighing system?
Multi-point weighing is a listed application, and it is the usual answer when a platform is larger than a single sensor supports. The considerations change once you do it: the cells need to share load reasonably evenly, the structure has to be stiff enough not to redistribute weight as it flexes, and the outputs are summed, so the system needs corner trimming to make the reading independent of position across the whole deck. Tell us the platform size, the capacity and how the structure is built and we will talk through whether one sensor or several is the right approach.
Questions From The Field
My sensor reads nothing at all. How do I tell if it is dead?
Start with a meter rather than a calibration. Disconnect the sensor and measure across the excitation pair and the signal pair separately, comparing each against the figures on this page. An open circuit or a reading far outside the published bands — below 300 ohms or above 1,000 — indicates a broken gauge or a damaged lead rather than an application problem. If both resistances are healthy, the fault is almost certainly in the wiring, the instrument or the mechanics, and the sensor itself is the last place to look.
Readings changed after the equipment was moved to a colder room.
Check the zero first, since it drifts about twice as fast as span on this series. A move between environments produces a step change in both the sensor and everything bolted to it, and a re-zero once the equipment has fully settled at the new temperature will resolve most of it. Two cautions. Allow proper settling time — a large steel platform takes far longer than the sensor to reach equilibrium, so re-zeroing too early simply captures a moving value. And confirm the new location sits inside the compensated range, because outside it the internal network is no longer cancelling the right amount.
Can I open the sensor or add resistors to trim it?
No, and it is worth being blunt about why. The components inside were selected and trimmed for that individual sensor during manufacture, and they are the reason it holds its accuracy and temperature specifications at all. Adding resistance to the excitation or signal path changes the compensation, breaks the relationship the calibration assumes, and produces a sensor that appears to work while being quietly wrong. If a unit is not performing as expected, send it to us rather than adjusting it — the diagnosis is usually quick and frequently turns out to be the installation rather than the cell.
The scale reads high when a load is placed near the edge of a large platform.
Three candidates, in order. Platform size against the published maximum — at these capacities decks tend to grow during design, and a surface that has crept past the limit will behave exactly this way. Platform stiffness, because a large deck that flexes under an off-centre load changes where the force reaches the sensor regardless of the compensation. And the mounting, since a base that deflects under load produces a similar symptom. Work through those before suspecting the sensor, which is the least likely of the four.
Do I have to calibrate each unit I build, or can I use the nominal figure?
With a ten percent tolerance on rated output, a nominal figure will not give you a weight you can defend. Working from nominal is reasonable where the product needs a repeatable indication — a fill check, a threshold, a presence test. Where a number is displayed and acted upon, calibrate each assembled unit against a known mass. Doing it on the finished product rather than the bare sensor also captures your platform, your frame and your electronics, which is the combination the customer actually uses.
A forklift knocked the platform. What should I check?
Impact and static weight are very different things, and safe overload is a survival limit rather than a working allowance. Remove the platform and measure the unloaded output, comparing it against the zero balance specification — a zero that has shifted well outside it and stayed there points to permanent set in the beam. Then check a known weight still reads correctly, because a cell can settle to an acceptable zero and still have lost span. Finally inspect the mounting and the platform frame, since a hard sideways blow often damages the structure and the fasteners before it damages the sensor.
Readings creep slowly upward while a load sits on the scale.
Look at the mechanics before the sensor. Slow movement under a constant load usually means something in the assembly is settling: a fastener bedding in, a rubber foot or gasket compressing, an adhesive joint relaxing, or a frame taking up clearance. Temperature is the second candidate, particularly if the load was warm or the equipment is heating, and it produces a similar shape. Note the direction and the timescale, then remove the load and see whether the zero returns — if it does not, the settling is mechanical and the fixture is where to look.
What routine maintenance does an installed SPL need?
Four things at whatever interval suits the duty. Check the unloaded reading against the zero balance figure, since a growing offset is the earliest useful warning that something has changed. Confirm the platform is still free — debris under a deck, a fastener that has worked loose or a guard that has shifted into contact will all affect the reading before anything looks broken. Inspect the cable and its entry for damage, and keep it clear of anything that pulls or chafes. And verify against a known weight periodically, more often on equipment handled roughly than on a laboratory bench.