A load cell reports force electrically. Somewhere inside sits a piece of metal cut to a deliberate shape, wearing gauges that notice when it yields; the bridge behind them expresses that noticing as voltage, and a certificate earned against reference weights turns the voltage into pounds.
There is a distinction worth drawing early, though, because it changes which specifications actually matter: some load cells spend their working lives on a bench, and some spend them inside a machine.
A sensor used for testing gets installed for a run, watched, and taken out again. It lives in a controlled room, handled by people who understand it, and its calibration certificate is rarely more than a year old. Almost anything you'd want to know about it can be found by looking at it.
A sensor left permanently installed lives a different life entirely. It sits in a joint or a fixture for months or years, and during that time nobody looks at it. It accumulates whatever the surrounding environment produces — dust, coolant mist, humidity, wash-down spray, temperature cycling every shift. Its readings are trusted precisely because nobody is standing there checking them. For that sensor, robustness and environmental protection stop being nice-to-haves and become the properties the whole installation depends on.
This is where sealing enters the specification. Load cell protection comes in recognised levels. An environmentally sealed cell has the cavity around its gauges potted with epoxy or similar, with washers or O-rings where the cable enters — good against dust, debris, and moderate moisture, and appropriate for facilities where splashing happens but heavy wash-down does not. A welded seal adds protection around internal components but still leaves the cable entry unsealed. A hermetically sealed cell is genuinely air- and watertight, with the electronics and the cable entry both sealed, and it's what heavy wash-down or corrosive service actually requires. Knowing which level you have — and which your environment demands — matters more than most single accuracy figures.
Through-hole designs are especially likely to end up in the permanent category, because their whole purpose is to be captured inside an assembly around a bolt, stud or shaft. You install one, torque the joint, and it stays. That's why you find them monitoring bolted joints, clamping force in production equipment, robotic end-effectors, automated assembly stations, and machinery where a fastener's condition needs confirming rather than assuming.
The measurement mechanism is standard. What deserves attention on a permanently installed sensor is everything that happens after commissioning day.
Producing the signal. Bring the bridge to life at 10 VDC, squeeze the element, and each gauge's resistance edges away from where it started; symmetry is lost and millivolts appear. What you supplied sets the scale of what comes back, which is the reason sensitivity is written with volts top and bottom. The electronics do their work, the certificate does the translating, and force appears — provided the supply matches the one the certificate was written against. Depart from it and every number departs with it.
Why the annulus geometry needs even loading. A donut senses through the ring of material between its bore and its outside diameter. Force arriving on part of that ring rather than distributed around it produces a reading that reflects where the pressure landed as much as how much there was. The faces bearing on each side need to be flat, hard, and wide enough to cover the full annular area — and on an installation that will stay assembled for years, that bearing condition needs to be right the first time, because nobody is going to revisit it.
The bore is for clearance. Whatever passes through the middle should do so concentrically and without touching the wall. Contact there adds friction and side loading that the element registers alongside the axial force, and in a permanent installation a slow-developing rub — from settling, thermal movement, or vibration walking a component sideways — can appear as a gradual measurement drift that looks like something else entirely.
What long service actually threatens. Three things, mostly. Moisture reaching the gauges, which is what sealing exists to prevent and which shows up as a wandering zero. Temperature cycling, which is calculable from the published drift coefficients rather than mysterious, and which is worth working out in advance if the sensor lives somewhere that heats up every shift. And the joint itself relaxing as surfaces bed in and coatings settle — genuinely the thing you installed the sensor to detect, but easy to misread as instrument drift if you haven't separated the two.
Zeroing is a decision, not a formality. Because these sensors are captured in a joint that then gets tightened, when you zero determines what you measure. Zero before torquing and the reading shows the clamping force the joint develops. Zero after, and you've subtracted that clamp and will see only what's added on top of it. On a monitoring installation this choice is effectively permanent, so make it deliberately.
A sensor destined to be fitted and forgotten raises a different set of questions from one bound for a test bench. Walk us through the assembly and, just as importantly, the environment it will live in, and we can tell you which of those questions actually bite in your case.
Ask how long it stays in there. A sensor coming out after each test can be checked, cleaned, and recalibrated on a comfortable schedule. One that's buried in a machine for three years cannot, so environmental protection, mechanical robustness, and a stable zero become the specifications that decide whether the installation still means anything in year three.
Then characterise the environment honestly. Not the environment on the day of installation — the environment across a full year of production. Coolant mist, wash-down, humidity swings, dust, and shift-to-shift temperature cycling all count. Match the level of sealing to that reality: potted and O-ringed construction handles dust, debris and moderate moisture well, and a genuinely wet or corrosive environment needs hermetic sealing rather than optimism.
Size the bore before anything else. Measure what has to pass through and pick a hole that clears it properly — enough room that it never contacts the wall through assembly, thermal movement, or years of vibration. Where a series offers several bores per capacity, that's a real specification decision, and it's the one most likely to make an installation succeed or fail.
Check the outside diameter against the space. Counterbore, recess, or the gap between a nut and a flange — the sensor has to physically live there, and in retrofit work that dimension frequently decides the whole question before any electrical specification is considered.
Then size capacity, with assembly in mind. Working force well inside the range for resolution, credible worst case inside the safe overload rating — and remember that a wrench and a long lever can generate far more force than the joint was ever meant to see. Overload during installation is a real hazard on bolted-joint measurement, not a theoretical one.
Provide proper bearing surfaces. Hardened, flat, sized to cover the annular face. Where the existing hardware in an assembly can't do that, the answer is a pair of correctly specified washers rather than accepting what happens to be there.
And plan the monitoring side. Direction comes first: these are certified pushing, so raise any pulling component while you are still quoting. After that, the readout. For a monitored joint the interesting capability is rarely the display itself but the setpoint alarm behind it — continuous monitoring only earns its keep if something tells a person when a value moves, otherwise you have built an instrument nobody reads. Our amplifier signal conditioner modules and digital displays cover the range from a bare regulated supply through to alarms, analog output and logging. Where a machine carries several cells, Cal-Teds plug and play keeps each unit's calibration attached to the unit itself.
Four things let us specify this properly: what passes through the middle, how much room surrounds it, the forces involved, and an honest description of the environment across a full year — not just the day of installation. Get us those and we can recommend something that still reads true when you return to it. Most capacities are on the shelf and go out within a day of ordering, and there is a discount structure for educational institutions.
THB Series Load Cell Applications.
The Transducer Techniques THB Series through-hole donut load cell, characterized by their versatile design with multiple through-hole diameter options and made from heat-treated 17-4 PH stainless steel, are well-suited for various applications where precise force measurement is essential.
- Industrial Automation: THB Series load cells are integrated into industrial automation systems for monitoring and controlling forces in robotic applications, material handling processes, and manufacturing operations. They ensure precise assembly and quality control in industries such as automotive, electronics, and consumer goods manufacturing.
- Robotics and Automation: These load cells are used in robotics and automation systems for force sensing applications, such as robot end-effector force control, pick-and-place operations, and quality checks on assembly lines.
- Force and Torque Measurement: THB Series load cells are employed for force and torque measurement tasks in research, testing, and quality control across various industries. They provide accurate data on forces and torques applied in both tension and compression modes.
- Materials Testing: In materials testing laboratories, THB Series load cells are used for various testing applications, including tensile and compressive testing of materials such as metals, plastics, ceramics, and composites. They help assess material properties and product quality.
- Automotive Testing: Automotive manufacturers and testing facilities use THB Series load cells for quality control and testing of vehicle components, including suspension systems, steering mechanisms, and brake systems. These load cells contribute to evaluating the performance and safety of automotive parts.
- Aerospace and Aircraft Testing: The aerospace industry relies on THB Series load cells for structural testing of aircraft components, flight control systems, and materials used in aircraft construction. They play a crucial role in ensuring the reliability and safety of aerospace equipment.
- Product Development: Engineers and researchers use THB Series load cells during product development and prototyping. They assist in evaluating the performance, durability, and structural integrity of new designs and components.
- Biomechanics and Medical Devices: In biomechanics research and medical device testing, THB Series load cells measure forces applied to the human body or medical devices during experiments and analysis. This data is vital for understanding physiological processes and validating medical equipment.
- Educational Laboratories: Educational institutions incorporate THB Series load cells into engineering and physics laboratories to teach students about force measurement principles and conduct experiments related to mechanics and materials science.
- Custom Machinery and Equipment: Manufacturers and research facilities integrate THB Series load cells into custom-built machinery and equipment designed for specialized testing and manufacturing processes that require precise force measurement.
The Transducer Techniques THB Series through-hole donut load cells, with their versatile design and multiple through-hole diameter options, are valuable tools across a wide range of industries and applications where accurate and reliable force measurement is crucial for safety, quality, and performance assessment.
Frequently Asked Questions
What are the four THB bore options, and how do I pick one?
The suffix identifies the through hole: -P is 0.128 inches (nominally 1/8"), -Q is 0.193 inches (3/16"), -R is 0.266 inches (1/4"), and -S is 0.391 inches (3/8"). Measure what needs to pass through, then choose the bore that clears it properly — genuine clearance, not a slip fit, since the component should never touch the bore wall during assembly or in service. All four options carry identical electrical specifications, so the suffix tells you about fit and nothing else.
What capacities does the THB Series cover?
Five: 100, 250, 500, 1,000, and 2,000 lb. With four bores available across each of them, that's twenty configurations from a single series — and because they're all priced the same at $630, bore and capacity are genuinely independent decisions. You never have to accept the wrong hole to get the right force range, or vice versa.
What does it mean that the THB's strain gauges are sealed?
It means the gauges are protected against most industrial environments rather than left exposed to whatever the surroundings produce. In practice, sealing of this kind guards the sensing electronics against dust, debris, and moderate moisture — the conditions found in most working facilities. It's a meaningful advantage for a sensor that will be installed inside machinery and left there. Worth being precise about the limits, though: this is environmental protection, not hermetic sealing, so heavy wash-down or genuinely corrosive service is a different requirement. Tell us about your environment and we'll confirm whether the THB suits it.
Why is the THB's outside diameter 1.50 inches?
The larger body is what makes the bigger bores and higher capacities possible. On a donut, the sensing structure is the ring of material between the bore and the outside diameter — open the hole up to 3/8 inch and you need more outside diameter to leave enough material doing the work. At 1.50 inches the THB accommodates fasteners and shafts a smaller donut simply couldn't clear, while still being compact enough for most assemblies.
What accuracy does the THB Series offer?
Nonlinearity and hysteresis are each 0.25% of rated output, nonrepeatability is 0.1% of rated output, and zero balance is 1.0% of rated output. For monitoring applications, the nonrepeatability figure usually matters most, since it describes how consistently the same load produces the same reading — which is what lets you compare today's measurement against last quarter's and conclude something real from the difference.
How much does a THB deflect under load?
0.002 inches at rated output. On a through-hole installation that number has a practical consequence beyond stiffness: the sensor is captured inside a clamped joint, so how much it compresses affects how the joint itself behaves. Two thousandths of an inch is small enough that adding a THB to a bolted stack-up doesn't meaningfully alter the joint's characteristics once it's torqued.
What is the THB made from?
Heat-treated 17-4 PH stainless steel with bonded foil strain gauges. The alloy earns its place here: 17-4 PH is a precipitation-hardening stainless whose heat treatment forms fine strengthening particles throughout the metal, and that strength is what allows a 3/8 inch hole through a 1.50 inch body while still carrying 2,000 lb. The stainless base material also contributes corrosion resistance, which matters for a sensor expected to stay installed.
Is the THB compression only, or can it measure tension?
The standard calibration is compression, which reflects how donut cells are normally used — captured in a joint and squeezed as it's tightened. Tension calibration is available as an option and should be specified at order time if your application pulls rather than clamps, since the load path differs between the two directions and a compression certificate documents compression alone.
How does temperature affect a THB reading?
Compensation covers 60° to 160°F, with a safe operating range of −65° to 200°F. Inside the compensated band, output drifts 0.005% of load per °F and zero drifts 0.01% of rated output per °F. On a permanently installed sensor that arithmetic is worth doing up front against your expected daily and seasonal swings — it's the difference between recognising a normal thermal excursion and mistaking one for a joint that's losing preload.
Can I get the THB with the plug-and-play TEDS option?
Yes, OPT-TEDS is available. It stores the unit's calibration data at the connector so a compatible instrument reads and applies it automatically. With twenty configurations in this series that look broadly similar once fitted, and with sensors often installed in multiples across a machine, that's a practical way to keep the right calibration attached to the right sensor rather than depending on records and labels.
Questions From The Field
Should I choose a THA or a THB?
Bore size and space usually decide it. The THA has a 1.00 inch outside diameter with two bore options up to 3/16 inch, and suits tight installations around small fasteners. The THB's 1.50 inch body opens up four bores to 3/8 inch and capacities to 2,000 lb, so it handles genuine structural fasteners the smaller series can't clear. Two other differences favour the THB where it fits: a tighter zero balance specification, and sealed gauges for protection in industrial environments. If the THB physically fits and your fastener needs a larger hole, it's usually the better choice.
Can I leave a THB permanently installed in production machinery?
That's a reasonable use for it, and the sealed gauge construction is what makes it so — protection against dust, debris and moderate moisture is what a sensor needs to survive years inside a machine. Two things to settle first. Confirm your environment is genuinely within that protection level, because heavy wash-down or corrosive exposure calls for hermetic sealing instead. And plan how you'll verify it over time: a sensor nobody looks at still needs a calibration interval, and building periodic verification into your maintenance schedule is what keeps the data defensible in year three.
My reading has drifted over several months. Is the sensor failing or is the joint relaxing?
Separating those two is the whole diagnostic problem in long-term monitoring, and there's a workable order to it. First calculate what temperature alone would explain from the published coefficients and compare it against your measured change. Then consider timing: joints typically relax fastest soon after assembly as surfaces bed in, then stabilise, so a decline that's slowing suggests joint behaviour while one that's steady or accelerating suggests something else. Finally, if you can safely re-torque and re-zero, seeing whether the sensor returns to its original reading under a known load separates instrument drift from genuine preload loss.
Which bore should I pick if my bolt sits between two sizes?
Go to the next size up. A bore that's marginally tight either won't accept the component or will leave it contacting the wall, and bore contact adds friction and side load that the sensor can't separate from the force you want. With four options between 1/8 and 3/8 inch the steps are reasonably close together, so moving up rarely costs much clearance. All four share identical specifications, so there's no performance penalty for the larger hole.
What washers should I use with a THB?
Hardened, flat, and large enough in diameter to bear across the full annular face of the cell rather than pressing on part of it. This matters more on the THB than on a smaller donut precisely because the annulus is wider — a washer sized for the bolt rather than for the sensor can easily contact only the inner portion of the ring. A soft washer is the other common problem, since it indents progressively under load and shifts the contact area as force builds.
I need to monitor several bolts on one flange. How should I set that up?
Decide first whether you need per-bolt visibility or a combined total. If a single fastener losing preload would matter — and on a flange it usually does, since that's how leaks and fatigue start — you need a cell on each bolt you care about and a channel to read each one. If you only need confidence that overall clamp is holding, instrumenting a representative subset costs considerably less. Either way, setpoint alarms are worth configuring, because continuous monitoring only helps if something tells you when a value moves.
My THB is in a machine that gets washed down. Is that a problem?
Potentially, and it's worth checking rather than assuming. Sealed gauge construction protects well against dust, debris and moderate moisture — splashing and humidity are within its scope. Directed wash-down spray is a different level of exposure, and the cable entry is typically the vulnerable point rather than the sensor body. Describe the wash-down regime to us, including pressure and chemicals, and we'll tell you honestly whether the THB is appropriate or whether you need a hermetically sealed design instead.
Do I need to re-zero a THB every time the joint is disturbed?
Yes, treat re-zeroing as part of any reassembly. The clamping condition never reproduces exactly — torque distribution, surface contact and seating all vary slightly — and the resulting small offset is normal rather than a fault. Zero in the fully assembled condition, and be consistent about whether you zero before or after final torque, since that choice determines whether your readings show total clamping force or only the load added on top of it. Changing that convention partway through a monitoring programme will make your historical data incomparable.