Interview questions related to weighing

 The Instrumentation Insider: Article #7 – The Wild World of Weighing and Force (Gravity Never Sleeps)

Welcome back, mass-movers. We've measured almost everything a process can throw at us—pressure, temperature, flow, vibration, even the chemical composition of a gas stream. But there's one fundamental physical property we haven't tackled yet: weight.

Gravity is the one process variable that never changes. It's always there, always pulling, and it never needs calibration. But the instruments that measure it? Oh, they're a different story. Load cells, belt scales, weighbridges, tank weighing systems—they're the unsung heroes of inventory control, batching, and custody transfer. And they're finicky. They drift with temperature, they get mechanical stress from pipe connections, and they absolutely hate lightning strikes.

We're talking about strain gauges, deadload, corner correction, belt tracking, and why that 100-ton silo reads 102 tons after a rainstorm. Let's get weighing.

1. What is a load cell and how does it actually work?

Answer: A load cell is a block of metal (usually steel or aluminum) with strain gauges bonded to it. When you apply a force, the metal deforms microscopically. The strain gauges (which are tiny resistors) stretch or compress, changing their electrical resistance. This change is measured in millivolts per volt (mV/V). Apply the load, measure the resistance change, convert it to weight. It's that simple. The trick is making it accurate and repeatable.

2. What's the difference between "Compression" and "Tension" load cells?

Answer: It's all about how you apply the force.

· Compression: You push down on the cell. The cell gets shorter. Common under tank feet, silos, and floor scales.

· Tension: You pull on the cell. The cell gets longer. Common in hanging hoppers, crane scales, and overhead batching systems.

  Some cells can do both, but most are optimized for one. If you use a compression cell in tension, the mounting hardware may not be designed for it, and the reading will be non-linear.

3. What is "Deadload" and why does it ruin my zero?

Answer: Deadload is the unwanted weight that the cell always sees—the weight of the vessel, the piping, the agitator, and the insulation. When you calibrate a tank, you "tare out" (zero out) the deadload so the instrument only reads the product weight. The problem? Deadload changes. If you add insulation to a pipe, that's new deadload. If you replace a heavy agitator motor with a lighter one, that's less deadload. Every time the physical structure changes, you must re-zero.

4. My load cell reading wanders by a few kilos throughout the day. Why?

Answer: Temperature compensation failed. The strain gauges are sensitive to temperature changes. A good load cell has internal temperature compensation resistors, but if the cell is directly under a hot pipe or outside in the sun, the compensation can't keep up. Also, thermal expansion of the mounting structure—the steel expands in the sun, pushes against the cell, and adds mechanical stress. It's called "thermal zero shift." Shade the load cell and re-zero at operating temperature.

5. What is "Corner Correction" on a platform scale?

Answer: A platform scale has multiple load cells (usually 4, one in each corner). If you place a weight exactly in the center, all four cells share the load equally. But if you place the weight in the corner, that one cell takes most of the load. "Corner Correction" is the process of adjusting the individual trim pots (potentiometers) on the summing junction box so that the total weight reads the same regardless of where you place the weight. You need a known test weight and a lot of patience. If you skip it, the scale is accurate only in the center—and useless everywhere else.

6. My belt scale is drifting. Is it the load cells?

Answer: Probably not. Belt scales are notorious for mechanical issues. The most common problem is belt tension changes—if the belt stretches, or the take-up pulley moves, the force on the weighing idler changes. Also, material build-up on the idlers or the scale frame adds false deadload. The second most common issue is belt speed calibration—if the speed sensor reads wrong, the totalizer multiplies the load by the wrong speed. Clean the belt, check the idlers, and calibrate the speed sensor first.

7. How does a belt scale calculate tonnage?

Answer: It does math, and a lot of it. The load cells measure kg per meter of belt (the belt loading). The speed sensor (usually a prox on a tail pulley) measures meters per second. The integrator multiplies: Load × Speed = Mass flow (kg/s). Then it integrates that over time to get total tonnage. If either the load or the speed is wrong, the total is wrong. They're both critical. If you trust one more than the other, you're missing half the equation.

8. What is "Strain Gauge" technology and why do I need a matching amplifier?

Answer: A strain gauge only changes resistance by a fraction of an ohm under full load. The mV/V output is tiny (2-3 mV/V at full scale). If you run that directly to a PLC input, you'll see nothing but noise. You need a signal conditioner or amplifier that excites the bridge with a precision voltage (usually 5-10 VDC), amplifies the micro-signal, filters it, and outputs a standard 4-20mA or digital signal. You can't skip the amplifier—it's the brain of the whole system.

9. My load cell says "overload." What does that mean?

Answer: It means you've exceeded the cell's rated capacity—the mechanical limit before damage occurs. Most cells are rated for 150% of full scale for safe overload, and 200% for ultimate (breakage) load. If you have a 1000 kg cell and you put 1200 kg on it, you're in the overload zone. The output will be non-linear, the zero will shift, and if you do it repeatedly, you'll permanently damage the strain gauges. You need a bigger cell, or you need to reduce the load.

10. What is the difference between a "Hydraulic" and an "Electronic" load cell?

Answer:

· Hydraulic: Uses a piston and fluid. Load pushes the piston, which pressurizes the fluid. Cheap, robust, and explosion-proof, but drift with temperature, and not very accurate (1-2% error). Used in rough applications.

· Electronic (Strain Gauge): More expensive, far more accurate (0.1% or better), but sensitive to temperature and electrical noise. Used for custody transfer and batching.

  Hydraulic is for "do I have 20 tons or 25 tons?" Electronic is for "do I have 20,000 kg or 20,005 kg?" Choose accordingly.

11. Can I install a load cell under a tank with existing piping?

Answer: You can, but you'll regret it if you don't add flexible piping connections. The load cell must support only the vessel's weight. If you have rigid pipes attached to the tank, those pipes will exert horizontal and vertical forces on the vessel, and those forces will be translated to the load cells as "fake weight." You must install flexible hoses or expansion joints on all inlet/outlet lines before you even think about weighing. This is the #1 cause of inaccurate tank weighing.

12. What is a "Weighbridge" and why do they need a pit?

Answer: A weighbridge is a heavy-duty platform scale for vehicles—trucks, railcars, etc. It's typically 18 meters long and rated for 50 tons or more. The "pit" is the concrete recess in the ground where the load cells and the platform sit, flush with the road surface. It protects the cells from weather, allows vehicle access without ramps, and provides a stable foundation. The downside? Pits fill with mud, water, and debris, which need regular cleaning. The electronic version (above-ground) is cheaper but less durable.

13. My digital load cell isn't communicating. Is it dead?

Answer: Digital load cells (RS-485, CANbus, etc.) are great—they output a digital weight value directly, no amplifier needed. But they're also more complex. If it's not communicating, check your termination. RS-485 needs a 120-ohm termination resistor at each end of the daisy-chain. Also, check the address—each cell on the bus must have a unique address (set via dip switches). If two cells have the same address, the bus locks up. It's not dead—it's just lost.

14. What is "Zero Tracking" and do I need it?

Answer: Zero tracking is an automatic algorithm that slowly adjusts the zero to compensate for minor drift (like a bit of dust accumulating on the scale). It's great for floor scales in clean environments. But it's dangerous in a process tank—if you slowly fill the tank, and the zero tracking is too fast, it may "track" the incoming weight and think it's still zero. You should only enable zero tracking on check-weighers and retail scales, never on process control tanks. Set it to "Off" or a very low rate.

15. How do I calibrate a load cell system without actual weights?

Answer: You can't—not accurately. The gold standard is deadweight calibration—putting known test weights on the system and trimming the output. But if you can't get test weights (e.g., a 100-ton tank), you use hydraulic calibration (a calibrated pressure cylinder that applies a known force to the load cell). This is acceptable for rough checks. Some people use "electrical calibration" (injecting a precision mV/V signal), but that only checks the amplifier—it doesn't check the mechanical mounting or the load cell itself. Deadweights are the truth.

16. My weigh feeder is oscillating. What's going on?

Answer: It's a control loop issue—similar to the PID tuning we talked about in Article 4. The weigh feeder controller is trying to maintain a setpoint flow rate. If the gain is too high, the belt speed fluctuates wildly, and the load cells see these changes, creating a feedback loop. Turn down the gain (Proportional) and slow down the Integral. Also, check the belt tension—if the belt is slipping, the speed sensor and the load cell are fighting each other. They need to be in sync.

17. What is a "Batching System" and why does it need a "Dribble" phase?

Answer: Batching is when you load a specific recipe—e.g., 500 kg of Flour, 200 kg of Sugar, 50 kg of Yeast. You use a large valve (bulk) to get close to the target, then a small valve (dribble) to top off accurately. The dribble phase is critical because there's always material in flight (from the valve to the scale) that hasn't hit the scale yet. The controller shuts the bulk valve early, enters dribble, and watches the weight settle. If you skip dribble, you'll consistently over-batch by the amount of material in flight.

18. My tank weigh system is reading negative. How?

Answer: You have a pipe pulling up on the tank. Maybe the pipe support settled, or the expansion joint is compressed. The pipe is physically lifting the tank, reducing the load on the cells. The cells see the upward force, subtract it from the deadload, and the reading goes negative. Walk around the tank and check every pipe connection. There should be a gap in every pipe support—if you can't slide a piece of paper under the pipe, it's rigidly connected and pulling the tank. Shut the valve and fix the support.

19. Why does my load cell have a "Creep" specification?

Answer: Creep is the gradual change in output over time under a constant load. If you put 1000 kg on a scale and walk away, the reading might slowly drift from 1000 to 1002 over the next 10 minutes. That's creep. It's caused by the metal relaxation and the creep of the strain gauge adhesive. Good load cells have creep specified as <0.02% of full scale per hour. You can't eliminate it—you must either live with it or use a digital filter in the PLC to ignore it. But don't ignore it completely—it may be a sign of mechanical overloading.

20. What is the #1 rule of load cell installation that everyone forgets?

Answer: The mounting surface must be flat and level. If you bolt a load cell to a warped or uneven surface, the cell will be mechanically stressed even before you apply any weight. This causes an initial zero offset, and it makes the cell behave non-linearly because the force is applied at an angle, not straight down. Use a spirit level and a feeler gauge. If the base plate has a 1 mm gap, shim it. If you can't shim it, don't bother calibrating—the accuracy will never be acceptable. Start with a flat foundation. Everything else is a waste of time.

Weighing systems are brutally honest. They don't lie about how much material is in your tank, and they don't care about your feelings. If something is mechanically wrong, the numbers will tell you. Learn to listen to the mV/V signal, respect the mechanical mounting, and always, always check the pipe connections.

Next up Article: "Valves and Actuators – The Muscle of the Process (Making Things Move)." We're finally giving the instruments something to do—open, close, throttle, and modulate. We're talking control valves, positioners, solenoids, and why that pneumatic valve is always late to the party.

Now go level that base plate. The scale isn't going to fix itself.