Understanding Load Cell Resistance and Its Impact on Performance: A Comparison Between 350 OHM and 2800 OHM Load Cells
At Morehouse, we have been asked about the differences between load cell input and output resistances, which typically range from 350 OHM to 2800 OHM.
This blog aims to provide a deeper understanding of load cell resistance, what this means for your load cell, and its expected performance.
Figure 1 Morehouse Shear Web load cells. The one on the left has a 350 Ω resistance, and the one on the right has 1400 Ω resistance.
Load cells, such as the ones pictured above, are used in multiple industries and for various measurements. These can vary from reference standards to calibrating other force measuring devices to the general purpose that is used in everything from your car to the scale at the grocery store that keeps telling you to “place your item in the bagging area.”
One of the key specifications of a load cell is its electrical resistance, typically measured in ohms (Ω). Common values range from 350 Ω to 2800 Ω, and while this might seem like a technical detail, it plays a crucial role in how the load cell performs in different applications. In this blog, we’ll explore the significance of load cell resistance, particularly the differences between 350 Ω and 2800 Ω load cells, and what that means for performance in real-world situations.
What is Load Cell Resistance?
Load cell resistance refers to the electrical resistance of the strain gauge bridge inside the load cell. A load cell generally consists of a Wheatstone bridge circuit formed by four strain gauges.
When a load is applied to the cell, these strain gauges change resistance, producing a small voltage corresponding to the applied load.
Many of today’s load cells are designed with a Wheatstone Bridge Configuration.
A Wheatstone bridge circuit is composed of four resistive elements (strain gauges) that change resistance when a load is applied. The input load cell resistance and output load cell resistance are measured at different points in this circuit, which leads to inherent differences between the two.
Input Resistance: This is the resistance measured between the two excitation terminals (often labeled as +EX and –EX). It represents the total resistance of the circuit that the excitation voltage is applied to and can be used to calculate the nominal power consumption of the load cell.
Output Resistance: This is the resistance measured between the two signal terminals (often labeled as +SIG and –SIG). It represents the resistance through which the measurement signal (proportional to load) is obtained. This value normally has no effect on power consumption.
Input resistance on most load cells is higher than output resistance due to the addition of temperature compensation circuitry.
And often a load cell will appear to work even if you mix up excitation and output terminals.
However, this will affect both temperature compensation and calibration.
350 OHM Load Cells: Industry Standard
Most industrial load cells, especially those in the medium to high-capacity range, use a 350 Ω bridge circuit.
This value has been an industry standard for decades and balances performance and power consumption well.
Plus, the cost of the multiple strain gauges used is typically inexpensive.
Due to thermal variations in the environment and the heating effects of the gages on the material and cost, a trade-off was needed between low noise and errors.
In the early days of load cells and strain gauges, many standards were around 120 Ω bridge as it was likely easier to make.
However, the heating of the element impacted the smaller load cells and strain gauges, resulting in instability caused by the thermal effects.
Many tiny load cells would be hot to the touch as they would heat up quickly if not ran at lower voltages.
The lower voltage reduced output and decreased resolution.
Miniature load cells perform much better in the 21st century.
Modern indicators handle lower voltages with less noise and strain gage production has borrowed techniques from the semiconductor industry to make gages more reliable and with higher resistances.
Thus, we believe the 350 Ω bridge circuit became the standard as it balances the trade-offs quite well.
Note: Because a 350 Ω bridge is more standard across many designs, it does not mean the load cells or torsion cells with a 350 Ω bridge is better than one with a 2800 Ω bridge.
Key Characteristics of 350 Ω Load Cells:
Power Consumption: Lower resistance load cells require more power to operate. This means they draw more current from the excitation voltage provided by the load cell amplifier or signal conditioning equipment.
A 350-ohm bridge balances these extremes and typically delivers an output in the 2-4 mV/V range, which is sufficient for most signal amplification and processing circuits.
If we look at the formula for the current where I is the current, V is the Voltage, and R is the resistance. We find that a 350 Ω load cell draws approximately 0.029 Amps.
However, because they draw more current, 350 Ω load cells tend to require power supplies that handle the higher current draw requirements.
This is typically not an issue for most power supplies unless multiple load cells are connected to a single indicator or power supply.
For example, it is very common in the weighing industry for multiple load cells to be connected in parallel through a junction box and read by a single indicator.
However, connecting them in parallel lowers the effective input resistance seen by the indicator or power supply.
In this case, the effective input resistance must be calculated and compared and the instruments available supply current.
Luckily indicators dedicated to the weighing industry often tell you how many 350 Ohms load cells can be connected so the user doesn’t have to do any math.
The 350 Ω bridge can make the cell an acceptable candidate for applications where strong signal integrity is essential, such as in industrial environments with high electrical noise.
Compatibility: Most industrial weighing systems are designed around 350 Ω load cells, making them widely compatible with standard instrumentation and easier to implement without requiring specialized hardware.
Advantages of 350 Ω Load Cells:
Widely compatible with industrial systems.
The lower current draw relative to lower-resistance bridges reduces thermal noise, which can be a significant source of error in long-term or high-precision measurements.
Less sensitive to environmental noise which means the shielding and type of cabling used if it is 6-wire and below 28 gauge usually has little impact on the measurement result as long as the load cell and indicator use a 6-wire configuration.
Morehouse has another blog on 4 versus 6-wire. https://mhforce.com/load-cell-cable-length-errors/
Challenges:
Higher power consumption can be a concern in battery-powered energy-sensitive applications.
Applications that combine multiple load cells may benefit from using input resistances above 350 Ohm.
1000 - 2800 OHM Load Cells: Higher-Resistance Option
Compared to 350 Ω load cells, 2800 Ω load cells offer distinct advantages, especially in applications where lower power consumption and improved temperature stability are essential.
Key Characteristics of 1000 - 2800 Ω Load Cells:
Power Efficiency: 2800 Ω load cells consume less current due to their higher resistance. This makes them ideal for applications where power availability is limited, such as in battery-powered remote monitoring systems.
Using Ohm’s law a 2800 Ω load cell draws approximately 0.0036 Amps.
Temperature Stability: Load cells with higher resistance are often more stable over a wider temperature range (less self-heating).
This is because less current flow generates less heat, reducing thermal effects that could otherwise impact the measurement accuracy.
Advantages of 1000 - 2800 Ω Load Cells:
Lower power consumption.
Better for remote, battery-powered applications.
Reduced self-heating effects, offering more consistent performance across varying temperatures.
It also reduces warm-up times and short-term drift, and it is an important consideration when using load cell material with less thermal conductivity.
Reduced self-heating is especially advantageous in miniature and low-capacity load cells. Higher resistance can eliminate the need to reduce excitation voltage in these applications, thus retaining a higher output signal.
Challenges:
Some load cells might not be compatible with legacy industrial systems designed for 350 Ω load cells.
Cost can be a factor as the strain gauges are often more expensive.
How Resistance Affects Load Cell Performance
Higher resistance gages are generally more expensive due to things like higher precision patterning, larger sizes, more expansive materials, and processing
- Power Consumption
One of the most significant impacts of load cell resistance is its influence on power consumption. A higher-resistance load cell (2800 Ω) will consume less power, which can be critical in applications where energy efficiency is a priority.
On the other hand, lower-resistance load cells (350 Ω) are more power-hungry yet are a time proven for heavy-duty industrial systems where reliable and robust signal transmission is crucial.
This is where the 1400 Ω load cell resistance of the Morehouse budget load cell might be a sweet spot for field use using batteries.
Figure 2 Morehouse Wireless Load Cell Adapter.
Morehouse has wireless options for almost any load cell. The option shown is a wireless Bluetooth module that uses a 3V battery to power the load cell. The signal can be read using almost any Bluetooth device.
- Application Suitability
350 Ω load cells are generally well suited for harsh industrial environments where signal strength is crucial. In contrast, 1400 Ω load cells are more often found in environments where energy efficiency is more important than signal strength, such as in battery-powered or wireless systems.
- Temperature Stability
Due to reduced current flow, 1400 Ω load cells tend to exhibit less self-heating than 350 Ω cells. This can make them more stable over a wide temperature range, which is important in applications where temperature fluctuations are expected. Self-heating can introduce errors in the measurement, so reducing it by using higher-resistance load cells can be beneficial in precision applications.
Which Load Cell is Right for You?
When choosing between 350 Ω and 1400 Ω or even 2800 Ω load cells, the decision ultimately comes down to your application's specific requirements.
If you work in a high-noise environment and are not concerned about power consumption, a 350 Ω load cell is likely your best option.
On the other hand, if you need to prioritize power efficiency or are working in temperature-sensitive or remote applications, a load cell resistance of 1400 Ω or higher could be the better choice.
Figure 3 Morehouse Budget Cell with a Load Cell Resistance of 1400 Ω.
If you want a load cell that sits in the center, our budget cell with a load cell resistance of 1400 Ω might be the optimal choice. – It’s also made of stainless steel which is more corrosion resistant.
The bridge resistance alone will not tell you about how well the load cell will perform for your specific application.
Load Cell Resistance Conclusion
Load Cell Resistance is only one specification of many that might matter.
In addition, many specification sheets do not cover how well the load cell will perform when repositioned or used in a different environment than how it was calibrated.
Morehouse has another article on Choosing the Right Load Cell for Your Application as well as a webinar on 4 steps for Choosing the Right Load Cell Calibration System that can help you make a more informed decision.
The difference in load cell resistance between 350 Ω and 2800 Ω might matter, or it might not.
Resistance also impacts the noise in the system.
For example, a higher resistance may allow a higher excitation voltage and, therefore, a better signal-to-noise ratio.
On the other hand, too high of a resistance may pick up more electromagnetic interference.
With some experimentation, the correct balance of resistance, indicator filtering, cabling technique, shielding, and grounding can provide the required accuracy in even tough environments.
By understanding these differences, you can make more informed choices and optimize the performance of your measurement system.
More Information about Morehouse
We believe in changing how people think about force and torque calibration in everything we do.
This includes setting expectations and challenging the "just calibrate it" mentality by educating our customers on what matters and what may cause significant errors.
We focus on reducing these errors and making our products simple and user-friendly.
This means your instruments will pass calibration more often and produce more precise measurements, giving you the confidence to focus on your business.
Companies around the globe rely on Morehouse for accuracy and speed.
Our measurement uncertainties are 10-50 times lower than the competition, providing you more accuracy and precision in force measurement.
We turn around your equipment in 7-10 business days so you can return to work quickly and save money.
When you choose Morehouse, you're not just paying for a calibration service or a load cell.
You're investing in peace of mind, knowing your equipment is calibrated accurately and on time.
Contact Morehouse at info@mhforce.com to learn more about our calibration services and load cell products.
Email us if you ever want to chat or have questions about a blog.
We love talking about this stuff. We have many more topics other than expressing SI units!
Our YouTube channel has videos on various force and torque calibration topics here.
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