shunt calibration Archives

Shunt calibration is a process of calibrating a measurement instrument using a shunt calibration resistor. The shunt calibration resistor is connected in parallel with the measurement instrument to provide a known resistance value, which is used to calculate the instrument’s accuracy.

In shunt calibration, a known current is passed through the shunt calibration (cal) resistor, which generates a known voltage drop across the resistor. This voltage drop is measured using the measurement instrument being calibrated, and the instrument’s accuracy is calculated based on the known resistance value of the shunt calibration resistor and the measured voltage drop. They create a simulation of load and verify the health of the sensor. Commonly, they are used to scale instruments.

The accuracy of the measurement instrument can be calculated by knowing the shunt resistor’s precision level and applying Ohm’s Law, which states that the current passing through a resistor is proportional to the voltage drop across it and inversely proportional to its resistance value.

Shunt calibration can be used to calibrate force measurement devices, including load cells. Interface provides shunt calibration resistors in our accessories line as “loose” resistors. They are also available with engineered to order requests for designs into cables, connectors and even within the load cell.

Shunt calibration is an important process for ensuring accurate and reliable measurements in various industrial, commercial, and scientific applications. It allows measurement instruments to be calibrated quickly and cost-effectively, and it improves the accuracy and reliability of the measurement data.

What is a shunt calibration resistor?

A shunt calibration resistor is a resistor that is connected in parallel with a measurement instrument to provide a known resistance value. The purpose of the shunt calibration resistor is to calibrate the instrument to accurately measure the current passing through it. Shunt calibration resistors are often used with load cells to improve the accuracy and reliability of their measurements.

How are shunt calibration resistors used with load cells?

Load cells typically generate a small electrical signal in response to applied force or weight. This signal is amplified and processed by a signal conditioning circuit before a data acquisition system or controller uses it. The signal conditioning circuit can utilize an internal shunt calibration resistor on the instrumentation side, or activate a resistor located upstream in the system.

Shunt calibration resistors located either in the sensor, cable, or instrument will be switched into the circuit during the shunt calibration process, shunting and diverting current in the process. This shunting effect unbalances the Wheatstone bridge, simulating loaded output from the sensor. Because the resistance value is known, sensor span output and thus instrument scaling can be accurately verified. This electrical simulated signal negates the need for physical force or torque calibration of the system.

The shunt calibration resistor provides a known resistance value, which is used to verify the health and output of the load cell, ensuring accurate system measurement of the applied force or weight. The resistor diverts a small portion of the load cell’s excitation current. The value of the shunt calibration resistor is carefully selected based on the load cell’s characteristics and the desired measurement accuracy.

Shunt calibration uses the shunt resistor to force a load cell bridge to provide a fake signal output. It allows one to check for sensor health and whether the signal behavior has deviated from an original calibration certification with initial shunt output data.

This forced signal output allows for the attached instrument to be scaled. This could be setting signal conditioner scaling: When the load cell reaches max calibrated force, is the mV/V input properly scaled for the exact 5V, 10V or 20mA conditioner output? The other setting option is displayed units of measurement on a display: Is the load cell’s calibrated 3.999mV/V output at 100 lbs displaying 100 lbs on the display?

Shunt resistors are sized by resistance value to provide approximately two-thirds or three-quarters full scale output signal. Having this recorded value on the calibration certification the instruments can be scaled as necessary for full scale, and future shunt checks can ensure nothing is changing with the health of the circuit.

Interface Shunt Calibration Resistors – RCAL Resistors

Interface shunt calibration resistors, known as RCAL Resistors, are an accessory product. They are made from the highest components and processes to ensure the specifications for your Interface products perform to meet their published specifications. Available RCAL Models include RS-100-30K, RS-100-40K, RS-100-60K, and RS-100-120K are available.

Interface RCAL Resistors are high precision components and provide an effective, method for checking the calibration of a load cell system in the field or when a means of applying actual forces is unavailable.

Designed to work with Interface products.
Made with the highest quality components.
Created to maintain the specification of the product.
Precision wire-wound
5 ppm/°C, 0.01%

U.S. dimensions and capacities are provided for conversion only. Standard product has metric capacities and dimensions. U.S. capacities available upon special request and at an additional cost.

What are the benefits of using shunt calibration resistors?

There are several benefits of using shunt calibration resistors in measurement applications:

Calibration: Shunt calibration resistors can be used to scale measurement instruments, ensuring that they provide accurate calibrated unit readings. Shunt calibration can often substitute the need for physical force or torque system calibration
Convenience: Shunt calibration can provide a quick and easy system health check either before or immediately after a test. Confirming stable and consistent shunt readings can ensure data integrity in between regular scheduled physical calibration intervals.
Cost-effective: Using a shunt calibration resistor is an inexpensive one time investment vs time and cost associated with pre or posttest physical calibrations. This brings the freedom for frequent and quick system calibration checks with minimal equipment down time.
Flexibility: Shunt calibration resistors can be used with a wide range of measurement instruments, allowing for greater flexibility in measurement applications. Additionally, many instruments allow shunt resistors to be interchangeable for support of varying sensor outputs.

Overall, shunt calibration resistors are a practical and convenient alternative to physical system calibrations. Shunt calibration resistors can be packaged into all Interface load cells with support across most of the available instrumentation as well. Frequent system health and signal stability checks are vital to ensuring consistent integrity with test data and shunt calibration resistors bring such empowerment for extraordinarily little initial investment.

Contributor: Brian Peters

Additional Resources

Metrologists and Calibration Technicians 101

System Level Calibration Validates Accuracy and Performance

Shunt Calibration for Dummies – Reference Guide

Shunt Calibration 101

Regular Calibration Service Maintains Load Cell Accuracy

Top Five Reasons Why Calibration Matters

Calibration is a critical stage to ensure proper accuracy and reliability of any force measurement device. There are many ways to calibrate and different types of calibration. In the standards of maintaining our quality and precision requirements, Interface calibrates every test measurement device we manufacture including our load cells and torque transducers. Every device is shipped with the most detailed calibration certifications in the industry.

With our experience and expertise, we understand that sharing what we know is beneficial to our customers and partners in the test and measurement industry. One of the means by which we do this is through a series of technical white papers. A popular white paper that was written years ago still stands the test of time, as it provides a deep dive on the topic of shunt calibration. Click on the title “Shunt Calibration for Dummies,” to access the full white paper.

What is Shunt Calibration?

Shunt calibration is a technique for simulating strain in a piezo-resistive strain gage Wheatstone bridge circuit by shunting one leg of the bridge. The bridge may be internal to a discreet transducer or composed of separately applied strain gages. The resulting bridge output is useful for calibrating or scaling instrumentation. Such instrumentation includes digital indicators, amplifiers, signal conditioners, A/D converters, PLC’s, and data acquisition equipment. Care must be taken to understand the circuits and connections, including extension cables, in order to avoid measurement errors.

Benefits of Shunt Calibration

The biggest reason to use shunt calibration is the flexibility and low-cost it offers the user. In this method of calibration, the bridge circuit is already there, and you don’t need to make and break cable connections to run it. This means that a shunt calibration can be applied conveniently and at any time during the test program. It is often used in situations where the user is calibrating control system equipment that will be communicating with a transducer or to confirm that the transducer is functioning properly.

Expected Shunt Calibration Repeatability in Modern Transducers

An important question that comes up regarding calibration is what type of repeatability can I expect from shunt calibration? Included below are the specifications outlining expected repeatability:

Procedure for a repeatability test performed:

100 Klbf Load Cell specimen loaded in compression.
12 test cycles of 4 mV/V hydraulically applied physical load and 1 mV/V Shunt Cal on two bridge legs.
Rb = 350 ohm, Rs = 88750 ohm, 20 ppm/°C, internal to load cell.
Measurements over 3 days.
Interface Gold Standard HRBSC instrumentation.

Results of test

Std Dev of physical load measurement: 0.004%.
Std Dev of Shunt Cal: 0.001% pos, 0.001% neg.

The topics that are illustrated in examples and discussion points for this white paper include:

Basic Bridge Circuit and Formulas
Resistor Examples
Tolerance
Cables
Errors
Permanent Zero Balance Shifts
Transducer Toggles
Instrumentation
Procedures for Repeatability in Tests

If this is a topic that of interest, download this technical reference guide for further exploration and calculation examples in shunt calibration.

As a leader in calibration services, Interface has an A2LA ISO 17025 accredited calibration lab located at the company’s headquarters in Arizona. Many depend on Interface for expert recalibration, which we recommend to do annually for optima maintenance. Our own calibration lab has the broadest capability and highest quality of calibration and repair services available. We understand the criticality of proper calibration and traceability and have the experience and expertise necessary to meet your exacting needs.

Additional Calibration Resources

Extending Transducer Calibration Range by Extrapolation

Additional Interface Calibration Grade Solutions

Gold Standard® Calibration System

This refreshed white paper is a tribute to the contributions of LaVar Clegg.