WEIGHING SYSTEM CALIBRATION USING A LOAD CELL SIMULATOR

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1 WEIGHING SYSTEM CALIBRATION USING A LOAD CELL SIMULATOR By William Hess Director of Engineering May 2007 IM Export Circle Huntsville, AL (256) FAX NO. (256) Website: cotiinc@ aol.com

2 COTI, INC. 122 Export Circle Phone Huntsville, AL Fax sales@cotiinc.com Website: - TABLE OF CONTENTS Page I. INTRODUCTION II. LOAD CELL EQUIVALENT CIRCUIT III. SYSTEM WIRING IV. SPAN (GAIN) CALCULATIONS V. SPAN (GAIN) SETTING VI. DEAD LOAD SETTING VII. DETERMINING THE DEAD LOAD WEIGHT FOR A TANK FIGURE LOAD CELL EQUIVALENT CIRCUIT THEVENIN EQUIVALENT CIRCUIT FOR A LOAD CELL OHM LOAD CELL SYSTEM WIRING OHM LOAD CELL SYSTEM WIRING SYSTEM SET UP FOR LOAD CELLS OTHER THAN 350 OR 700 OUTPUT RESISTANCE IM 1004

3 WEIGHING SYSTEM CALIBRATION USING A LOAD CELL SIMULATOR A. INTRODUCTION This paper describes a method for calibrating single or multiple load cell systems using a load cell simulator and some fixed resistors instead of using calibrated test weights. This method, if done with simulator and some fixed resistors instead of using calibrated test weights. This method, if done with care, can result in weighing accuracy s of 0.25% of full scale. This calibration method is useful for two reasons: 1. As a pre-calibration to speed up the final system calibration in the field. 2. To calibrate systems that would be very difficult using conventional calibrated test weights, etc. B. LOAD CELL EQUIVALENT CIRCUIT A typical strain gage load cell circuit is shown in Figure 1. The nominal output resistance of a strain gage load cell is commonly 350, 480, 700 ohms. Coti s nominal load cell output resistances are: LC Resistance 350 ohms 480 ohms 700 ohms 1000 ohms MODEL CISB / CISB SS CP175 CI-5103 CI-BLC-C / CI-BLC-T CI-23-1 / CI-23-SS CP26S5 CI-16 CI-BLC-C / T CI-SSB CI- 58 CP26S3-10 The exact load cell output resistances may vary slightly due to production tolerances. The output resistance of a 350 ohm load cell is usually 351 to 352 ohms and a 700 ohm load cell is typically 702 to 703 ohms. Other load cell output resistances are possible, if in doubt, use an ohmmeter to verify the actual load cell resistance of the load cells that you are using. The actual load cell output resistance, instead of the nominal resistance, can be taken into account for slightly increased overall accuracy. The Thevenin Equivalent circuit for a load cell is shown in Figure 2. This load cell equivalent circuit is a convenient model to use since it shows that a load cell is a equivalent voltage source in series with the output load cell resistance. When using a load cell simulator, it is best to split any series resistance needed for the simulator output signal. An Page 1 of 4

4 unbalanced load cell simulator will shift the zero readings for the system being calibrated. C. SYSTEM WIRING A. Refer to Figure 3 for 350 ohm load cells. B. Refer to figure 4 for 700 ohm load cells NOTE: If a 350 ohm simulator is used when calibrating a 700 ohm system, (2) each 175 ohm resistors must be used, in series with the output signal leads, to make the 350 ohm simulator equivalent to a 700 ohm simulator. The resistors used should be 1% tolerance metal film type. C. If load cells with an output resistance other than 350 or 700 ohms are used, use the appropriate series resistors to simulate the set of load cells (see Figure 5) D. SPAN (GAIN) CALCULATIONS A. Set the span (gain) of the weighmeter or signal conditioner before the dead load is set. The span may interact with zero (dead load), but changing the zero settings (dead load) will not change the span calibration of Coti s equipment and most equipment by other manufactures. B. Determine the span level in a mv/v signal level. Mv/v signal = mv/v (fs) x percent LC utilization of the load cells (live load) Example: (4) each, 3 mv/v, 5000 pound load cells are used. The system live load is 7500 pounds. (4) each, 5000 pound load cells are equal to (1) each, 20,000 (3 mv/v) pound load cell for calculation purposes. Mv/v signal = 3 mv/v X 7,500 pounds 20,000 pounds = mv/v If the load cells have different sensitivities, use the average mv/v figure. Load Cell 1 = 3.02 mv/v Load Cell 2 = 2.97 mv/v Load Cell 3 = 3.01 mv/v Load Cell 4 = 2.99 mv/v Load cell average mv/v = ( ) / 4 Average load cell output = mv/v 2 of 4

5 E. SPAN SETTING Since most load cell simulators will not have a mv/v step, convert the live load signal as follows: mv/v = 7,500 pounds 1.1 mv/v = 1.1 x 7,500 = 7,333 pounds Thus, set the simulator to 1.1 mv/v and adjust the system weighmeter for a reading of 7,333 pounds. This setting is equal to a calibration setting of 7,500 pounds at mv/v signal level. NOTE: Also refer to the weighmeter installation and service manual. F. SPAN SETTING WITH MULTIPLE LOAD CELLS For multiple load cells use the following procedure: 1. Calculate the live load signal as in step D. 2. Connect the simulator to the junction box as in figures 3, 4, or 5 3. You can use the actual load cells in all but one (1) summing board input or you can use fixed resistors in the signal leads for each load cell input. 4. Since the simulator is connected to only one (1) input, multiply the simulator setting by the number of load cell inputs. Example: A. The calculated load cell signal from the tank is 1.2 mv/v. Number of Load Cells Simulator Settings 1.2 mv/v 2.4 mv/v 3.6 mv/v 4.8 mv/v G. DEAD LOAD SETTING After the span is set correctly, set the LC simulator to 0.0 mv/v. Adjust the dead load switches so that the display reads a minus weight that is equal to the weight of the empty tank. 3 of 4

6 H. DETERMINE THE DEAD LOAD WEIGHT FOR A TANK 1. To estimate tank dead load, measure the output signals of the installed load cells, average their output signals and ratio the result with total load cell capacity. Example: A tank having four supports and 5,000 lb capacity load cells with 10 volts excitation. LC1 = 4.0 mv LC2 = 5.0 mv LC3 = 4.5 mv LC4 = 5.5 mv 19.0 mv LC average signal = 4.75 mv Excitation voltage = 10v LC mv/v rating = 3 mv/v 10V (excitation voltage) X 3 m/v (load cell output) = 30 mv (the full load cell output signal at capacity) mv (average output) / 30 mv (load cell output signal at full scale) = 15.8% (of load cell capacity with empty tank). 20,000 pounds (load cell capacity) X.158 (% of load cell capacity with empty tank) = 3167 pounds (estimated tank dead load). 4 of 4

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