DayTronic 3770 Instruction Manual - Page 14

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Model 3770

CALIBRATION (cont.) – Shunt Calibration Equivalent Calculation

If dead weight calibration is not practical and the transducer calibration data is unknown, the

Equivalent Input value for the factory-installed calibration resistor can be approximated as follows,

assuming that the mv/v sensitivity rating of the transducer and the bridge resistance is known.

Sample Calculation: Assume that K = 3.000 mv/v for a 5000-pound load cell (full scale) with a bridge

resistance of 350 ohms and using the internal 59K Ohm shunt resistor.

When + CAL is depressed, input sensor is at zero, display is on the 5000 range, adjust the readout for 2472

Display Calibration

analog controls which allow the user to alter the display reading to a value that is suited for the engineering units display

reading required. The additional span controls allow the user the flexibility to adjust the display reading as referenced to the

analog signal or referenced to the defaulted front panel display dip switches. In a standard – default "OFF" switch position

of the Coarse Span control, the display has a full scale reading set by the display range switches of 5000, 10000 or 20000

counts. At the selected reading the full scale analog output signal will be 5 VDC or 10 VDC, as selected by the Analog

Controls of the unit. To accomplish the display adjustment change, the user would first set the desired display range and

decimal location using the display dip switches, adjust - calibrate the analog output to the desired amplitude level and then

adjust the display Coarse and Fine span controls for the required digital readout.

Tech Tip:

The inability to balance correctly where the unit output reads totally off scale and the zero controls have no

authority can very likely be the result of a damaged or defective transducer or cable. This possibility can be confirmed (or

eliminated) by substituting a transducer and cable known to be in good condition or by simulating a balanced transducer, using

either a commercially available transducer simulator or the simple star bridge arrangement shown below. The star bridge

simulates a conventional four-arm bridge in an exact condition of balance. To construct a star bridge connect four resistors as

shown; use 180-ohm resistors to approximate a 350-ohm bridge.

Neither the resistor values nor temperature

characteristics are critical. Solder two

excitation resistors together, and then solder

the two Signal resistors together. Next, connect

the two junctions together using a separate

wire as shown. There is a good reason for this

method of construction, and it should be

followed. Connect the substitute or simulated

transducer to the module I/O connector using

a short 4-wire cable configuration as shown in

Figure 4. Attempt to balance the substitute or

simulated transducer. If conditions now appear

to be normal, the transducer or cable is at fault.

If the previous difficulties persist, the meter is

at fault.

X = 25000 x Rb

Where

X = Equivalent Input, % of full scale

Rb = bridge resistance, ohms

K = transducer sensitivity, mv/v full scale

Rc = calibration resistance, ohms (59 K installed)

25000 x 350

59000 x 3

he 3770 instrument display has separate adjustment controls that are independent of the

DC STRAIN GAGE PANEL METER

K x Rc

= 49.44% of full scale = 2472 pounds

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