Lakeshore Learning Materials 642 - Manual (Page 108)

Lake Shore Model 642 Electromagnet Power Supply User’s Manual
7-16 Service
7.11 CALIBRATION
Lake Shore maintains a fully automated calibration fixture for the Model 642 that measures existing performance of the
unit and then recalibrates the using NIST traceable resistance and voltage standards. In addition, non-calibration features
are verified for proper operation by a full battery of extensive tests. Although this testing is very complete, it requires the
return of the Model 642 to Lake Shore to perform the recalibration. In some instances, however, it may be necessary to
recalibrate the Model 642 in the field. Calibration information for the following operating parameters is provided:
• Output Current.
• Output Current Reading
• Output
Voltage Reading
• External Programming Voltage Reading
These calibrations are done through the computer interface and the calibration constants are stored in the non-volatile
memory in the instrument. The cover does not have to be removed to calibrate the instrument. The remaining features of
the Model 642 do not require calibration to operate within their specified tolerances.
Uncalibrated (default) values for gains are 1 while offsets are 0. Operation with these values is possible but the accuracy
will be reduced to as much as ±2% of full scale, generally. If the Model 642 is used in closed loop operation and
programmed through the external input, regular calibration may not be required.
7.11.1 Calibration Interface
Computer interface commands are included in the Model 642 specifically for calibration. These commands work with
either the IEEE-488 or RS-232C interface. Refer to Section 7.13.4 for a complete description of each calibration
command.
It is always recommended to read out old calibration coefficients using the CALZ? and CALG? interface queries before
attempting to calibrate. This will give the operator experience with the interface command, data formatting, and typical
values. If the old values are saved, they can be reloaded in the case of accidental loss of data during calibration. New
calculated calibration coefficients should be very similar to the old values. Discrepancy between the old and new values
of more than 0.1% of gain calibration coefficients or 0.1% of range for zero coefficients could indicate an error in the
calibration procedure or a hardware failure. Do not attempt to recalibrate a damaged instrument.
The instrument will use the new calibration coefficients as soon as they are sent with the either the CALZ or CALG
interface command but they are not saved permanently until the CALSAVE command is issued. If a mistake is made in
the calibration process, turn the instrument power off and on again before CALSAVE is issued to restore the old
calibration constants. Once CALSAVE is issued, old values cannot be retrieved from the instrument.
If calibration coefficients are left at default or are outside of the normal calibration range, the following error message
will appear in the instrument display when the instrument is turned on: “Calibration Invalid”. This error message must be
bypassed to allow calibration of the instrument. Press both Enter and Escape keys simultaneously to bypass the error
message. Operation in this state is possible but at least one calibration is known to be out of proper range and
measurement is likely to be erroneous.
Simple communications program examples are shown in Sections 5.1.5.2 and 5.2.7.1. Some time should be spent
becoming familiar with the calibration commands before beginning a calibration. Although the calibration factors are
sent to the Model 642 over the computer interface, this procedure is written to obtain the Model 642 readings solely by
visual observation of the front panel.