Lakeshore Learning Materials 642 - Manual (Page 109)

Lake Shore Model 642 Electromagnet Power Supply User’s Manual
Service 7-17
7.11.2 Calibration Equipment
1. Calibration Shunt Resistor – The output current of the Model 642 must be measured externally as the primary
reference for calibration. When current is measured, it is the result of the current through the known resistance of the
calibration resistor. This resistor must be accurately measured and its actual value (R Shunt) used to determine the
actual current flow. For example, if the resistor is measured at 0.99661 m
Ω , the actual current flowing is calculated
by the following equation.
I
shunt
= V
shunt
/0.99661 × 10 -3 or I
shunt
= 1000 × V
shunt
/0.99661
This resistor must withstand the full current of the Model 642 and do so with a minimum of heating that can easily
change the resistance and therefore the current measurement. At 70 A, this resistor only dissipates 4.9 W. Even so, it
is highly recommended to mount the resistor on a heat sink with forced air-cooling to minimize temperature rise and
related value changes during calibration. (Alpha PSBWR0010F is suggested)
2. Magnet Load – The Model 642 is designed for a nominal magnet load of 0.5-ohms and 0.5 H. For calibration
purposes, the magnet can easily be used as the calibration load. The calibration shunt resistor is wired in series with
the positive output terminal using a cable of similar current handling capability as the normal magnet lines. An
alternative to this is the use of a resistor bank with total resistance of 0.5-ohms capable of safely dissipating 2,450 W
of heat (the equivalent of a medium sized space heater). This procedure assumes the use of the water-cooled magnet
as a calibration load.
3. DC Voltmeter (DVM) – The voltmeter must measure VDC accurately to 10’s of µV if resolution to 10’s of mA
(from the Model 642). The Agilent Model 34401 DMM or better is suggested.
7.11.3 Calibration Procedure
The following calibration steps should be performed exactly in the order provided. Pay close attention to the use of
“CALZ” vs “CALG” commands. They can easily be confused and will certainly create unexpected results if accidentally
interchanged. Zeroing calibrations use “CALZ” commands while Gain (Span) calibrations use “CALG” commands.
7.11.3.1 Calibrate Current Output Zero
The 1 m Ω shunt resistor is wired in series with the calibration load from the positive output terminals with #4 AWG
wire. Cable length is relatively unimportant but should be less than 5 feet to the shunt resistor. Voltage across the shunt
resistor is to be monitored by the DVM. The DVM input connections must both be isolated from earth (power line)
ground.
1. Send “CALZ 10, 0” to set the output offset constant to 0.
2. Set the Model 642 output current to 0 A.
3. Measure the actual voltage across the shunt and record (V
shunt
).
4. Calculate the zero offset constant: -(V
shunt
/R
shunt
).
5. Send “CALZ 10, zero offset constant”.
6. Reset the Model 642 output current to 0 A (loads the new offset setting).
7. Verify the actual output current to be less than ±1 mA.
8. Send “CALSAVE” to write this calibration to non-volatile memory.
7.11.3.2 Calibrate Current Reading Zero
1. Send “CALZ 0, 0” to set the current reading offset constant to 0.
2. Measure the actual voltage across the shunt and record (V
shunt
).
3. Get the Model 642 output current reading (by front panel or interface) and record (I reading).
4. Calculate the zero offset constant: – (I
reading
– (V
shunt
/R
shunt
)).
5. Send “CALZ 0, zero offset constant”.
6. Verify the Model 642 output current reading to match the actual output current within ±0.0005 mA.
7. Send “CALSAVE” to write this calibration to non-volatile memory.