Quadra 4 Operator Manual
Tomtec
181
3.0 Protocol Or Program Security
The flexibility in programming the Quadra 4 has many advantages. The ease of
alterations can affect the performance of specific programs. The solution for this
problem is provided by the program checksum. The program checksum is a 32 bit
number that adds all of the digital steps in a program. It is recalculated each time that
program is called for operation. If the checksum matches the original validated
program, the Quadra 4 will execute exactly the same program. The accuracy is one
bit in 2
32
or 4 trillion sum.
A checksum for every program is calculated at the time the program is called. The
checksum is composed of two separate additions - the program itself and the Quadra
4 parameters. If the checksum matches the check sum originally recorded for a
validated program, it assures the operator the Quadra 4 will run the exact same
protocol.
If there is a mismatch, the checksum can be shown in its two parts - program and
parameters. If the program checksum has a mismatch, the original program may be
reloaded. This assumes an original validated copy is retained. If the mismatch is in the
instrument parameters, then it is necessary to check each one for a mismatch since
they are individually set.
While the Quadra 4 calculates the checksum each time any program is called, it does
not verify that it matches a previous record. This must be accomplished external to
the Quadra 4 normal software configuration.
4.0 Calibration For The Quadra 4
On the Quadra pipettor heads, the stepper motor moves the pistons a precise
distance with each step. Calibration is simply telling the stepper motor how many
steps are required for each volume to be displace All of the Quadra pipettor heads
use a similar method for calibration. The 450µL head is used as an example.
The first step is to determine how many steps are required to deliver a measured
volume. In the "factory settings”, the steps per microliter factor is set to 16 steps per
microliter. The "backlash" factor must be set to zero. With the display set to transfer
10µL, the software will move the pistons 160 steps. Both to aspirate and dispense.
The volume dispensed is accurately weighed. Assume water is the liquid and the
weight was 0.0089 grams or 8.9µL. Thus, 160 steps gave 8.9µL or 17.977 steps, per
microliter. This becomes the calibration factor for 10µL.
An air displacement pipettor covering a wide volume range, such as the Quadra 4
(10µL to 450µL), does not have a linear calibration curve. The dead air volume in the
pipettor, at a minimum, is equivalent to the tip volume (450µL). The physical laws of
nature govern the compressibility of this air column and its displacement of liquid. The
higher volumes from 100µL to 450µL are nearly linear. The physical factors have a
greater impact on the smaller volume in the range. Thus, the accuracy of calibration is a
function of how many points on the curve are measured and how many are
interpolated. This is determined empirically.
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