Calibration; Why Should You Calibrate The Mark 22; Materials Required For Calibration; Temperature Calibration - Martek MARK 22 Operation Manual

5 - CALIBRATION

5.0 W S
HY HOULD
M 22?
THE ARK
Electronic instruments
physical or chemical reactions require peri-
odic calibration in order to maintain their
accuracy. This is done by comparing the mea-
surements gathered by the instrument to a
reference or standard that is well-known and
widely accepted.
The most commonly used method for
calibration involves two points: zero point
and the span point.
two points is to establish the slope of the line
of measurement between the zero and span
points. The accuracy of an instrument using
a two-point calibration is therefore directly
related to the ability of the sensor and the
instrument to maintain linear output over the
full scale of measurement.
5.0.1 Materials Required for
Calibration
The Mark 22 can be calibrated using so-
lutions or electronic circuitry that simulates
the electrical properties of solutions. In ei-
ther case, the method chosen should be trace-
able to National Institute of Standards and
Testing, NIST (Formerly National Bureau of
Standards).
To calibrate the Mark 22 using solutions ,
the following items will be required:
1. Laboratory-grade
2. Temperature Bath (ice and warm wa-
ter will also work)
3. Conductivity Reference Solution
24
Y C
OU ALIBRATE
that measure
The purpose of these
thermometer
4. pH Buffers 7 and either 4 or 10.
To maintain the specified measurement
accuracy, always calibrate the Mark 22 in the
range of the measurement to be performed.
Pre-mixed conductivity solutions may be
purchased from scientific equipment distri-
bution warehouses or the solutions can be
mixed in the laboratory by consulting a
chemical handbook which compares potas-
sium chloride concentrations to electrical
conductivity values.
Items 1-3 of the above list are not neces-
sary if the temperature and conductivity are
calibrated electronically.
5.1 T
EMPERATURE
Temperature must always be calibrated
before conductivity and pH since tempera-
ture is used in the Nernstian and solution
compensation for pH, and for temperature-
corrected conductivity values and solution
compensation for conductivity.
Temperature can be calibrated either elec-
tronically inside the Mark 22 (Auto Calibra-
tion) or physically using a temperature bath
(Manual Calibration) to set the two point
calibration of zero and span.
In an electronic calibration, a particular
physical value can be determined by using
its direct known resistance value and elec-
tronically substituting this value as the ac-
tual physical measurement. For calibrating
temperature, the Mark 22 utilizes precision
resistors in its circuitry to simulate 0
50 C.
o
C
ALIBRATION
C and
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Martek Instruments, Inc.