Specifications

32
MODEL 30 pH/DO SECTION 6.0
CALIBRATION
6.2 CALIBRATING TEMPERATURE
6.2.1 Purpose
Temperature is important in the measurement of dissolved oxygen, pH, and ORP for different reasons.
Temperature affects the determination of oxygen in two ways.
1. The analyzer uses temperature during automatic air calibration. From the temperature, the analyzer calculates
the vapor pressure of water and, using the barometric pressure, calculates the partial pressure of atmospher-
ic oxygen. Once the analyzer knows the partial pressure of oxygen, it calculates the equilibrium solubility of
oxygen in water using a temperature-dependent factor called the Bunsen coefficient. Overall, a 1°C error in the
temperature measurement produces about a 2% error in the solubility calculated during calibration and about
the same error in the final measurement.
2. The oxygen sensor is a membrane-covered amperometric sensor. Oxygen from the sample diffuses through
the membrane and is consumed at an electrode immediately behind the membrane. The reaction produces a
current directly proportional to the concentration of oxygen. The permeability of the membrane depends on
temperature. Therefore, both temperature and dissolved oxygen least affect the sensor current. To account for
changes in the sensor current caused by temperature alone, the analyzer automatically applies a membrane
permeablility correction. Because the membrane permeability changes about 4%/°C at 25°C, a 1°C error in
temperature can produce about a 4% error in oxygen reading.
Temperature affects the measurement of pH in three ways.
1. The analyzer uses a temperature dependent factor to convert measured cell voltage to pH. Normally, a slight
inaccuracy in the temperature reading is unimportant unless the pH reading is significantly different from 7.00.
Even then, the error is small. For example, at pH 12 and 25°C, a 1°C error produces a pH error less than ±0.02.
2. During auto calibration, the Model 30 recognizes the buffer being used and calculates the actual pH of the
buffer at the measured temperature. Because the pH of most buffers changes only slightly with temperature,
reasonable errors in temperature do not produce large errors in the buffer pH. For example, a 1°C error caus-
es at most an error of ±0.03 in the calculated buffer pH.
3. The Model 30 can be programmed to calculate and display pH at a reference temperature (25°C).The max-
imum change in solution pH with temperature is about ±0.04 pH/°C, so a 1°C temperature error does intro-
duce a small error. However, the major source of error in solution temperature compensation is using an incor-
rect temperature coefficient.
Temperature affects the measurement of ORP in a complicated fashion that is best determined empirically.
Without calibration the accuracy of the temperature measurement is about ±0.4°C. Calibrate the sensor/analyzer
unit if
1. ±0.4°C accuracy is not acceptable
2. the temperature measurement is suspected of being in error. Calibrate temperature by making the analyzer
reading match the temperature measured with a standard thermometer.