Temperature Compensation - Hanna Instruments HI 720 Instruction Manual

TEMPERATURE COMPENSATION

If the setup item b.01 is set to ATC, then an automatic temperature compensa-
tion of the conductivity readings will be performed using the temperature values
acquired through the Pt100/Pt1000 input.
If the temperature probe is not connected or it provides an invalid temperature
(outside the -30 to 130°C range), the instrument will generate a "broken tem-
perature probe" error, which will be handled as stated in the error configuration.
In this case the temperature compensation will be automatically switched to the
MTC option. The instrument continues to monitor the Pt100/Pt1000 input to
track the Pt100/Pt1000 error closing. When this error is closed, the setup item
b.01 will automatically change back to ATC.
If the user set the parameter b.01 to MTC, then a manual temperature compen-
sation will be performed in any case, even if a temperature probe is connected.
The user can also select the temperature compensation algorithm through the
setup item b.10: Linear, NaCl or User defined table.
Linear Temperature Compensation
The linear temperature compensation is performed according to the following
formula:
C = C
comp
where
• C is the compensated conductivity value
comp
• C is the actual conductivity value (not compensated)
act
• T is the measured temperature
• T is the reference temperature (selectable at 20 or 25°C).
ref
The temperature coefficient ( ) must be manually adjusted by the user when
changing the reference temperature. If
coefficient with T =20°C must be calculated as follows:
ref
For example, if =1.90 %/°C, then
NaCl Temperature Compensation
For the NaCl algorithm, the compensation formula is the same as for the linear
method, but with depending on the temperature value according to the IEC
746-3 BII table. Note that the table reports
HI720 adjusts that values according to T
User Defined Table for Temperature Compensation
The compensation formula is the same as for the linear method, but the instru-
ment calculates the temperature coefficient based on the table defined by the
user through the setup items b.31 to b.34.
46
/ [1+ (T-T )/100]
act ref
is the coefficient with T =25°C, the
ref
= / (1 - /20).
= 2.10 %/°C.
values referred to 18°C and the
as follows: = /[1 +
ref 18
Up to 10 actual conductivity/temperature couples can be entered to define the
curve for the temperature compensation.
The default values for the temperature compensation table are:
Couple
This table corresponds to a constant
The couples must be entered according to these conditions, otherwise a "Tem-
perature table error!" message is displayed when the "User" option is confirmed
for the temperature compensation algorithm:
• if T < T
1
• T < T
min ref
• two subsequent temperatures must differ at least 1°C
Based on the defined table, up to 10 temperature coefficients are automatically
computed as follows:
where C(T
ref
with T < T
m
Then, if the current temperature is T, with T
T ) being two subsequent couples configured in the user table, then the tem-
n+1
perature coefficient will be:
(T -18)].
18 ref
When the measured temperature is lower than the minimum temperature in the
table (T ), then the coefficient will be calculated by replacing T with T
1
When the measured temperature is greater than the maximum temperature in
the table (T
Actual Conductivity
1 500 μS/cm
2 600 μS/cm
3 700 μS/cm
4 800 μS/cm
5 900 μS/cm
6 1000 μS/cm
7 1100 μS/cm
8 1200 μS/cm
9 1300 μS/cm
10 1400 μS/cm
= 2%/°C.
, then C < C
2 1 2
< T
max
= (C - C(T ))/[(T
n n ref n
) is the actual conductivity at T , and is computed as follows:
ref
C(T ) = C + (C - C )*(T
ref m m+1 m
< T
ref m+1
n
(T) = + ( - )*(T - T
n n+1 n
), then the coefficient will be calculated by replacing T with T
10
Temperature
0.0 °C
5.0 °C
10.0 °C
15.0 °C
20.0 °C
25.0 °C
30.0 °C
35.0 °C
40.0 °C
45.0 °C
-T )*C(T )]
ref ref
- T )/(T - T )
ref m m+1 m
T T , with (C , T ) and (C
n+1 n n n+1
)/(T -T )
n n+1 n
.
1
.
10
47
,