Hanna Instruments HI 720 Series - Manual (Page 24)

46 47
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 Actual Conductivity Temperature
1 500 μS/cm 0.0 °C
2 600 μS/cm 5.0 °C
3 700 μS/cm 10.0 °C
4 800 μS/cm 15.0 °C
5 900 μS/cm 20.0 °C
6 1000 μS/cm 25.0 °C
7 1100 μS/cm 30.0 °C
8 1200 μS/cm 35.0 °C
9 1300 μS/cm 40.0 °C
10 1400 μS/cm 45.0 °C
This table corresponds to a constant β = 2%/°C.
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
1
< T
2
, then C
1
< C
2
• T
min
< T
ref
< T
max
• two subsequent temperatures must differ at least 1°C
Based on the defined table, up to 10 temperature coefficients are automatically
computed as follows:
β
n
= (C
n
- C(T
ref
))/[(T
n
-T
ref
)*C(T
ref
)]
where C(T
ref
) is the actual conductivity at T
ref
, and is computed as follows:
C(T
ref
) = C
m
+ (C
m+1
- C
m
)*(T
ref
- T
m
)/(T
m+1
- T
m
)
with T
m
< T
ref
< T
m+1
Then, if the current temperature is T, with T
n
T T
n+1
, with (C
n
, T
n
) and (C
n+1
,
T
n+1
) being two subsequent couples configured in the user table, then the tem-
perature coefficient will be:
β(T) = β
n
+ ( β
n+1
- β
n
)*(T - T
n
)/(T
n+1
-T
n
)
When the measured temperature is lower than the minimum temperature in the
table (T
1
), 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
10
), then the coefficient will be calculated by replacing T with T
10
.
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
comp
= C
act
/ [1+ β(T-T
ref
)/100]
where
• C
comp
is the compensated conductivity value
• C
act
is the actual conductivity value (not compensated)
• T is the measured temperature
• T
ref
is the reference temperature (selectable at 20 or 25°C).
The temperature coefficient ( β) must be manually adjusted by the user when
changing the reference temperature. If β is the coefficient with T
ref
=25°C, the α
coefficient with T
ref
=20°C must be calculated as follows: α = β / (1 - β/20).
For example, if β=1.90 %/°C, then α = 2.10 %/°C.
NaCl Temperature Compensation
For the NaCl algorithm, the compensation formula is the same as for the linear
method, but with
β depending on the temperature value according to the IEC
746-3 BII table. Note that the table reports β values referred to 18°C and the
HI720 adjusts that values according to T
ref
as follows: β = β
18
/[1 + β
18
(T
ref
-18)].
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.