Delta OHM HD32MT.3, Operating Manual

Page: 28 / 62

HD32MT.3
- 28 -
V1.2
9 MEASURES OF ANALOG INPUTS
The values of the analog inputs acquired by the data logger are always voltage in V.
If an analog input is connected to a voltage transmitter, the data logger acquires directly the
voltage supplied by the transmitter.
If an analog input is connected to a current transmitter, the data logger captures the value of
the potential difference across the internal 50 Ω shunt resistance. If the transmitter has output
0...20 mA, the data logger acquires values in the range 0...1 V. If the transmitter has output
4...20 mA, the data logger acquires values in the range 0.2…1 V.
The value recorded in the data logger is the result of a linear transformation applied to the
value of the input voltage in V acquired:
Recorded value = offset + (gain x input voltage in V)
The offset and gain parameters can be set with commands ADCONOFFS_x and ADCON-
VGAIN_x (x=A,B,C) respectively (see paragraph 11.10) or, in alternative, with the help of Me-
teo Studio software.
By setting offset = 0 and gain = 1, the value recorded is exactly the value of the input voltage
in V.
If the input is connected to a current transmitter and you want to record the value of the cur-
rent, instead of voltage value acquired, set offset = 0 and gain = 20.
Generally, the value of voltage in V acquired by the data logger corresponds to the value of a
physical quantity, for example temperature, humidity, pressure, etc. It is therefore of greatest
interest directly store the value of the corresponding physical quantity rather than the value of
the analog input signal. To record directly the value of the quantity in the data logger, simply
enter the Offset and Gain parameters with the values calculated by the following relations:
( ) ( )
is fs
is fs fs is
V - V
V G - V G
 
=
offset
is fs
is fs
V - V
G G

=
Gain
Where: G
is
= initial scale value of physical quantity
G
fs
= full scale value of physical quantity
V
is
= input voltage (in V) corresponding to the initial scale of the physical quantity
V
fs
= input voltage (in V) corresponding to the full scale of the physical quantity
The value will be recorded in the same units of measurement of G
is
and G
fs
values. This unit of
measurement is the one to be defined with the command ADCONVUNIT_x (x=A,B,C), see par-
agraph 11.10.
The use of non-zero Offset and Gain different from one, produces accurate results only if the
output of the sensor varies linearly with the measured physical quantity. Otherwise, the rec-
orded value will be affected by an error that depends on the response of the sensor. For sen-
sors having non-linearity that cannot be neglected, it is advisable to record the value of the
analog input signal and apply appropriate corrections during the processing of the recorded da-
ta.
Example 1
Suppose you want to record the values of temperature measured by a temperature transmitter
with output 0...1 V and measuring range -40 ... +60 °C.
The values required for the calculation of offset and gain are therefore:
G
is
= -40 °C G
fs
= +60 °C V
is
= 0 V V
fs
= 1 V
The values of the parameters offset and gain are: