User manual for meter with U/I input SRP-77
8.
THE ALARM LED
The
ALARM LED
(
AL
) is turned on when input signal is out of the permissible input
range. See parameters:
“tyPE”
,
“Lo r”
and
“Hi r”
in paragraph
„InPt” menu
.
9.
OVER-CURRENT PROTECTION
The current input of the device is equipped with over-current protection circuit. This circuit
protects the standard resistor to damage. Maximum input current is set to 40mA (typical).
When temperature of the standard resistor falls, the protection circuit will turn off himself
automatically, and the device will measure the input current again. Due to thermal
phenomenon in the standard resistor, precision of the measurement may be lower, during few
minutes (up to the standard resistor temperature will falls to the environment temperature).
10.
DISPLAYED VALUES CALCULATION
For simplicity of examples it's assumed that current input is selected, and all calculations
are related to this input. When voltage input is selected calculations are similar (be sure of
particular ranges and units).
The first step to compute the result of measure is the calculation of the normalized result
(it means result of 0-1 range). To do it, the begin of the input range (0 mA for 0-20mA range,
and 4mA for 4-20mA range) must be subtracted from measured value. Next, received result
must be divided by the width of the input range ( it means 20mA for 0-20mA range, and 16mA
for 4-20mA range). So normalized result can be expressed by expressions:
I
n
=
I
inp.
−
4
16
for
4
÷
20
mA range
I
n
=
I
inp.
20
for
0
÷
20
mA range
where I
inp.
Means input current (in mA), and I
n –
normalized result.
If measured value exceeds the nominal input range (0-20mA or 4-20mA), and do
not exceed the permissible input range, then received normalized I
n
result will
exceed 0-1 range, e.g. input range 4-20 mA, input current = 3 mA – the normalized
result is equal -0,0625, and for input current = 22 mA, the normalized result is equal
1,125. In such cases presented expressions are still correct.
10.1.
ADDITIONAL CALCULATIONS (USED CONVERSION CHARACTERISTIC)
The manner of the additional computation of the displayed result depends on selected
conversion characteristic. All presented charts are connected with the input range 4 - 20 mA.
33
i
