transmitted value is going to be the error in the digital value plus a voltage error of the output. For
the receiving end (data recorder, logger, panel display), there is a quantization error plus an
accuracy error when the analog voltage is converted to a digital value. The best resolution of a 12
bit A/D on a 0 to 5 scale is 1.25 mV. If the scale is wider or the number of bits is less, then the
resolution is even coarser.
This suggests that most users will want to customize the output range to maximize the accuracy of
their equipment over the range of interest. The command to set the Analog Output range is the
aXARzf!
Where a is the address character, XAR is the extended command to set the
analog range, z is the pressure in psi that is to correspond to 0.000 VDC, and
f is the pressure in psi that is to correspond to 5.000 VDC.
If the user wanted the output of the ACCUBUBBLE to be 5 to 10 psi then the following command
would adjust the range.
0XAR+5+10!
If the user wanted the ACCUBUBBLE to output V1 volts at pressure P1 and V2 volts at pressure
P2, then the following formulas would be used to determine z and f.
z
P
V P
P
V
V
=
−
−
−
1
1
2
1
2
1
(
)
f
P
V
P
P
V
V
=
+
−
−
−
1
5
1
2
1
2
1
(
)(
(
)
)
For example, suppose we want the ACCUBUBBLE to output 2V at 20 ft of water and 4V at 40 ft of
water. First we must convert feet of water to psi by dividing by 2.3073. This gives V1=2V,
V2=4V, P1=8.668, P2=17.336. Therefore
z = 8.668 - ( 2*(17.336 - 8.668) / (4-2) ) = 0
f = 8.668 + ( (5-2)(17.336 - 8.668) / (4-2) ) = 21.67
Our command would therefore be:
0XAR+0+21.67!
Converting Voltage to Pressure
The formula for converting the analog output voltage to pressure is:
Pressure = analog output * Slope + offset
where the slope is (5 volt pressure value - 0 volt pressure value) / 5
and the offset is the 0 volt pressure value.
For the above illustrated range of 2V at 20 feet of water and 4V at 40 feet of water the offset for
reading the pressure in psi would be
z
which was 0. The slope would be (
f - z
) / 5 which is: 4.334.
To compute the output in feet of water for this example, the slope would be 10 and the offset would
be 0.
The slope and offset for different units when the output is configured for the factory default of 0 to
22 psi is given in the following table:
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21
Содержание Accububble 56-0131-50-1
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