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error. If the range is narrowed to 980 to 1030 mB then 1 mV error corresponds to a 0.01 mB error. The
narrower range would also mean that it would take 100 mV of noise to generate a 1 mB error instead of the
10 mV that would be required with the 600 to 1100 mB range. Therefore to maximize the accuracy of data
collected or displayed via the analog output, adjust the output range of the ACCUBAR
®
to match the range
of pressures 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 mB (hPa) that is to correspond to 0.000
VDC, and f is the pressure in mB (hPa) that is to correspond to 5.000 VDC.
If the user wanted the output of the ACCUBAR
®
to be 800 to 1100 mB then the following command would
adjust the range.
0XAR+800+1100!
If the user wanted the ACCUBAR
®
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 you want the ACCUBAR
®
to output 2V at 29" Hg and 4V at 31" Hg. First you
would convert "Hg to mB by multiplying by 33.864. This gives V1=2V, V2=4V, P1= 982.056, P2=
1049.784. Therefore
z = 982.056 - ( 2*(1049.784-982.056) / (4-2) ) = 914.328
f = 982.056 + ( (5-2)(1049.784-982.056) / (4-2) ) = 1083.648
The command would therefore be:
0XAR+914.328+1083.648!
Converting Voltage to Pressure
The formula for converting 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 29" Hg and 4V at 31"Hg the offset for reading the pressure in mB
would be
z
which was 914.328. The slope would be (
f - z
) / 5 which is: 33.864. To compute the output in
"Hg for this example, the slope would be 1 and the offset would be 27.
