Here is a better answer. I previously said that Absolute Pressure was not able to be adjusted. I was wrong. I didn't read the manual far enough and only saw the first part about adjusting Relative Pressure. So I figured that only Relative Pressure was able to be adjusted and the Absolute Pressure was factory set, which it its. And they really don't want you messing with it unless you really know what you are doing and you think it is necessary.
So it is possible to adjust the Absolute Pressure that was factory set. To get into this menu you hold down TEMP and MAX/MIN buttons down for 5 seconds. You proceed to the next calibration setting by pressing SET. You adjust each setting with the normal WIND+ and PRESSURE- buttons. Press ALARM to restore given setting to factory default.
Although you are allowed through this advanced calibration menu to adjust Absolute Pressure the addendum mentions that it still recommends you don't mess with this setting and instead just calibrate Relative Pressure only via instructions in section 6.3. (Figure 23 in the manual has this Note in the Absolute Pressure setting section).
With that said I found another weather station manual made by Ecowitt which is very similar in capability to the WS-2902A and is just another similar Fine Offset clone with a different console. In Ecowitt's manual there is more details in understanding Absolute and Relative Pressure calibrations. I found it well laid out, easy to follow so I will copy paste that section below from the HP3500B manual. Shame the WS-2902A manual doesn't explain things this well.
Follow these steps and your Absolute Pressure and Relative Pressure should work properly.
*** This is actually very good information that anyone can use with any weather station of any brand that lets them adjust both Absolute Pressure and Relative Pressure offset.
7.3.1 Calibration of barometric pressure settings.Calibration of barometric pressure requires some additional
understanding, which we will provide here. Also provided is a step by
step procedure for calibrating correctly.
Absolute barometric pressure, can be calibrated at manufacturing time
by comparing with a precise instrument that measures pressure at the
same location. In practice, sometimes small adjustments of a few hPa
may be needed. The relative pressure represents what the air
pressure would indicate if your station was at sea level and depends
on the altitude of your console and cannot be known in advance. This
is why it needs an adjustment.
There are different manners in which to handle this adjustment. We will
outline a reliable procedure below, which requires adjusting both
pressures. The method assumes that you have an official airport
sufficiently nearby to act as a reliable reference. Usually distances of
up to 25 miles work reliably, but this is not always true and depends on
geography. We start by assuming that your station’s absolute pressure
reading is correct and needs no offset correction.
The procedure below assumes you are starting from the console’s
factory setting. With those settings, ABS and REL should, at this time,
be displaying the same value. We also assume, for the moment, that
ABS pressure is 100% correct.
1. For this procedure we will get the most precise results if our
display for pressure is in hPa units. Even if you do not want to use
those units eventually, set the console to use them for now.
2. Determine the altitude, or elevation above sea level, of your
station’s console. This measurement is necessary to account for
the difference in air pressure caused by the elevation of your
console. Elevation above sea level reduces the absolute pressure
measured by your sensor. Determine this altitude using a GPS, or
look it up using a tool such as this web site:
https://www.freemaptools.com/elevation-finder.htm. You can
input your location’s GPS coordinates, or manipulate the map to
your location. Click on “Estimate Elevation” and observe the result.
For an example we will use a console location at 42 ft. above sea
level.
3. This tool will provide the ground level elevation at your location, so
you will need to add the right amount for how high above ground
level your console is. If you are on a ground floor and have the
console on a desk, you’ll have to add something like 3-4 ft. If you
are using a GPS system that tells you elevation, make sure it is
right next to the console and you’ll be able to read the correct
elevation right from the GPS results without further adjustment.
4. With the correct altitude/elevation in hand you will need to
determine the correct offset. To be added to the absolute pressure
reading in order to compute relative pressure (sea level
equivalent). Correction tables can be found on-line in many places.
One example is the table found at the web site at
https://novalynx.com/manuals/bp-elevation-correction-tables.pdf.
Locate your elevation in the first column and read the correction in
the third column. This table, however is rather coarse, making it
hard to be precise. An alternative is an on-line calculator such as
the one found here:
http://www.csgnetwork.com/barcorrecthcalc.htmlFor our example of 42 ft. above sea level we input 42 ft. of
elevation and a standard pressure of 1013.25 hPa/mb and press
calculate. We find an “absolute barometer value” that should be
-1.5626061222588443 hPa lower than at sea level. The inverse
(because relative pressure is higher than absolute pressure) of
this number will be our “REL PRESS OFFSET” value. Use the
settings procedure to increase REL by +1.6 (nearest rounded
value we can input).
5. Now we need a reliable reference for pressure at sea level. Locate
the official identifier for the nearest airport. Refer to “World Airport
Codes” at
https://www.world-airport-codes.com or a similar
reference. Enter your location or nearby airport name, and press
“Search.” Select the correct airport from your search results and
click on it. For example, search for “Mountain View” and click on
“Moffet Field.”
6. From the resulting page find the ICAO code, if listed. Otherwise
use the IATA code. For the example above, you would find IATA
code “NUQ.”
7. Now go to a web site like AVIATION WEATHER CENTER (for US
locations) at
https://www.aviationweather.gov/metar?gis=off and
enter the code you found in step 2, and click “Decoded” (to make
the next step easier) before requesting the METAR information.
For the example we would enter “KNUQ” and find a result output
like: “30.09 inches Hg (1019.0 mb) [Sea level pressure:
1019.1 mb]”
8. Go to the calibration settings page and observe the “REL
Barometer value (this is the value we just adjusted in step 4
above). Compare the REL value with the value from the airport. IN
our example, the REL display was 1022.9 where we expected
1019.1. This then tells us that our displayed REL pressure is
1022.9 – 1019.1 = 3.8 hPa different from the reference source.
9. Since we assumed the absolute pressure measured was correct,
and we presumably calculated the elevation related offset
correctly, we must conclude that the absolute pressure was not
correct after all. It appears to be 3.8 too high, so we’ll now enter a
correction of -3.8 in the settings for “ABS Barometer” until it reads
a value 3.8 hPa lower than before. This kind of correction is
entirely normal as during manufacturing small shifts in the
pressure sensor readings can be introduced.
10. For a more precise procedure, locate a very precise barometer
that you can place right next to the console, you would adjust
“ABS Barometer” until the ABS pressure reads identical. You
would then still adjust “REL barometer” until it displays the value
from the reference airport. This procedure would also produce the
correct relative pressure, but due to a precise calibration of the
absolute pressure, it too is correct.
The first procedure above generally works quite well, but for stations at
fairly high altitudes (e.g. 5,000 ft. or higher) it may be more incorrect
than at lower altitudes. In such cases comparisons with other known
correct, and nearby at similar altitude, stations may help.
Now that calibration is complete, feel free to change the pressure units
to whatever you like.
Note: Airport METAR data is often only updated every 10, 15 or even
30 minutes. If you use the information in the procedure above,
you may be looking at pressure data that is out of date by as
much as the update interval. To get best results observe
several times and figure out the update interval and then use
two values for the procedure: one taken immediately after an
update, another taken about halfway through the interval.
Note: It is also a good idea to observe some more after the calibration
procedure is complete to make sure the numbers are correct.