TECHNICAL DESCRIPTION 3
During testing a way to better and to simplify at the same time Barometric Pressure compensation vs. Temperature was found. Compensation to cell output voltage has been eliminated and compensation is directly applied to the Barograph mmHG math channel, as shown below.
3.1 BAROGRAPHY MATH CONVERSION EXPRESSIONS
Barometric pressure, after the invention of Torricelli’s Mercury Barometer, is expressed in mmHg (height of Mercury column in mm). To convert mmHg to HPa (Hectopascals) mostly used today:
HPa = mmHg * 1,333 
Barometric pressure (B.P.) is influenced by height above sea level (S.L.). For this reason Barometers and Barographs can be calibrated in two ways: either to read B.P. referred to S.L. regardless of height (Relative Pressure Pr) or to read B.P. at the height above S.L. (Absolute Pressure Pa). All measurements shown in our posts are done at Absolute Pressure at the height of my location, which is 220 m. above S.L. Pressure variation with height is not linear and the conversion expressions are:
Pr = Relative Pressure (Pressure at sea level) - HPa.
Pa = Absolute Pressure (Pressure at height Ha) - HPa.
Ha = height above sea level in metres.
Pa = Pr * (EXP(-Ha/7000)) = Pr * e(-Ha/7000) NOTE: expression following e is the exponent 
Pr = Pa / (EXP(-Ha/7000)) = Pa / e(-Ha/7000) NOTE: expression following e is the exponent 
Expressions ,  and  are easily calculated by an Excel spreadsheet. For example let Ha=140 m; Pr=1011,9 HPa,
Pa = 1011,9 * e(-140/7000) = 1011,9 * e(-0,02) = 1011,9 *0,980199 = 991,9 HPa
3.2 PICOLOG CHANNEL SETUP
Setup information and parameters for all Picolog channels are shown in Table 1 (uploaded).
Channel descriptions are given in logical sequence and not according to Table 1. (REVIEW NOTE: Table 1 has been amended twice to eliminate errors. With this note I am posting VER. 3 which should be O.K.)
CHANNEL 1, Sensor Temperature, is EXT. 1. Scale is 10 – 30°C and the calibration equation (set in “Options”) has the form: X * T, where T is the calibration factor. Before placing the LM35 sensor into the oil column, check reading against a mercury thermometer and find calibration factor. I found: T=99.
CHANNEL 5, Cell Supply Voltage, is EXT. 2. As already explained, this voltage must be accurate and have a long term stability of 100 uV or better. The ideal value is V = 4,990 mV, which can be obtained by trimming the adjust pot on circuit of the LT1021 Voltage Reference IC with the help of an accurate DVM. This voltage is reduced by a factor of 2 by the DrDAQ resistor input network to the value of 2,495 mV: hence the channel equation X*2. Channel scale is 4,900 to 5,000 mV. The absolute value of the supply voltage is not so important, as long as it does not drift with environmental changes and time.
CHANNEL 7, Cell Voltage, is EXT. 3. As written in Post TECHNICAL DESCRIPTION 2, the reference point of Cell Voltage, with dimensions shown, should be 1,000 mV @ 18°C and 730 mmHG or 973.1 HPa. With other dimensions and/or different height and/or different conditions, one could find a pressure different from 730 mmHg, but system must be set up in any case, by finding a given cell voltage at known conditions: it does not matter if the reference voltage is different, as long as conditions (Pressure, Temperature and Height) are known. This operation must be attempted after having allowed the system to settle vertically for a minimum of seven days. Pressure data can be obtained from any local weather forecasting internet site and/or from an accurate mercury barometer. In general sites give updated Pr and Pa values every few minutes. Knowing Ha, Pr data from the internet site can then be easily converted to the local Pa value using .
To set up channel 7, first calibrate voltage value shown by Picolog by comparison with an accurate DVM. In my case the calibration factor is 1,007. By using the expression: X*1007 in Picolog, in my system the cell voltage readings in Picolog and on the DVM are the same. Then all said conditions must be noted.
CHANNEL 4, Ambient Temperature, is the DrDAQ own temperature channel. This channel also must be calibrated against an accurate Mercury thermometer: in my case the correction equation is: X*0.8, as the DrDAQ temp. reading is 20% higher than true. Bear in mind that, in operation, Cell Temperature will always be higher than Ambient Temperature, because the LM35 sensor normally sits 1.5 metres higher than the DrDAQ.
More setup information on next post.
- Table 1, amended VER. 3 - Setup expressions & Data
Last edited by Glovisol
on Mon May 04, 2015 10:35 am, edited 3 times in total.