**PARAMETER DERIVATION**

**1.**If the pressure uncertainty of ± 12.5 hPa is not acceptable, calibrate the Barograph by means of parameters

**k**and

**r**. To calculate the calibration parameter

**k**, first the current local Barometric pressure (reference pressure) must be known. This info can be obtained from a Torricelli mercury barometer or from a metereological site on Internet. On Internet normally barometric pressure is given at sea level: to convert it to your altitude H, use the following expression:

Slp = Sea Level Pressure found on Internet, hPa

H = Altitude in metres.

G = 7000 in metres (universal reference parameter)

**Shp**= Pressure at altitude H

Then:

**Shp**= Slp * EXP (-H/G) where the expression EXP means e^(-H/G)

EXAMPLE: Slp = 1015,2 hPa; H = 230 metres;

**Shp**= 1015,2 * EXP(-230/7000) = 982,4 hPa.

**2.**To find calibration parameter

**k**, we proceed as follows.

Using [1] we compute the sensor output voltage if the sensor were calibrated:

**Vo**= Vcc*((0,01067*

**P**)-0,32667) [1]

Then we measure the actual sensor output

**Vo**’: the actual sensor calibration parameter is

**k = Vo/Vo’**

EXAMPLE: suppose our reference pressure is: Shp = 982,4 hPa. Remember that

**P**in [1] is in kPa, then Shp must be divided by 10 and becomes 98,24 kPa. Then:

**Vo**= 5*((0,01067*98,24)-0,32667) = 3,6078 V = 3607,75 mV this is the output voltage the sensor would have if it were accurately on the spot.

Now we turn on the Baro circuit and we read,

**not**3607,75 mV, but

**Vo’**= 3,6403V = 3640,3 mV

Using the inverse function [2] we can find the pressure as read by the sensor:

**P**= ((Vo/Vcc)+0,32667)/(0,01067) [2]

**Po’**= ((3,6403/5)+0,32667)/0,01067)*10 = 988,50 hPa

Thus the static error of this particular sensor is: 988,5 – 982,4 = + 6,1 hPa

Therefore constant

**k = Vo/Vo’**= 3607,75/3640,3 = 0,9911. This correction brings our Barograph on the spot.

**3.**To find parameter

**r**, used to find parameter

**h**, as shown in Part 3, we measure sensor output at different Vcc voltages: 4,8 ; 5,0 ; 5,2 V and we record the sensor output voltages. Then we calculate the average deviation and finally we divide the average deviation by the supply voltage variation, to obtain the sensor

**output voltage**deviation per unit of

**supply voltage**variation: this is parameter

**r**. Using one of the two Excel examples:

Derivation of

**r**for Sensor KP-235 #1:

For Vcc = 4.800 mV ------------Vo = 3.571,4 mV ---------ΔVo- = -149,10 mV

For Vcc = 5.000 mV ------------Vo = 3.720,5 mV ---------ΔVo = 0.00 mV

For Vcc = 5.200 mV ------------Vo = 3.854,7 mV ---------ΔVo+ = +134,20 mV

ABSOLUTE AVERAGE +/-mV = ((ΔVo+) - (ΔVo-))/2 = (134,20 +149,10)/2 = 142 mV

Measured ratiometric error factor:

**r**= 142/200 =

**0,7083**

While the procedure above provides the actual Ratiometric Error, it must be noted that the formula for Ratiometric Error percentage given in § 2.5.1, page 13 of the KP-235 data sheet, is not correct.

At the moment the Barograph is giving excellent performance and is undergoing a one week test (168 hours). Test results will be posted in PART 5.