**THEORY OF OPERATION & PICOLOG SETUP WITH THE 4.6V PRECISION PSU OFF THE USB BUS**

To avoid mistakes and to simplify the setting up it is best to repeat the entire procedure of Part 3, with the relevant changes.

**All calculations are carried out in the Excel file, KP-235 Math 6 (4.6V) which is attached & can be downloaded**. In this file you will also find the following graphs for 4.6V operation: Vo Vs. P; P Vs. Vo; Error Vs. Baropressure and Factor

**r**Vs. Baropressure.

The sensor transfer function is:

Vo = Vcc*((0,01067*P)-0,32667) [1]

The inverse sensor transfer function is:

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

where Vo is sensor output in V, Vcc is supply voltage in V & P is Barometric pressure in kPa and must be multiplied by 10 to obtain hPa.

Description of various DrDAQ channels in Picolog now follows.

1. In EXT. 1 (Channel 1 input) :

**Raw Sensor Output**. This is the cell output, reduced by a factor of 2. To read the original voltage in mV we must multiply by a factor of 2000.

Parameter formatting: mV

Digits width: 3

Decimal positions: 2

Min. Value: 0

Max Value: 6000

Scaling equation is: X*2000

2. In Calculated Parameter:

**Calibrated Sensor Output**. Here we correct the reading of the Raw Sensor Output in two ways:

a) Remembering that the sensor reading may be off to a maximum of ± 12.5 hPa, obtaining the actual Baropressure read from a calibrated barometer or from Internet data and using transfer function [1] we calculate a calibration factor

**k**. With the sensor used this was k = 0,988. The procedure to obtain

**k**is described in previous Part 5.

b) In another (following) Calculated Parameter Channel we shall find the variable factor

**h**used to nullify the Ratiomentric Error.

Therefore:

Variable

**A**: Raw Sensor Output

Variable

**C**: PSU Error

(

**A***0.988)+

**C**where

**C**=

**h**

Parameter formatting: mV

Digits width: 4

Decimal positions: 1

Min. Value: 3000

Max Value: 4500

3. In Calculated Parameter:

**Raw Baropressure**. Here we calculate the uncorrected Barometric pressure from the raw sensor output, using the transfer function [1]. Note that 4600 is nominal voltage in mV and factor 10 transforms kPa into hPa.

Therefore:

Variable

**A**: Raw Sensor Output

(((

**A**/4600)+0.32667)/0.0167)*10

Parameter formatting: hPa

Digits width: 4

Decimal positions: 1

Min. Value: 850

Max Value: 1150

4. In Calculated Parameter:

**Calibrated Baropressure**. Here we calculate corrected Barometric pressure from the calibrated sensor output, using again the transfer function [1]. Note that 4600 is nominal voltage in mV and factor 10 transforms kPa into hPa.

Therefore:

Variable

**A**: Calibrated Sensor Output

(((

**A**/4600)+0.32667)/0.0167)*10

Parameter formatting: hPa

Digits width: 4

Decimal positions: 1

Min. Value: 850

Max. Value: 1150

5. In Calculated Parameter:

**Baropressure Sensor Error**. Here we keep the difference between raw & corrected Baropressures under control. Note that error can be positive or negative. With our sensor is always positive because its raw reading value is above actual value (calibration k = 0.988).

Therefore:

Variable

**A**: Raw Baropressure

Variable

**B**: Calibrated Baropressure

**A**-

**B**

Parameter formatting: hPa

Digits width: 3

Decimal positions: 1

Min. Value: -12.6

Max Value: +12.6

6. In Calculated Parameter:

**Calibrated Baropressure mmHg**. Here we simply convert hPa into mmHg. Any convenient conversion is possible, e.g. inches of Mercury.

Therefore:

Variable

**C**: Calibrated Baropressure

**C**/1.33

Parameter formatting: mmHg

Digits width: 3

Decimal positions: 1

Min. Value: 650

Max Value: 850

7. In EXT. 2 (Channel 2 input) :

**Precision PSU Output**. Here we monitor the 4600 mV output of the Precision PSU off the USB Bus. Voltage is reduced by a factor of 3.0667 to 1250 mV by R18/R19, thus 3067 in the scaling equation.

Parameter formatting: mV

Digits width:5

Decimal positions:1

Min. Value: 4585

Max Value: 5015

Scaling equation is: X*3067

Alarm - : 4590

Alarm + : 4610

8. In Calculated Parameter:

**PSU Error**. This is really the calculation of the Ratiometric Error factor

**h**due to deviations in the Precision PSU output and is calculated from a factor

**r**, now derived mathematically,

**as shown in the Excel file KP-235 Math 6 (4.6V) and in previous Part 6**. Because of the stable Precision PSU voltage, we have restricted the range of Vcc to ± 5 mV, but, as was to be expected, the

**r**equation stays the same and it will correct even if the precision PSU output will exceed the ± 5 mV. This is what one would call a belt & braces approach. As we have seen this

**h**parameter is used in above entry 2, Calibrated Sensor Output.

Therefore:

Variable

**C**: Precision PSU output (Vcc)

Variable

**D**: Raw Baropressure

h = (4600 –

**C**)*

**r**= (4600 –

**C**)*((0.001067*

**D**)-0.3265)

Parameter formatting: mV

Digits width: 2

Decimal positions: 1

Min. Value: -20

Max Value: +20