THEORY OF OPERATION & PICOLOG SETUP

Sorry guys, but before describing the setting up of the Picolog, it is necessary to know the why and the because of what we do in the software…..All calculations are carried out in the Excel file, KP-235 Math 2, which is attached & can be downloaded. 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.

These functions are plotted in the mentioned Excel file. The plot of Vo vs.P, with the tolerance band, is attached.

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 NOTE: corrected on 27/11/2015

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 equations [1] and [2] we calculate a calibration factor k. With sensor # 2 this was k = 0,988.

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*k)+C or (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 5000 is nominal voltage in mV and factor 10 transforms kPa into hPa.

Therefore:

Variable A: Raw Sensor Output

(((A/5000)+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 5000 is nominal voltage in mV and factor 10 transforms kPa into hPa.

Therefore:

Variable A: Calibrated Sensor Output

(((A/5000)+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 5000 mV output of IC102. Voltage is reduced by a factor of 4 to 1250 mV by R102/R102, thus 4000 in the scaling equation.

Parameter formatting: mV

Digits width:5

Decimal positions:1

Min. Value: 4985

Max Value: 5015

Scaling equation is: X*4000

Alarm - : 4990

Alarm + : 5010

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 parameter r, which in turn is unique for any sensor and is easily derived by measuring the output deviation for a given PSU output deviation from 5 V, as shown in the Excel spreadsheet. Two KP-235 sensors were measured (see Excel file): for Sensor #1 r = 0,7083 and for Sensor #2 r = 0,7550. As we have seen this h parameter is used in above entry 2, Calibrated Sensor Output.

Therefore:

Variable C: Precision PSU Output

h = (5000 – C)*r = (5000 – C)*0.755 (r of Sensor #2 is used)

Parameter formatting: mV

Digits width: 2

Decimal positions: 1

Min. Value: -20

Max Value: +20

In Part 4 we shall see the derivation of parameters k and r in detail.