Campbell-scientific 109-L Temperature Probe Manual de usuario descargar pdf (Pagina 18)

Model 109 Temperature Probe

8. Operation

8.1 Sensor Schematic

FIGURE 8-1. 109 Thermistor Probe Schematic

8.2 Measurement and Output Linearization

CRBasic instruction Therm109() measures the 109 probe thermistor and

automatically converts the result to temperature. With reference to the

previous FIGURE 8-1, 109 thermistor probe schematic, a precise excitation

voltage is applied at the Vx line and the voltage drop across the 24.9 kΩ

resistor is measured at the Vs line.

The ratio of measured voltage (Vs) to excitation voltage (Vx) is related to

thermistor resistance (Rs) and the 24.9 kΩ bridge resistor as described in the

following equations:

Vs/Vx = 24900 Ω / (Rs + 24900 Ω)

Solving for Rs:

Rs + 24900 Ω = 24900 Ω • (Vx/Vs)

Rs = 24900 Ω • ((Vx/Vs) – 1)

The relationship of Rs to temperature is tabulated in Appendix C, Thermistor

Resistance and Temperature, but is calculated by Therm109() using the

Steinhart-Hart equation, described as follows:

T = 1 / ((A + (B • ln(Rs))) + C • ((ln(Rs)))^3)) – 273.15

where:

T = temperature in Celsius

= 1.129241E–3

= 2.341077E–4

= 8.775468E–8

Coefficients provided by Measurement Specialties™.

1 2 ... 13 14 15 16 17 18 19 20 21 22 23 ... 29 30

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Campbell-scientific 109-L Temperature Probe Manual de usuario Pagina 18