Campbell Scientific CR10 CR10 Measurement and Control - Page 100

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES The following calculations are based on using a Geokon model 4500 Vibrating Wire sensor. An individual multiplier and offset must be calculated for each sensor used in a system. MULTIPLIER The fundamental equation relating frequency to pressure is P=-FxG+B where P = pressure, PSI G = the Gage Factor obtained from the sensors calibration sheet in PS|/digit. The units of a digit s1g H72119-3;. B = offset Fx=l2Hz2(10'3), where f is frequency. lnstruction 28 measures period, T, of the vibrating wire in milliseconds (ms) and returns a measured value, X, of X = 1/(T2(ms)2) = f2(1 O-6)Hz2 A multiplier of -1000 in Instruction 28 converts the measurement to digits, as shown below. -Fx = -XC103) = -f2(10-3)Hz2 To calculate the multiplier, convert Geokon's gage factor, G, to the desired units (i.e., feet of water per digit) and multiply by -1000. TEMPERATURE CORRECTION The temperature correction is applied as follows. Pt=P+C*(t1 -ts),where Pt = Pressure corrected for temperature, oC C =Temperature coefficient, PSI/"C (from Geokon calibration sheet) t9 & t1 = lnitial and current temperatures, oC. The temperature coefficient, C, must be converted to units compatible with the gage factor, G. 7-16 WELL MONITORING EXAMPLE ln this example the vibrating wire sensor is to monitor water table height (Figure 7.16-2). The desired data is the distance from the lip of the well to the water surface. The sensor is vented to atmosphere to eliminate measurement errors due to changes in barometric pressure. The water level is expected to stay within 40 to 80 feet of the lip the 50 psi pressure sensor is placed approximately 100 feet below the lip of the well. The calibration data from Geokon is provided in Table 7.16-1. TABLE 7.16-1 Calibration Data for Sensor 3998 Gage Factor (psi/digit) Temp. Coeff. (psi/'C) 0.0151 -0.0698 The multiplier, m, is calculated to convert the reading to feet of water. rn = 0.0151 (psi/digit) " 2.3067 (ft of water/psi) . -1000 = -34.831 ft of water/digit After the probe reaches thermal equilibrium, the initialtemperature, to, is measured to be 24"C. The water column above the sensor is referred to as the "Reading". The Reading decreases with increasing "Distance" from lip of wellto water surface so the Distance is computed by subtracting the Reading from the Offset as shown in Figure 7.16-2. The "lnitial Distance" to the water surface is measured with a chalked line to be 47.23 feet below the lip. The first time the program is executed, the program calculates the offset (Offset = Distance + Reading) required to obtain a reading of 47.23 feet. The otfset is stored in Location 4 and applied to subsequent measurements. NOTE: Following program compilation in the "0 Mode, all input locations are set to zero. This fact is utilized to detect the first execution following a program compilation. The example assumes the sensor has been connected as shown here.