SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES

FIGURE 7.8-1. Wiring Diagram for Rain Gage with Long Leads
7.8TIPPING BUCKET RAIN GAGE WITH LONG LEADS

A tipping bucket rain gage is measured with the Pulse Count Instruction configured for Switch Closure. Counts from long intervals will be used, as the final output desired is total rainfall (obtained with Instruction 72, Totalize). If counts from long intervals were discarded, less rainfall would be recorded than was actually measured by the gage (assuming there were counts in the long intervals). Output is desired in millimeters of precipitation. The gage is calibrated for a 0.01 inch tip, therefore, a multiplier of 0.254 is used.

In a long cable there is appreciable capacitance between the lines. The capacitance is discharged across the switch when it closes. In addition to shortening switch life, a transient may be induced in other wires packaged with the rain gage leads each time the switch closes. The 100 ohm resistor protects the switch from arcing and the associated transient from occurring, and should be included any time leads longer than 100 feet are used with a switch closure.

 

 

PROGRAM

01:

P3

Pulse

01:

1

Rep

02:1 Pulse Input Chan

03:

2

Switch closure

04:11 Loc [:RAIN mm ]

05:

0.254

Mult

06:

0

Offset

7.9100 OHM PRT IN 4 WIRE HALF BRIDGE

Instruction 9 is the best choice for accuracy where the Platinum Resistance Thermometer (PRT) is separated from other bridge completion resistors by a lead length having more than a few thousandths of an ohm

7-6

resistance. In this example, it is desired to measure a temperature in the range of -10 to

40°C. The length of the cable from the CR10 to the PRT is 500 feet.

Figure 7.9-1 shows the circuit used to measure the PRT. The 10 kohm resistor allows the use of a high excitation voltage and low voltage ranges on the measurements. This insures that noise in the excitation does not have an effect on signal noise. Because the fixed resistor (Rf) and the PRT (Rs) have approximately the same resistance, the differential measurement of the voltage drop across the PRT can be made on the same range as the differential measurement of the voltage drop across Rf.

If the voltage drop across the PRT (V2) is kept under 50mV, self heating of the PRT should be less than 0.001°C in still air. The best resolution is obtained when the excitation voltage is large enough to cause the signal voltage to fill the measurement voltage range. The resolution of this measurement on the 25mV range is +0.04°C. The voltage drop across the PRT is equal to Vx multiplied by the ratio of Rs to the total resistance, and is greatest when Rs is greatest (Rs=115.54 ohms at 40°C). To find the maximum excitation voltage that can be used, we assume V2 equal to 25 mV and use Ohm's Law to solve for the resulting current, I.

I = 25mV/Rs = 25mV/115.54 ohms = 0.216 mA Next solve for Vx:

Vx = I(R1+Rs+Rf) = 2.21V

If the actual resistances were the nominal values, the CR10 would not over range with Vx = 2.2 V. To allow for the tolerances in the actual resistances, it is decided to set Vx equal to 2.1 volts (e.g., if the 10 kohms resistor is 5% low, then Rs/(R1+Rs+Rf)=115.54/9715.54, and Vx must be 2.102V to keep Vs less than 25mV).