Campbell Manufacturing CR10 7.11100 OHM PRT IN 4 WIRE FULL BRIDGE, SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES, FIGURE 7.11-1.Full Bridge Schematic for 100 ohm PRT

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES

FIGURE 7.11-1. Full Bridge Schematic for 100 ohm PRT

7.11100 OHM PRT IN 4 WIRE FULL BRIDGE

This example describes obtaining the temperature from a 100 ohm PRT in a 4 wire full bridge (Instruction 6). The temperature being measured is in a constant temperature bath and is to be used as the input for a control algorithm. The PRT in this case does not adhere to the DIN standard (alpha = 0.00385) used in the temperature calculating Instruction

16.Alpha is defined as ((R100/R0)-1)/100, where R100 and R0 are the resistances of the PRT at 100°C and 0°C, respectively. In this PRT alpha is equal to 0.00392.

The result given by Instruction 6 (X) is 1000

Vs/Vx (where Vs is the measured bridge output voltage, and Vx is the excitation voltage) which is:

X = 1000 (Rs/(Rs+R1)-R3/(R2+R3))

The resistance of the PRT (Rs) is calculated with the Bridge Transform Instruction 59:

Rs = R1 X'/(1-X')

Where

X' = X/1000 + R3/(R2+R3)

Thus, to obtain the value Rs/R0, (R0 = Rs @

0°C) for the temperature calculating Instruction 16, the multiplier and offset used in Instruction

6are 0.001 and R3/(R2+R3), respectively. The multiplier used in Instruction 59 to obtain Rs/R0 is R1/R0 (5000/100 = 50).

It is desired to control the temperature bath at 50°C with as little variation as possible. High resolution is needed so that the control algorithm will be able to respond to minute changes in temperature. The highest resolution

is obtained when the temperature range results in an output voltage (Vs) range which fills the measurement range selected in Instruction 6. The full bridge configuration allows the bridge to be balanced (Vs = 0V) at or near the control temperature. Thus, the output voltage can go both positive and negative as the bath temperature changes, allowing the full use of the measurement range.

The resistance of the PRT is approximately

119.7ohms at 50°C. The 120 ohm fixed resistor balances the bridge at approximately

51°C. The output voltage is:

Vs = Vx [Rs/(Rs+R1) - R3/(R2+R3)] = Vx [Rs/(Rs+5000) - 0.023438]

The temperature range to be covered is ±50

±10°C. At 40°C Rs is approximately 115.8 ohms, or:

Vs = -802.24x10-6 Vx

Even with an excitation voltage (Vx) equal to 2500 mV, Vs can be measured on the +2.5 mV scale (40°C = 115.8 ohms = -2.006 mV, 60°C = 123.6 ohms = 1.714 mV). There is a change of approximately 2 mV from the output at 40°C to the output at 51°C, or 181 µV/°C. With a resolution of

0.33µV on the 2.5 mV range, this means that the temperature resolution is 0.0018°C.

The 5 ppm per °C temperature coefficient of the fixed resistors was chosen so that their 0.01% accuracy tolerance would hold over the desired temperature range.

The relationship between temperature and PRT resistance is slightly nonlinear one. Instruction 16 computes this relationship for a DIN standard PRT where the nominal temperature

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