Honeywell 9782 Series manual Example of a Conductivity Loop

Conductivity/Resistivity Analyzer/Controller

10.8Entering Values for Lead Resistance Compensation (Wide Range Only)

Introduction

If you use standard Honeywell cell lead lengths of 7 or 20 feet connected directly to the Analyzer/Controller, no compensation for lead resistance is necessary. Similarly, if a junction box is used to extend the leads up to 150 feet, no compensation is required. However, if longer leads are used (greater than 150 feet), signal quality can be adversely affected unless you enter information that will permit the 9782 to compensate for lead resistance.

If you use a single wire gauge (12, 14, 16, or 18 AWG) in a length up to 1500 feet, simply specify the gauge and length as described in Table 10-9.

If mixed wired gauges are used, or lead length or wire gauge are not within the stated ranges, the 9782 can still perform the compensation. However, you must first calculate the lead resistance, then put it in terms of the available settings for AWG gauge and length.

The resistance of each available gauge choice (in copper wire) is:

12 AWG = 1.6 ohms per 1000 feet

14 AWG = 2.5 ohms per 1000 feet

16 AWG = 4.0 ohms per 1000 feet

18 AWG = 6.4 ohms per 1000 feet

For example, suppose each lead between the cell and Analyzer/Controller consists of 500 feet of 12 gauge wire and 1000 feet of 18 gauge wire.

9782

1000 ft of 18 AWG

Junction

Box

500 ft. of 12 AWG

Honeywell

Conductivity Cell

Figure 10-1 Example of a Conductivity Loop

Because there are two different types of wire used in each lead to the cell in this example, the total lead resistance is calculated as follows:

(2 x 0.5 x 1.6) + (2 x 1 x 6.4) = 14.4 ohms

Since the 9782 only allows entry of one wire gauge type, we allow for the worst-case condition by dividing the total resistance by the resistance per thousand feet of the higher resistance gauge wire. In our example this would be:

14.4 ohms ÷ 6.4 ohms per thousand feet of 18 AWG wire = 2,250 feet

The length to enter is one-half this number, 1125 feet, because the 9782 Analyzer/Controller already accounts for the fact that there is always a pair of conductor wires in the system loop. Therefore, in our example we would use the procedure in Table 10-9, and specify the wire gauge as 18 AWG and the length as 1125 feet.

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