Welcome back to Foxcroft.com Blog, this is the second part of the blog that was posted on 5/20/11.

      The cell electrodes are connected to the amplifier section of the electronics, where the low level amperage is boosted and ranged to a usable signal. Specifically, 0 to 5 volts DC. The 0 to 5 VDC is then input to and isolation amplifier, which provides electrical isolation and an electrical safety barrier. The 0 to 5 VDC output of the isolation amplifier is then doubled to 0 to 10 VDC, which is used in the final 3 output stages of the circuit. These are the LED display, the alarm circuit and the 4-20 milliamp DC output signal. The amplifier board is powered by a dual bi-polar regulated DC power supply, which provides two sets of +12 and -12 volts. The LED display provides an instantaneous numerical reading of the chlorine residual in parts per million.The alarm circuit provides two user adjustable alarm level settings that activate two relay outputs, which can be used to control external devices or alarm annunciators. The FX-1000p amperometeric chlorine analyzer has been design with process control applications in mind, and as such, is an excellent choice for use in process control of chlorine residuals in freshwater, wastewater, salt water and food processing.

      Amplifier board calibration is done at the factory, with a default range of: 0 to 5 ppm. The unit can easily be re-ranged in the field. The analyzer can be ranged anywhere from a low of 0 to 0.5 ppm to a high of 0 to 60 ppm. On-site standardization (chlorine residual calibration) is done when the unit is commissioned, and thereafter needed or desired, using an accurate chlorine residual titrator (or test kit), and chlorinated and non-chlorinated sample of the process waters being analyzed.

     If you would like more information or a quote please click on the following link:

     Foxcroft Equipment & Service Company's Amperometric Chlorine Analyzer's residual reading is determined by measuring the amperage produced by oxidized chlorine in the measuring cell. The amperage level is directly proportional to the level of free chlorine available in the sample stream. When reading total chlorine, the unit is actually reading converted free iodine from potassium iodine that is added to the cell for Total Chlorine readings.

     The cell itself is constructed such that the sample that runs through it does so at a continuous and fixed rate. Any additional flow is allowed to overflow to waste from a drain below the overflow weir. The sample stream runs down though the lower block, where it mixes with vinegar (and Potassium iodide where Total Chlorine is being analyzed), and then flows upward into the measuring cell. Within the cell, the vinegar provides pH buffering to a value of 4.0, and aids in keeping the cell clean. If Potassium Iodide is being used to read Total Chlorine, it chemically converts any free and combined chlorine residuals into free iodine, which is then read as a total chlorine reading. The cell utilizes a mixer and 150 pvc balls to ensure even chemical mix, clean the electrode and consistent readings. Very pure grades of gold and copper are used in the construction of the electrodes to enhance signal strength. Sample flow leaves the cell from the top and runs out a secondary drain to waste.

See part II to follow

Written by: Gavin McCulloch

This is a commonly asked question, as is “when do I need to recalibrate my analyzer?”

 The FX-CL series amperometric chlorine residual analyzer is designed to operate continuously, 24 hours a day, 365 days a year. The system requires little routine maintenance other than changing the vinegar buffering agent bottle as it becomes depleted and occasional calibration checks.  

 As part of a standard quality control procedure, we recommend the analyzer readings be verified using an accurate chlorine residual test instrument once every 7 days. In many applications, using a portable DPD colorimetric residual analyzer for routine calibration checks is sufficient to meet reporting requirements. For the utmost precision or in critical applications we recommend comparison with an amperometric titrator.

Calibration is typically done on an as needed basis, or at the minimum frequency specified by regulatory requirements. Barring any changes to the process or analyzer, the analyzer should be calibrated at least once per year.

You Need to Calibrate When:

If any of the following situations occur you should first verify residual measurements, make any required corrective measures, and recalibrate the analyzer.

• If readings are not within ± 0.1 mg/L or ± 15% of a routine grab sample measurement.

• If there is a change in sample flow rate to the analyzer.

• If readings gradually drift up or down and there have been no apparent changes to the process.

• If readings suddenly or unexpectedly change by a large amount. (NOTE: if the residual drops dramatically or to zero, first make sure you have sample flowing into the measuring cell, as evidenced by a discharge from the left drain under the measuring cell).

• If the analyzer has been off line long enough to discolor or oxidize the copper negative cell.

• If there is a change in the sample or if the analyzer is sampling a different source.

• If the measuring range is changed.

• After performing maintenance.

Calibration Guidelines:

You cannot simply enter calibration values with the analyzer running dry, with values that you expect to see, or with values from previous calibrations.

Calibration is always performed using water from your process.

Always set the zero point first.

When calibrating the zero point you must use dechlorinated process water, not distilled or deionized water.

Allow the measuring cell time to stabilize after switching water sources during calibration.

The standard, or span calibration value should be determined at about half of the analyzer's operating range.

Allow the measuring cell to stabilize to the process for 2-24 hours upon initial startup, after returning an offline analyzer to service, or after replacing the negative cell.

In an effort to conserve water more of our customers are asking about reducing the sample flow rate into their amperometric chlorine residual analyzers.

Discussions with customers sometimes reveal that the sample flow is turned down so far that the water is draining from the sample cell in drops or a trickle.

There are two major problems this causes:

1. The gold positive electrode can be damaged. Water flowing through the sample cell cools the electrode. Without sufficient flow the electrode overheats, deforms, and stops functioning properly.

2. You will get unreliable readings with any amperometric chlorine residual analyzer if a minimum consistent flow isn't maintained because chlorine is consumed at the electrode and must be replaced.

Also bear in mind that to measure the same residual as your process it's important to send the sample to the analyzer as quickly as possible, especially when using the analyzer to control chlorine feed.

To see how the recommended flow from the (left) measuring cell drain should look please check out the video below. A blue "PA Rural Water" pen is placed next to the drain for size reference.

You may notice the overflow from the drain on the right is less than the drain from the measuring cell on the left. That's OK, maintaining the measuring cell drain flow is more important.

For those who prefer numbers, the recommended sample flow into a standard analyzer is 500 ml/min. The minimum sample flow into a standard analyzer is 250 ml/min. The minimum flow rate from the measuring cell drain is 130ml/min.

If you must minimize water usage we recommend using the FX-1000P-RM flow rotometer. With this unit you bypass the overflow weir and feed the sample into the measuring cell. By using the FX-1000P-RM's pressure and flow control, flow into the analyzer can be reduced to 2-1/2 to 3 GPH (157 to 189 ml/min).

Flow consistency is just as important as flow rate. Amperometric chlorine analyzers are flow sensitive. If you reduce the flow to your analyzer and make no other changes to your system the residual will decrease as well.

So if you decide to reduce the flow to your analyzer, don't set it below the minimum recommended flow rate and don't forget to recalibrate!