While starting up a new FX-CLv2 chlorine analyzer we sometimes get a call for help from a customer: "I need to return this analyzer. I calibrated, it was running fine, and then the motor stopped. My old FX-1000P didn't do this, what's wrong?"

First we explain the analyzer is working as designed - the No Flow sensor in the measuring cell didn't detect sufficient flow to the measuring cell, so it signaled the processor to turn off the mixing/stirring motor. The buffer feed motor is also turned off during no flow conditions so as not to waste the pH buffer.

It also displays a "No Flow X" alarm on the local touch screen display.

It also energizes the no flow alarm relay, indicated by the LED, to alert you to the flow interruption via your SCADA or remote monitoring system:

After hearing this, the customer will usually say that their old analyzer didn't do this, and they'd like the old style instead. Why is this included anyway? Can we bypass this?

The customer is right, the "old" FX-1000P did not have a no flow sensor; when flow was lost, usually due to a bubble blockage, the motor kept running. The customer wouldn't realize there was a problem until......

Without a continuous supply of water the electrode eventually overheats. The measuring electrode expands due to thermal expansion, the gold tube portion of the electrode then cracks or distorts, causing a water leak between the gold tube and copper post on which it's mounted.

This "short circuit" would cause constant, wildly fluctuating residual readings, prompting a tech support call. After a few questions examination of the electrode would confirm it was destroyed and required replacement, costing over $1,000 dollars.

When we started the re-design of our FX-1000p chlorine analyzer to the digital FX-CLv2, we attempted to resolve or avoid as many issues as possible to make the new analyzer as trouble-free as possible.

We included the optical No Flow sensor in the measuring cell to prevent destruction of the measuring electrode. It works as stated above with an interruption in flow; when flow is restored it turns both motors on, resets the no flow alarm, and displays that flow is restored on the touch screen display with the blue water drop and "FLOW" message.

To the question of "can this sensor be bypassed?", the answer is yes, but you'll first need to sign a waiver to confirm that you want this safety device disabled and will be responsible for the cost of a positive measuring electrode replacement, even during the warranty period.

Their "old analyzer" on the same sample stream probably did experience similar blockages; except that without the sensor and alarm present they didn't know it until the residual dropped to near zero (due to the chlorine being consumed at the electrode and not being replaced) or the electrode was damaged.

So why are the blockages occurring for some customers, or only in certain locations? Usually the flow interruption is caused by a collection of air or gas bubbles that block water entry into the measuring cell. Bubbles can be caused by a pump cycling off and on, but this typically occurs when sampling groundwater with entrained gas or air bubbles. We don't usually see this issue with surface water or wet well monitoring. As the water rises from the well or aquifer, it de-pressurizes, any micro-bubbles present expand in size and collect in the port in the lower block that holds the measuring electrode.

The solution is to use the overflow bypass fitting provided with each new system, or to use the fitting in combination with our flow rotometer.

Many think reagentless chlorine sensors are always the best tools for online monitoring and control of free or total chlorine residual.

They have no moving parts and low consumables cost: just replace the electrolyte every 3-6 months and the membrane cap every year.

With no toxic reagents or buffer solutions, and low maintenance, chlorine sensors seem to be the perfect choice for any water system.

If your water does this to equipment, your "low maintenance" sensor will turn into a cost of ownership nightmare.

Untreated groundwater with elevated iron, solids or calcium levels clogs the micro porous membrane, preventing chlorine from reaching the electrode. In addition to fouling the electrodes, too much iron will prevent 3-electrode sensors from producing an output.

The sensor shown above needed membrane cleaning and electrolyte replacement every 3 days due to excessive iron levels, resulting in:

• Inability to use a new SCADA system to control chemical feed.
• Increased chlorine usage and cost.
• Higher direct labor maintenance cost, and less time to perform other tasks.
• Higher consumables cost.

There's a proper tool to do every job. As you wouldn't use a pipe wrench to replace a spark plug, you don't use a reagentless membrane sensor to measure chlorine in dirty water.

We define dirty water as containing any of the following and thereby being unsuitable for membrane covered chlorine sensors:

• Wastewater
• Potable water with iron, calcium, manganese, turbidity or total dissolved solids above US EPA Drinking Water Standards
• Hydrogen Sulfide
• Corrosion Inhibitors

To avoid selling you something that doesn't work, Foxcroft reviews your application first and recommends the better of two types of instruments that best suit your process.  

Our bare electrode model FX-CLv2 excels in dirty or clean water; and our reagentless chlorine sensor models are for free or total chlorine in filtered, clean water.

Don't assume that the most heavily marketed product is always best for your application. We don't hesitate to recommend probe based systems, if there is a high probability it will work without undue attention and maintenance.