Find answers to technical questions about Foxcroft instruments here by browsing these categories.

Chlorine Analyzer FX-CLv2

  • Why did the mixing motor stop?

    There are three possible reasons for the mixing motor to stop:

    1. Insufficient flow was detected by the no flow sensor, causing the analyzer to turn off the both the mixing motor and buffer pump motor.
    2. Excessive friction / binding between the copper negative cell and the mixing paddle, or between the bottom of the paddle and lower block.
    3. Worn out motor.

    To troubleshoot the source of the problem, first see if the no flow alarm indicator is showing a red "X". If so, the pump motor should also have been stopped. This indicates an insufficient flow to the measuring cell.

    If the blue water drop is showing or if the buffer pump is working this indicates a problem within the measuring cell or the mixing motor.

    To verify insufficient flow, measure the flow coming from the left drain hose under the measuring cell, it should be 100-120ml/min. Note that flow from the right overflow drain is immaterial in diagnosing this problem.

    The cause of the no flow condition could be due to bubbles blocking the internal port, or debris. A bubble blockage can usually be broken by inserting a wire down through the flow nozzle and into the port in the lower block.

    Removing the flushing plug while water is running will also dislodge bubbles as well as any debris.

    If  flow is not the issue, the motor stoppage is likely caused by a worn negative cell. The inside diameter of the copper ring enlarges over time due to erosion by chlorine as well as friction. The mixing cleaning balls become lodged between the mixer paddle and negative cell; the increased friction raises current to the motor.

    Our latest current protected motors will shut down if current exceeds 200mA, in older analyzers the inline motor fuse will blow if current exceeds this limit.

    To fix you must replace any worn parts in the measuring cell.

    Motor stoppage no caused by insufficient flow is your clue to perform maintenance on the measuring cell.

    Lastly, if none of the conditions above apply, replace the motor.

  • Why is the residual displayed always 0ppm?

    This rare problem is caused during calibration by entering the zero point as zero and entering the standard, or span calibration value as zero instead of the grab sample analysis result.

    The analyzer is being told that both endpoints of the calibration slope are zero, therefore it outputs zero

  • Why is the residual displayed vastly different from grab sample?

    This is caused by improper calibration, typically we find that a negative slope was entered.

    The software defines the ppm displayed based on a positive slope, the starting point is zero and the end point (A.K.A. span or standard) is a positive number greater than zero.

    NOTE: The slope endpoints are determined by the volts DC (VDC) generated by the measuring electrode, not by the ppm you enter.

    The negative slope is produced if the VDC when you enter 0.00 in zero point calibration is higher than the VDC when the grab sample result is entered. (See our blog post here for more details). In the photo in our blog post shows the zero point is higher than the endpoint of the calibration slope: the VDC was 1.457 when the zero was set, and 0.443 when the span value was entered.

    This is usually caused by not allowing sufficient time for chlorine to be flushed from the system when setting the zero point. For reference the VDC at zero in our factory calibration is always between 0.05 and 0.003VDC.

    In some cases non-chlorine substances in the water generate a relatively high VDC reading of 0.100 or more. If so, set the zero at the lowest observable stable VDC, and perform your span (standard) calibration value at about half of the range, minimum, rather than your typical residual.  Setting the span value at a VDC too close to the zero point VDC will result in erratic readings.

    Pump a higher residual solution from a 5-gallon bucket if you can't alter your system residual. for reference we use a Little Giant model NK-2, item 527003.


  • How do I scale the 4-20 mA output?

    You can't, and it's unnecessary.

    We use a higher quality, more accurate chip than most to produce the mA output. In initial development testing we found the output to be accurate and stable to 5 digits to the right of the decimal point (ie. 5.99999), whereas most SCADA systems use 2  significant digits to the right of the decimal for mA input.

  • How often should I change the negative cell and positive electrode?

    The negative cell wall will erode as a function of residual and the aggressiveness of your water.

    In typical drinking water residuals the negative cell should last 1 to 1-1/2 years. Higher residuals of 5-10pm in some wastewater treatment will require replacement every 6 months.

    The positive electrode is rarely if ever replaced unless it is punctured or cracked.

Support Services

  • What type of support do you offer?

    Support is provided by phone or email.

    We no longer provide field service by a factory technician.

    Field installation and repair services are provided by authorized representatives where available.

  • Do you offer trial systems?

    We offer free trials of the model FX-CLv2 bare electrode chlorine analyzer in which you only have to pay shipping charges. Please contact us for more information.

    Trial systems using fluoride, chlorine and pH/ORP sensors are generally not available. It is cost prohibitive to offer a new sensor for each prospective customer to test.

  • Do you accept credit cards?
    Yes we accept credit cards as well as purchasing cards issued by municipalities.

Fluoride Analyzers

  • Readings fluctuate when I touch the sensor or cable, why?
    Our fluoride sensors have a high impedance, touching the sensor or touching and moving the cable can induce a static charge that will temporarily disrupt the reading. You should avoid touching the sensor during operation and calibration
  • How long do sensors last?

    Our customers measuring fluoride in drinking water get 4 to 5 years of service from a single sensor, without any maintenance beside cleaning if necessary.

    Life span of acid service sensors varies by application and averages 1 to 2 years or slightly more.

  • Can I use the FX-300-F for reporting?

    No. While the same ion selective electrode technology is used, the sample is not conditioned with ionic strength adjusting buffers as specified by standard methods.

    The analyzer is intended for process monitoring and control.

Gas Detection

  • Why does my detector alarm with no gas leak?

    The two most common causes are:

    1. The zero point shifted. This occurs over time and in response to large temperature changes.

    2. The sensor is reacting to a different, non-target gas. One of our customer's chlorine detector reacted to fluoride chemical stored in the room.

    Click here for instructions to reset the zero point and general information.

  • How do I adjust the zero point?
    Click here for instructions on how to adjust the zero point, as well as general information on electrochemical gas sensors.
  • How does humidity affect the sensor?

    Gas sensors are relatively unaffected by humidity if conditions are not condensing within a range of 15% to 90% RH. The sensor will show a transient response to rapid changes in humidity which should go away after 30 seconds.

    The gas sensor includes an aqueous electrolyte and a porous diffusion barrier. The means the sensor can both absorb water from the atmosphere and dry out. At continuous operation at high temperatures and 90%-100% RH the sensor can become prone to leakage as the free space in the sensor slowly fills with water. The sensor can gradually be restored to balance without permanent damage by exposure to lower relative humidity.

    Likewise, continuous operation at 0-15%RH will cause the sensor to dry out, which can cause the acid electrolyte to attack the seals. This occurs if the volume of electrolyte decreases by more than 40%. If not left in this condition too long the sensor can be restored by exposing the sensor to RH humidity above 15%.

  • How does temperature affect the sensor?

    Both the baseline (zero point) and output span are affected by temperature. The baseline approximately doubles with every 10°C increase in temperature. The output span will increase slightly up to about 10% before leveling off with gradual increase in temperature. A transient spike can occur with rapid changes in temperature; this should go away after 30 seconds.

  • Why does the sensor drift?

    Over time all electrochemical gas sensors will experience a shift of the zero, or reference point. When this happens the gas concentration readings will also shift accordingly, producing inaccurate readings or alarming at incorrect levels.

    Note that sensor responsiveness will vary with environmental conditions.

    Typically gas exposure refers to the target gas, however exposure to interfering gases can cause similar effects as target gas exposure.

    Causes of gas sensor zero point drift include:

    • Chemical degradation of the sensor over time.
    • Temperature, affecting both the span and zero point
    • Repeated use in extreme high or low temperature or humidity conditions, or environments with high levels of airborne particles. Temperatures above the rated limit stress the seals, which will cause electrolyte leakage.
    • Exposure to high levels of gas, or exposure to concentrations that exceed the range of the sensor.
    • Low level gas exposure for extended periods of time.
    • Continuous exposure to solvent vapors or highly corrosive gases.
    • Rough handling or jolting of the sensor and electronics

    Exposure to these conditions may damage the sensor to the point that it can no longer operate properly over its full range or be able to calibrate properly.


Have another question?

Feel free to send us a message or call us toll free at 800-874-0590.