Sensor Care

Sensor Care and Maintenance

How often do I need to clean and calibrate my sensor?

The most common question we are asked is how often must you clean and calibrate a Turtle Tough sensor. While this question is virtually impossible to answer, a Turtle Tough sensor will have up to 10 times the stability of mainstream sensors. Most of our customers can extend calibration intervals by 2 to 5 times, whilst still maintaining their required accuracy and performance.

How often a sensor requires cleaning and calibration depends upon:

  • The process conditions and how quickly the major constituents are poisoning the sensor
  • The desired accuracy for the measurement to maintain acceptable process control
  • The amount of fouling or process build-up that will eventually affect sensor performance
  • How well the sensor is cleaned and maintained at the specified interval (ie. using the correct chemicals to decontaminate the sensor)

Turtle Tough sensors are specifically designed to improve these maintenance requirements by incorporating the following:

  • Turtle Tough sensors have very tough measurement elements that are slow to deteriorate and therefore drift is minimised
  • Turtle Tough sensors utilised open geometry designs and low fouling materials to reduce build-up and extend cleaning intervals
  • Turtle Tough sensors have very resilient materials of construction to withstand harsh cleaning regimes. Strong acids, alkaline, organic solvents and abrasive mechanical cleaning can be deployed to efficiently and effectively clean sensors returning them to optimum operating condition 

 

Cleaning and Calibration Frequency Guideline

 

Drift (pH units)TOUGH APPLICATION (ie: intensive chemical process)MODERATE (ie wastewater)EASY (ie clean water)

0.1

3x per week

1x per week

1x per 2 weeks

0.3

1x per week

1x per 2 weeks

1x per 2 months

0.5

1x per 2 weeks

1x per 4 weeks

1x per 6 months

 *Please note this is a general guideline only and should not be relied upon for accurate process control.

 

How do I determine the calibration frequency?

It is important to note that you can never rely on our guideline as to the basis for your calibration requirement. Every chemical process is unique and the accuracy required can only be determined by your site requirements and expert process control personnel. Each user will need to establish a drift profile on their own application before deciding on an appropriate calibration interval. Cleaning and calibration intervals can vary significantly across individual applications. To determine the drift characteristics you will need to periodically test the sensor against a known buffer or accurate grab sample to determine the rate of drift (ie the difference between the displayed value and the standardised solution). Test frequently at the half-life of your expected calibration frequency to determine how far the sensor has deviated from the standard solution. Once the sensor reaches the threshold of your accuracy requirement, that is a good indicator for establishing your calibration regime. It is recommended that a sensor is properly cleaned each time it is removed for calibration. Even if it does not appear dirty, you should following the cleaning process to remove microscopic contamination that will affect the performance of the sensor.

The recommendations given in this document are valid for most Turtle Tough pH and ORP sensors. Care and maintenance for your particular sensor may vary from that described here. Contact the factory for specific information regarding proper care and maintenance of your particular pH or ORP sensor for a given installation and application.

 

Maintenance Free

Turtle Tough Digital pH and ORP sensors are a completely sealed assembly and are sealed for life. This provides extreme process resistance as well as a maintenance-free sensor which greatly increases sensor life and reduces labour costs. A Turtle Tough sensor does not contain any o-rings, washers, gaskets, or serviceable components/assemblies. It does not require refreshing or refilling of electrolyte. The only requirement is regular cleaning following our recommended cleaning schedule for your application. Regular cleaning will prolong the life of your sensor and ensure hassle-free operation.

 

Cleaning

Cleaning requirements will vary depending on the application for which the sensor is used. The following should always be observed when cleaning:

  • Never scratch or aggressively scrub the pH or ORP (sensing) elements. These are delicate glass electrochemical electrodes. They can be easily broken by
  • mechanical force.
  • The reference junction is a solid-state material and can be cleaned with aggressive chemicals. See the list below of recommended cleaning solutions. This solid-state reference can also be cleaned effectively by using a sharp razor-edged tool.
  • Great care should be taken not to scratch the pH glass or ORP element during cleaning of the reference junction.

Common approved cleaning solutions include:

  • 5-15% Hydrochloric Acid – (For Alkaline deposits)
  • 5-15% Sodium Hydroxide – (For Organic Contaminants)
  • Surfactant (Non-ionic soaps such as micro-90)
  • 10% Ammonium Bifluoride

Cleaning of Reference Junction in Presence of Oils & Fats
Cleaning the reference is best accomplished in a mechanical way by the use of a suitable straight-edge razor. You want to scrape clean the surface of the reference junction taking care not to touch nor scratch the pH glass. This is possible because of our unique solid-state reference technology.

Cleaning of pH Glass in Presence of Oils & Fats
There are two main ways to remove oily/fatty type build-up. The first method is the more mild way and should be tried first.

  1. The first method is to use a dye and fragrance-free surfactant. We recommend using the MICRO-90 cleaner. This is a very effective means to remove such build-up without dehydrating the pH glass element.
  2. The second method is to use NaOH solution to chemically cleave the bonds in the oil/fat. This might be required if the first method of using just a surfactant is insufficient, either due to the nature of the build-up or the extent of the build-up. The NaOH solution is very effective at this task. The use of this kind of strong caustic cleaning solution will, however, dehydrate the pH glass and necessitate some reconditioning time with HCl acid. This will also neutralize any excess NaOH on the sensor. This should also be followed by final conditioning in the conditioning solution before recalibration.

 

Removing Silicate Contamination

In order to remove silicate contamination, you will need to use a strong acidified ABF cleaning solution. It is recommended that 10% ammonium bifluoride (ABF) is sufficiently acidified with hydrochloric acid (HCl) such that it removes the contamination from the surface. The amount of activation needed (that is to say how much acid is added to the 10% ABF stock solution just prior to cleaning) will depend upon the extent of the silicate fouling on the sensor as well as the frequency with which the cleaning is performed. This silicate contamination cannot be removed without this strong activated ABF cleaning because it is bound to the surface of the pH glass and reference. Using the strong acidified ABF cleaning solution the deposited silicates will become soluble and much more easily removed from the sensor tip. The Turtle Tough Ultra pH sensor with High HF resistant glass is one of the few in the world able to withstand this high HF cleaning regimen itself. The pH glass for this type of cleaning service will have a rather high impedance to ensure sufficient integrity and longevity.

WARNING: this must only be attempted if the sensor is fitted with the High HF resistant glass option.
IMPORTANT SAFETY: The above cleaning methods may involve the use of hazardous materials. The above recommendations do not purport to address all the safety measures required.

 

Conditioning for Calibration

After the sensor has been cleaned, it must be thoroughly rinsed with deionised water to remove any residual cleaning reagents. The sensor can then be soaked in pH 4 buffer to recondition the pH and reference elements. Some sensors will also require conditioning in saturated potassium chloride if the reference junction has been depleted of the ions in the solid-state conductive polymer (typical for clean water applications). Condition the sensor in saturated potassium chloride and/or pH 4 buffer for whatever period of time is required to achieve optimal calibration results.

 

Sensor Selection for Individual Process Lines

No sensor should be used beyond the indicated temperature and pressure limitations for that given sensor. Sensors should only be used for the application(s) that an authorised TurtleTough representative has recommended. If you are unsure that your sensor is recommended for a particular application, please contact the factory.

 

Avoiding Thermal Shock

For high-temperature applications where process liquid exceeds 70°C, you will prolong the life of the sensor by avoiding thermal shock. Thermal shock occurs when you rapidly change the temperature of the sensor from hot to cold or vice versa. This rapid expansion/contraction of sensor components can damage internal elements and cause micro-cracking that will accelerate the rate of deterioration. In extreme cases, it will crack the sensing element causing a total failure. To avoid this the sensor should be heated or cooled slowly during removal/insertion and cleaning and calibration processes.

TIPS: Always allow sensors to air cool back to ambient before inserting into a cold liquid (i.e. such as buffer or cleaning solution). When heating up the sensor, wherever possible, gradually heat the sensor.  If this is not possible in the process, heat the sensor in 2 stages prior to inserting the sensor in the process. One such method is placing the sensor in hot tap water (i.e.at 50-60 deg C) first and allowing it to get up to temperature, prior to inserting it into a hot process. This will greatly reduce the thermal shock and extend the life of the sensor.

 

Storage

The standard shelf life for all Turtle Tough pH and ORP sensors is one year from the date of shipment. Sensors stored longer than this period may still be functional but are no longer under warranty. Sensors should be stored in a cool, dry location with the sensor tip (where the pH/ORP element is located) oriented toward the ground. All sensors come standard with a conditioning solution in the cap. This conditioning solution is 50% pH 4 buffer and 50% saturated potassium chloride (mixed by volume). The sensor cap should be kept tightly affixed to the sensor body and sealed with common piping Teflon or PVC tape when the sensor is not in use. Sensors that are to be returned for a shelf-life warranty claim must have the original sensor cap and conditioning solution intact to be eligible for warranty replacement. Contact Turtle Tough factory before returning any sensor for warranty claim to obtain a valid RMA.

You may need to store your pH or ORP sensor if you don’t need to use it right away, or you are using it intermittently. Follow the storage steps below to ensure your sensor’s longevity.

STEP 1
Always store pH or ORP sensors in a cool, dry location.

STEP 2
All Turtle Tough sensors are delivered with our conditioning solution inside the cap. If you are storing the sensor, this conditioning solution must be present. Contact us if you need to purchase some or make your own by mixing 50% pH 4 buffer with 50% saturates potassium chloride. KCl is the same electrolyte used in our reference systems.

STEP 3
Sensors should always be stored with their rubber cap tightly affixed and sealed Teflon or PVC tape to ensure that the tip of the sensor remains moist.

STEP 4
The orientation of the sensor is of utmost importance. Always store with the sensor tip (sensing element) pointing downwards so the at the silver chloride solution stays in contact with the glass bulb, keeping it hydrated.

By following these simple steps, you will ensure that your Turtle Tough sensor remains in working order for 12 months or more.