Frequently Asked Questions

Industrial sensors are built for continuous measurement in the process: they're sealed against process fluids, tolerate temperature, pressure and solids, and need far less frequent calibration. Lab electrodes are made for clean samples and frequent calibration. More in industrial vs laboratory sensors.

Yes. The ULTRA, TOUGH and PRECISION series are available as pH or ORP sensors.

Digital Smart Sensors give you our digital signal or an open Modbus RTU signal, and analogue options are also available. Smart Sensors can also connect directly to your PLC or data acquisition system without a traditional transmitter. See our sensor technology.

Tell us about your process and our application engineers will recommend a sensor. Request a quote or use the quote assistant on that page.

A pH sensor measures how acidic or alkaline a solution is, using a glass membrane that responds to hydrogen ions, and reports it on the 0–14 pH scale. An ORP (oxidation-reduction potential, or redox) sensor measures how strongly a solution oxidises or reduces, in millivolts, using a metal sensing element instead of pH glass. Both use the same kind of reference system, which is why our ULTRA, TOUGH and PRECISION series come in either pH or ORP versions. Unlike pH sensors, ORP sensors can be installed at any angle.

Read about our industrial ORP sensors.

Wherever a process depends on oxidation or reduction: disinfection and dechlorination in water and wastewater treatment, cyanide destruction in gold processing, chromate reduction and other metal-finishing treatment, and chemical processes that add an oxidiser or reductant. ORP is often measured alongside pH, because many of these reactions depend on both. Tell us about your process and we'll recommend a sensor.

In most cases, yes. We build sensors with the electronics to work with most major transmitter brands, including Rosemount, Yokogawa, Endress+Hauser, ABB and Foxboro, as long as your transmitter accepts an analogue sensor. This avoids the cost of changing over your instrumentation, and our sensors typically outlast the OEM sensor several times over.

Some transmitter brands place limits on the sensor design. For the best life and performance we recommend a Turtle Tough sensor with a Turtle Tough TX series transmitter. See OEM replacement sensors.

The chemical and physical resistance built into a Turtle Tough sensor adds to its impedance, and many OEM transmitters aren't designed for it. They may reject the sensor early or show error codes while it still has plenty of life left. Turtle Tough transmitters are designed for the higher impedance of our sensors, so you get the sensor's full working life.

No, and that's deliberate. Our success comes from building a sensor for your application. An off-the-shelf copy would ignore that your process needs its own matching chemistry. Our sensors may connect to your transmitter, but the materials and dimensions won't be an exact replica of what you use now. We don't make replicas; we make sensors that work. Tell us about your application and we'll match the build to your process.

If your analyser only accepts a proprietary smart digital sensor, we can't offer a compatible sensor: retrofits need an analogue input. Instead, our own Smart Sensors and hardware can replace the system, or connect directly to your PLC. Talk to your Turtle Tough representative.

Yes. Our Digital Smart Sensors process the measurement on board and output an open Modbus RTU signal, so they can connect directly to your PLC, SCADA or data acquisition system without a traditional transmitter. Add an analyser or controller if you also need a local display, relays or 4-20 mA outputs.

The ULTRA TOUCH analyser, 6088 controller and Digital Smart EX transmitter communicate over Modbus RTU (RS-485). The 6088 controller adds an isolated, scalable 4-20 mA output and 4 programmable relays, and the EX transmitter offers optional 2 or 4 relays with an isolated 4-20 mA output.

Yes. The 6088 controller takes up to 4 Smart Sensors, and the DSS Smart Touchscreen Controller comes in 2, 3 or 6-channel versions that accept any combination of pH, ORP and dissolved oxygen sensors. Both recognise sensors automatically, so sensors can be hot-swapped for maintenance.

Yes. The Digital Smart EX Explosion-Proof Transmitter is rated for Class I, Division 1 and 2 areas (FM and CSA), with ATEX and IECEx approvals and an IP68 enclosure. It works with our SMART Series sensors.

That's the classic sign of a ground loop: the circuit is earthed at two points with different potentials, so current flows through the sensor's reference and shifts the reading, often making it noisy too.

To check: calibrate the sensor in buffer in a container with no electrical connection to the process, then run a heavy-gauge wire from the process liquid (or piping) into the buffer. If the reading changes or turns noisy, you have a ground loop.

Act quickly, as ground loops polarise and eventually destroy the sensor. Remove unintended ground connections, and if you can't, fit a signal isolator with input, output and power isolation. More in what is a ground loop?

Usually stray current or static, not a faulty sensor. pH and ORP sensors measure tiny voltages (just 59 mV per pH unit), so a small current can shift the reading a lot. Common causes are poorly earthed or shielded equipment, plastic pipework and plastic immersion rods, and nearby pumps, motors or variable speed drives.

Install sensors in metal fittings that can be properly earthed (stainless steel immersion rods rather than plastic), earth plastic pipework, and keep sensors as far as possible from high-current equipment. See installation recommendations.

• Inline: every Turtle Tough pH/ORP sensor can be installed inline. Choose twist-lock, sanitary (tri-clamp) or hot-tap fittings rather than front-end threads, because screwing a sensor in and out at every service twists and damages the cable.
• Hot-tap: the only inline method that lets you insert and remove the sensor while the line is live. Twist-lock and sanitary fittings need the line isolated.
• Submersible: select the waterproofing option when ordering, mount the sensor on a stainless steel rod for earthing, and never hang it by its cable unless you've ordered the cable protection option.

Inline sensors carry the temperature element at the tip, and fully submersible ones carry it in the back end, so specify the installation when you order. See installation recommendations.

Ideally at 45°, which stops air bubbles being trapped at the tip, or vertically tip-down. A pH sensor must be at least 15° above horizontal and never installed upside down. Dissolved oxygen sensors should be vertically tip-down. ORP and conductivity sensors work at any angle, but tip-down is still recommended: it avoids spills and stops solids settling in the sensor tip.

It depends on:

• the process conditions, and how quickly the main constituents poison the sensor
• the accuracy you need to keep acceptable process control
• how much fouling or build-up affects the sensor over time
• how well the sensor is cleaned at each interval (using the right chemicals to decontaminate it)

Turtle Tough sensors are designed to stretch these intervals. Their tough measurement elements deteriorate slowly, so drift is minimised. Open-geometry, low-fouling designs reduce build-up. Resilient materials withstand harsh cleaning, so strong acids, alkalis, organic solvents and abrasive mechanical cleaning can return a sensor to optimum condition.

A general guideline only. Establish the drift profile in your own application before setting a calibration interval, as intervals vary significantly between applications. See sensor care and maintenance.

Clean the sensor first, rinse it with deionised water and blot it dry with a lint-free cloth (don't rub, as that builds static). Then:

1. Immerse it in pH 7 buffer, let the reading stabilise, and set the meter to 7.
2. Rinse and blot dry, then use pH 4 (for acidic processes) or pH 10 (for alkaline ones) and set the meter to that value.
3. For a 3-point calibration, rinse again and repeat with the other buffer.
4. Rinse, blot dry and return the sensor to service or storage.

Always let the sensor reach the same temperature as the buffer before calibrating, use fresh buffers, and check accuracy against a known buffer rather than a portable meter. Full procedure: pH sensor calibration procedure.

A 2-point calibration (pH 7 plus pH 4 or 10) is adequate for most general applications. It's quicker, but it assumes a linear response across the whole pH scale.

A 3-point calibration (pH 7, 4 and 10) records the acidic and alkaline slopes separately, which can differ, so it's more accurate when you need high precision or measure across a wide pH range. Not every meter can do it: basic meters are usually limited to 2 points, while smart sensors store separate acid and alkaline slope data on board.

Check the slope and the change in offset after each calibration. A theoretically perfect sensor has a slope of 59 mV per pH unit, and a new sensor typically reads 56–59 mV.

For offset, track the change from new, not the distance from 0 mV. Turtle Tough sensors often have higher offsets from new because of their high-impedance components, and our digital smart sensors handle up to ±250 mV. A change of under 10 mV is as new, 10–20 mV is still good, 20–40 mV shows significant deterioration (still compensated by calibration), 50 mV or more means replacement should be considered, and 100 mV or more points to severe reference damage, often from stray current or chemical attack, which needs fixing straight away.

For critical processes you may choose to replace sensors earlier, for example at a 50 mV slope. Always clean the sensor each time you calibrate.

A pH or ORP sensor is a consumable electrochemical device: chemical exposure gradually erodes the glass and poisons or depletes the reference. Life varies enormously with the application, from weeks or months in demanding processes to months or years in easier ones, and can't be predicted reliably without experience in that process.

Turtle Tough sensors are built from materials matched to the process so they outlast standard sensors, and good cleaning and calibration extend their life further. The best way to know is a trial in your own process. Ask about our "same risk" trial, and see the savings calculator.

1. Store it in a cool, dry place.
2. Keep the conditioning solution in the cap. Every Turtle Tough sensor is supplied with it, or you can mix your own from 50% pH 4 buffer and 50% saturated potassium chloride (KCl).
3. Keep the rubber cap tightly fitted and sealed with PVC tape so the tip stays moist.
4. Store it tip down, so the solution stays in contact with the glass bulb.

Stored this way, a sensor stays in working order for 12 months or more. See how to store a Turtle Tough sensor.

Never scratch or aggressively scrub the platinum ORP element. The solid-state reference junction is tougher: it can be cleaned with aggressive chemicals, or carefully with a sharp razor-edged tool, taking care not to touch the sensing element. Approved cleaning solutions:

• 5–15% hydrochloric acid, for alkaline deposits
• 5–15% sodium hydroxide, for organic contaminants
• A non-ionic surfactant, such as Micro-90
• 10% ammonium bifluoride

Afterwards, rinse the sensor thoroughly with deionised water to remove any cleaning residue. Always follow the safety precautions for each chemical. See sensor care and maintenance for the full procedure.

A Turtle Tough sensor does cost more per unit than an OEM equivalent. Hand-making sensors from materials and designs matched to each application costs more. But the result outlasts an OEM sensor several times over, which saves significantly over its life.

Use our savings calculator to work out what you could save, and ask about our "same risk" trial: for a single trial sensor, you pay the same as you would for your OEM sensor.

We make all three technologies, because each suits different conditions.

• Optical (luminescent): doesn't consume oxygen, so it measures accurately even at very low concentrations. It's approved under ASTM D888-05, uses very little power, and stores its calibration in the sensor for plug-and-play replacement. A good fit for water, wastewater and aquaculture.
• Galvanic: our ULTRA Series galvanic smart sensors are built from tough thermoset plastics for harsh industrial processes where most sensors fail, with on-board diagnostics to simplify calibration.
• Amperometric membrane: our TOUGH M6M and M4M systems, proven for almost three decades in mining. They calibrate in air, hold calibration for 3–4 months, need only annual maintenance in most applications, and the sensors last up to 20 years in clean applications and 5 in harsh mining slurries.

Not sure which suits your process? Ask our engineers.

For a galvanic sensor: clean it, then dry it completely, especially around the cathode. Calibrate in air, suspended over a bucket of water so the air is humid. Wait until the temperature and reading have fully stabilised, and use automatic temperature compensation and the analyser's automatic calibration where available. Afterwards the slope should be between 1.5 and 2.2. Outside that range, the sensor was probably not dry, not at temperature equilibrium, or not rebuilt correctly.

Optical sensors store their calibration on board, so they can be swapped in without recalibrating. See dissolved oxygen troubleshooting.

The most common reason is salinity compensation: most portable meters don't have it, so turn it off on the analyser when comparing against a portable reading. Then check that the temperature is calibrated correctly, as a wrong temperature causes a big discrepancy, and check the slope (it should be 1.5–2.2). See dissolved oxygen troubleshooting.

The TOUGH Dissolved Oxygen Optical Smart DSS3 sensor is designed for water, wastewater treatment and aquaculture: a strong polypropylene body, quick-connect plugs for instant hot-swap, and Modbus RTU output. For rivers, lakes, drinking water and environmental monitoring, the TOUGH optical sensor with a POM and 316 stainless steel body is the usual choice.