Redox probes

Paleo Terra produces sturdy probes to measure redox potential in soils, sediments and surface waters. The probes are built from fiberglass reinforced epoxy tubes and will last in water and the soil environment for many years. The redox sensing Pt is > 99.95% pure. The probe and cable connection are filled and sealed with epoxy to make a robust waterproof whole.

A redox probe basically consists of a Pt sensor embedded in the fiberglass/epoxy probe and connected to a copper wire. The simplest standard probe is 8 mm in diameter, 30 cm long and has one Pt sensor placed at 2 cm above the pointed tip.

The regular size of the Pt is approximately 0.4 mm width by 12.5 mm length, yielding a surface area of 5 mm2. At an 8 mm diameter probe this forms a half ring around the probe.

The distance between the probe tip and a Pt sensor will often be as small as possible. A Pt sensor must be placed at least 2 cm above the pointed tip (3 cm on 12 and 16 mm diameter probes), or 1 cm above a flat tip. In some use cases, some extra probe length below a Pt sensor helps to prevent a probe from falling over, e.g. when a probe is used in a river to measure the redox potential only a few cm into the river bed.
At the top end of the probe, a minimal distance of 10 cm between the Pt sensor and the cable exit is recommended for a durable design. Reducing this distance down to 5 cm is possible, at the cost of a weaker cable connection.

In rare cases, if the probe hits a sharper object (e.g. a piece of gravel, a stone or some metal) under an unfortunate angle, the Pt can be pushed out of position. Although it will often still be possible to measure redox potential, and probes can often be repaired, a probe must be considered lost in some cases.

Multiple sensors

Passive redox probes can be fitted with multiple Pt sensors, at intervals of multiples of 1 cmCustom intervals are possible, with a minimum distance of 3 mm between sensors. A longer lead time and an extra fee per probe design apply.. On 8 mm diameter probes, a maximum of 10 sensors applies. Up to 18 sensors can be mounted on larger diameter probes. Every Pt sensor is connected to its own wire inside the cable.

Probe diameters and lengths

Standard length of the redox probes is 30 cm, but any length between 10 and 250 cm is possible (max. 150 cm for 8 mm diameter). Probes up to 90 cm length can be shipped economically via the regular postal service, longer probes can only be shipped via courier services.

The smallest diameter available (8 mm) is sturdy yet thin enough to push into many soils without the need to pre-auger. The 8 mm probes are most suitable when they can be installed by hand and are rigid up to about 40 cm. Longer 8-mm probes may bend, which can be unfavorable, although not for all use cases. 10-mm probes are more or less rigid up to 70 cm length. The fiberglass reinforcement of 12-mm probes is twice as thick and these are therefore much stronger than the 8- and 10-mm probes. 12-mm probes are recommended for longer lengths, difficult soils, where hammering may be necessary to install the probes, or other hostile circumstances like a riverbed moving cobbles. For extreme use cases, 16-mm diameter probes are also available.

Pre-augering may be necessary to limit the force exerted on a redox probe, but the pro's and cons of augering must be carefully weighed. To establish a good contact between the soil and the redox sensing Pt, it is often best not to pre-auger, or at least pre-auger using a smaller diameter. In sandy soils, augering with casings that will be removed later on to let the sand collapse onto the probes may be appropriate.

Cable options

A wire or cable is an integrated part of the redox probes to guarantee a waterproof design. The standard cable exit is from the top end of the probe like on most regular lab electrodes. Alternatively, a nylon top cap can be added to make the cable exit the probe from the side. This may be advantageous if the probe and cable must be buried completely, but as shallow as possible. The top cap also allows to hammer on top of a probe.

Standard cable on the probes is a PUR wheathering resistant cable. On single sensor probes Tasker RG58 PUR and SAB SD200C, igus CF240.PUR or similar on probes with multiple sensors.

It is recommended to always protect your cables in conduit. If you decide however to not use conduit but instead will leave the cable lying free, floating in the water or buried directly into the soil, consider the heavy duty igus CF2 cable. This is also the best option to pick if you know you may pull on the cable to retrieve the probes. Please note the CF2 cable, although robust, is not designed for direct burial nor for pulling. But if you know you will do either or both, it is the strongest option offered.

For single sensor probes that will be used indoor, or where the wiring is otherwise well protected, it may be possible to mount cheaper PVC cable: AWG16 with thick PVC sheath or RG174A/U coax wire. To guarantee a long term waterproof connection, at least 20 cm of probe above the Pt sensor is needed when choosing a PVC cable.

Standard cable length is 3 meter, but any practical cable length is possible. When determining your required cable length, don't forget to include some cable for routing into an enclosure if applicable.

Other options

The cable wire ends are terminated with ferrules to connect to standard terminals often found on dataloggers. Optionally 4mm banana plugs or BNC connectors can be mounted instead of the ferrules. If a cable is to be fed through conduit during installation, it is probably better to mount any connectors yourself when the cables are in place.

If a probe will be pushed into the soil many times, e.g. when mapping a larger area, the standard fiberglass pointed tip will wear out. A PEEK tip can be mounted on 8, 10 and 12 mm diameter probes to make the tip more durable (but the tip still won't last forever!) With a PEEK tip, the minimal distance between the tip and the lowest Pt is 3 cm for 8 and 10 mm diameter probes and 4 cm for 12 mm diameter probes.

In some cases, depending on probe and cable diameter, it is possible to integrate a thin (4mm) but strong rope. This rope may help to retrieve probes.

Prices

As long as your probe falls within the limits indicated you can automatically get the price by submitting the form below. For orders or prices of more complex probes, please contact me via email.

Probe diameter 8 mm10 mm12 mm16 mm
Probe length 10 .. 150 cm for 8 and 10 mm diameter, 10 .. 200 cm for 12 and 16 mm diameter
Pt sensor position(s) cm from pointed tip. No more than 4 sensors; separate multiple sensors with a comma. 2, 10 means you will have sensors at 2 and 10 cm from the pointed tip. On a probe of 30 cm length completely pushed into the soil this means you will measure at 20 and 28 cm depth.
Cable type
Cable length 1 .. 10 m
Wire termination use ferrules on wire endsadd 4mm banana connector(s)add BNC connector
Options add nylon top capadd PEEK tip

Please note!

A Pt redox sensor is a half-cell. To measure redox potential, a reference electrode and a suitable pH/mV meter or datalogger are needed.

All probes are built to customer specifications. Regular lead time 4 - 6 weeks.

More information

For all further enquiries, please contact:

Sander Smit
paleoterra@xs4all.nl
06 - 81640550