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 are stable in water and the soil environment over long periods of time. 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.

Passive probe

A passive 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, 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.

Active probe

A new active probe will be available for sale by the end of April 2017, with shipping starting end of May 2017. Inside the active probe, within a few cm from the Pt sensor itself, the redox signal will be amplified and digitized. Almost any small microcontroller board (like an Arduino) should be able to read the digitized signal. Paleo Terra will offer a simple Arduino-like board to act as a basic datalogger, but it will also be possible to connect the active probes to your own microcontroller system.

Amplification of the redox signal inside the redox probe makes it possible to attain a higher input impedance than available on the popular Campbell Scientific CR800/CR1000 research dataloggers (datasheet: 20 GΩ typical) or a redox specialized datalogger as the Hypnos (website: > 1 TΩ). Locating a pre-amplifier within 1 cm of the redox sensing Pt solves several issues that put a practical upper limit of 50-100 GΩ Even if the input stage of a datalogger has > 1 TΩ input resistance, the system input resistance is often compromised at several points. Bare printed circuit board (PCB) may have a resistance between traces of 1 TΩ, but only if the PCB is extremely clean and air humidity is very low. PVC insulated 0.14 mm2 wires of 10 m length have a resistance between wires of 'only' 2 GΩ. The insulation resistance of good terminal block connectors is also specified in the GΩ range.on the input impedance of 'traditional' passive probes connected to high-impedance instrumentation. The input resistance of an active probe will be > 1 TΩ More details will be available by the end of April 2017.

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, so a cable with enough cores must be chosen.

Probe diameters and lengths

The redox probes come in many different lengths. Standard length is 30 cm, but lengths up to 250 cm are 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. It is often economic to limit the length of the actual redox probe, and use the probe with extension pieces.

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. Still, in many cases, longer 8 mm probes are perfectly suitable. 10-mm probes are more or less rigid up to 70 cm length. 12-mm probes have a thicker fiberglass outer wall and are therefore stronger than the 8- and 10-mm probes. 12-mm probes are recommended for difficult soils, where hammering may be necessary to install the probes. For long lengths in difficult soils, 16-mm diameter probes may be required.

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 a fixed 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, the cable may exit the probe from the side, through the addition of a nylon top cap. The top cap allows to hammer on top of a probe. Also, the cable exit to the side may be advantageous if the probe and cable must be buried completely, but as shallow as possible.

Cable type cores diameter description
AWG16 thick PVC sheath 1 3 mm Basic wire for use in the field. Not shielded, do not use in EMI noisy environments. Choose color from blue, green and red.
RG174A/U coax wire 1 3 mm Basic shielded wire. Suitable for laboratory use. Put in conduit if used in the field.
PUR multicore cable 3
5
10
4 mm
4.5 mm
6 mm
PUR sheath, braided shield. Weathering resistant, suitable for outdoor use, preferably in conduit.
Robust PUR multicore 4
8
12
18
6 mm
8 mm
9 mm
13 mm
PUR sheath, very strong and flexible, braided shield. Weathering resistant. Best outdoor option if no conduit is used.

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 after installation.

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. For these cases a PEEK tip can be mounted.

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.

Pricing

Pricing of the redox probes depends not only on the components needed, but also on the complexity of a particular probe. Therefore, the prices below are only a rough indication. Please ask for a quote for your configuration.

Item Price indication
Standard passive probe
8 mm diameter, 30 cm length, 5 mm2 Pt at 2 cm from pointed tip, 3 meter coax cable
€ 41,00
Additional redox sensor (passive probes only) € 20,00 each
Longer probe € 0,25 per cm
Thicker probe diameter a few €
Nylon top cap € 15,00
PEEK tip € 20,00
Additional PVC AWG16 wire € 0,60 per meter
Additional RG174AU coax € 0,80 per meter
Additional PUR cable € 3,00 - € 7,50 per meter
4mm banana plug or BNC connector € 5,00 each

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