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Hot Hands at Wairaki

Hot Hands at Wairakei

Hot hands and corroded temperature probes were the bane of a Wairakei company’s testing routine.  Seismograph-monitoring medium
Teltherm noticed their equipment was in a state so they initiated a call to the end user.
The team discovered these probes were being used to measure the temperature in geothermal areas where the steam is really hot & the minerals are quite corrosive.
The Wairakei team also mentioned they weren’t enjoying getting their hands so close to the action either!
Teltherm created a long probe with a handle that was attached to a longer lead.
The lead was attached to a hand held digital readout that was encased in a waterproof bag.
A simple solution and the guys at Wairakei are finding the job a lot easier these days.

It can be difficult deciding what temperature sensor is right for you and your process and what is available out there in the market place.
In the Wairakei case we used a thermocouple – they are affordable and robust and suit numerous applications including the food and beverage sector.
Choosing a Temperature Probe
The common sensors available are thermocouples, resistance thermometers and thermistors.
For this particular article we will concentrate on thermocouples.

probeThermocouples can operate over a wide range of temperatures so your choice may be governed by the temperature you wish to measure. Potters for example may choose Type K or R as they work well at high temperatures.
Thermocouples are made up of differing types of wire and may or may not be contained in a sheath.
A handle and lead may be added & also a fitting that can be plugged into a read out device or a controller.

The purity of the metal is important as pure metals will result in better quality & more accurate thermocouples – you get what you pay for so don’t be fooled by cheap products.
Choosing the Right Thermocouple Type
The table below outlines some types of thermocouples and what they are made of. For best accuracy choose the thermocouple type that suits the temperature range of your process.
Note: Type K is widely used in NZ because of its affordability and manufacturer's often specify their Type K probes can be used from -100 /1200 °C and they can - however Type K thermocouples will generally perform better in the range of 95/1260 °C.
The most common thermocouple types, K, J, E, T, and N, are a good place to start when selecting a thermocouple.
Type Conductors - Positive Conductors - Negative Range °C
E Nickel-chromium alloy Copper-nickel alloy 95-760
J Iron Copper-nickel alloy 95-760
K Nickel-chromium alloy Nickel-aluminum alloy 95-1260
N Nickel-chromium-silicon alloy Nickel-silicon-magnesium alloy 95-1260
R Platinum—13% rhodium Platinum 870-1450
S Platinum—10% rhodium Platinum 980-1450
T Copper Copper-nickel alloy 0-350

You should select your thermocouple type based on two basic parameters:
  • Temperature range
  • Application environment e.g. chemical, abrasion, vibration, installation requirements.

Below are some general guidelines to consider when choosing a thermocouple – if you can gather this information it will help you choose the best device.
  • What are the maximum and minimum temperatures I need to measure?
  • How much can I spend?
  • What error tolerances are allowed?
  • What is the process atmosphere – is it corrosive?
  • How long does it need to last?
  • How fast does it need to read the temperature?
  • Will it be used full time or now & again?
  • Will it be exposed to bending or flexing during it's life?
  • How deep does it need to be immersed?
The Way the Thermocouple is Made is Important

Choose the correct construction to best suit your requirements.

Exposed Junction
The thermocouple wires are welded together and the insulation is sealed against penetration by liquid or gas. This type of thermocouple is the fastest to respond to temperature change but is the least robust and therefore is best suited to ambient atmospheres. E.g.I t would not suit the rigors of a wash down facility at a processing plant.

Grounded Junction
The thermocouple wires are welded together with (grounded to) the sheath. This provides more protection, and is used in cases of harsher atmospheric conditions, or when longer life is required.

Ungrounded Junction
The thermocouple wires are welded together and are insulated from the sheath. This provides the most protection for the thermocouple elements and is recommended for the longest life. Different types of sheath materials are available to offer greater protection. The response time of this type of thermocouple is usually slower.

You Will Need a Readout

Thermocouples and RTDs can be made to order or can be purchased ready made. controller
Thermocouples and RTDs require a readout to display the temperature. Readouts come in all shapes and forms such as controllers, recorders, thermometers and loggers. The probe can be situated for example in a cool store with a readout in a control room - or it can be connected directly to a hand held device. The options are numerous.

 Center 300
For improved accuracy – order an IANZ test certificate.
Ask for test points around your working temperature  – that way you will be provided with corrections that you can apply to your readings to improve the accuracy of your system.

Contact Teltherm at 09 633 0040 and they will help you through the process.


If you are not sure what you are after or are in a hurry, call us today and talk to an expert on industrial instruments.

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+64 9 633 0040
5 / 343 Church Street | Onehunga
Auckland 1061, New Zealand