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Dealing with Radiated and Conducted Interference, and Bad Earths

Most engineers will at some point experience issues with getting their wired datalogger to run through a test smoothly, with probes ‘spiking’ during what should be a steady state condition.

The most common cause of ‘spiking’ is probes intermittently becoming ‘open circuit’ and simply remaking the thermocouples is a quick and easy solution. Sometime ‘spiking’ may occur during a specific stage of a cycle, such as wash, where the motion of the wash effects a damaged or badly made thermocouple quite dramatically.

Occasionally however, it is the environment rather than the probes themselves that can be the issue. This often requires an element of investigation as in certain combinations of environments electro-magnetic interference can cause havoc with thermocouple signals. In these situations the leads in a temperature measuring system can act like an antenna if left unprotected. Thermocouple probes are intended and designed to sense temperature, but unfortunately they can act as a perfect antenna for electromagnetic radiation. The problem arises because the thermocouple circuit can pick up extraneous information, or noise, and pass it along to the data logger. In batch cycle processes employing pumps, motors, and electrical heaters, such as washer disinfectors and dry heat ovens etc., it is possible that emitted EMI can bombard unshielded thermocouple wire resulting in high frequency interference rendering the intended temperature signal unmeasurable.

bad earth graph

If pre-calibration has been carried out successfully, but probes persist in spiking during a test cycle, this usually points towards a noise issue. On some exceptionally ‘noisy’ sites, successful calibration may also be impossible.

All dataloggers have a Normal Mode Rejection and Common Mode Rejection Rating, which is shown in dB. However, if there are a lot of magnetic fields around the site, or the site has a “dirty” power supply from the mains, this may exceed the dataloggers ability to reject such noises when using thermocouples. Some dataloggers, such as the Fluke Netdaq, also filter on a specific frequency – this should always be set to the correct value (50Hz in the UK and Ireland)

Type T Thermocouple is only 39x10-6V per ⁰C change in temperature, which is very small. The datalogger needs to filter out all other noises whilst still reading the thermocouple accurately. A good example of noise is to put a mobile phone on top of the datalogger and make it ring. The magnetic field created is strong enough to cause the temperature readings to fluctuate. Other causes of interference which may be encountered are poorly shielded or faulty motors (for instance in a compressor or water pump), laptop power supplies, and solenoids switching. It should also be noted that coiled extension reels can magnify the effects of interference by acting as an aerial!

Earthing the datalogger will usually stop spiking due to environmental issues. Most dataloggers will have an earth point, and good practice dictates earthing to the same point as the machine being tested. However, if this does not resolve the issue, earthing to a different point or even removing the earth may be the answer.

On some occasions even earthing will not successfully prevent spiking. There are various other causes to investigate at this point.

  • The machine under test may be generating static within the chamber or at the sensor.
  • There may be a bad earth on the machine itself, in which case if this cannot be fixed immediately the logger and thermocouple tips would need to be insulated from the source (the machine) by removing the earth and putting an insulator (e.g. some thermocouple tape) between the metal of the tip and any metal it may touch in the machine.
  • The chemical used during a wash stage may be corrosive and over time effect the thermocouples. In these instances, using sealed thermocouples can be the answer, such as Teflon tipped or hermetically sealed probes.

As a rule of thumb, if thermocouples are stable during calibration and during tracking runs in a heat bath, but fail or spike during testing, the likelihood is that it is either a damaged thermocouple spiking as it flexes due to the mechanical action of the machine, or an environmental factor, as from a hardware perspective the source of heat is irrelevant (100 degrees in a thermal bath is an identical voltage input to 100 degrees in an autoclave).

If the probes are not stabilising during calibration it can be useful to start a trace when spiking is happening, and turn off nearby equipment sequentially to find the source of the interference.

If probes are spiking during a cycle, are all probes affected (a global issue) or just one (probably a faulty probe)

If probes always spike at a specific point in the cycle, what is turning on at that point (compressor etc.) – watching the pressure channel as well as the thermocouples can also help at this point.

Hermetically Sealed Thermocouple

Once the tip has been welded, the outer sheath is extruded over the junction and hermetically sealed or covered with a cap.

This Completely Prevents:
• Corrosion and damage to the tip
• Water tracking up the lead
• Interference caused by the wire touching the machine

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