HeaterMeter Probes

Bryan Mayland edited this page May 1, 2017 · 28 revisions

HeaterMeter uses resistive thermistor-based probes due to their availability and the fact that most BBQers already have some sort of meat thermometer. Maverick Industries makes several models of digital thermometers using thermistor probes with 2.5mm mono headphone jack connectors.

Use the Probe Range Comparison Tool to compare ranges of the probe types. The standard HeaterMeter build uses 10000 (10k) ohm resist.

Thermoworks Pro-Series

Highly Recommended

Thermoworks Pro-Series probes for their ChefAlarm system work well, and respond faster than ET-72/73 due to their smaller probe tips. Note these are thermistor-based probes, and not the high-precision Thermoworks thermocouple-based design. These probes are well built with better moisture resistance, spring strain relief, and larger temperature range too.

  • Cable length: 47 inches
  • Maximum working temperature: 644F
  • Maximum cable temperature: 700F
  • Isolated probes (not specified but verified in testing, subject to change)
  • 6 month warranty

Recommended HeaterMeter resistance: 10k ohm (standard)

Maverick ET-72/73

Labeled as

  • ET-7
  • ET-72
  • ET-73
  • ET-72/73

Highly Recommended

Available in two varieties, both "smoker probe" and "food probe" are the same internally and differ only in that the smoker probe is shorter, straight, and blunt compared to the food probe's pointed tip and 90 degree bend.

  • Standard 3ft length, rated to 410F maximum temperature
  • High Heat 6ft length, rated to 716F maximum temperature although some are only rated to 570F working temperature

Recommended HeaterMeter resistance: 10kohm (standard)

Maverick ET-732

Acceptable

Note how closely this model number resembles the ET-72/73 specification. When run all together as ET-732 they are internally different probes. The 2.5mm connectors on these probes are slightly longer than the ET-72/73 and when inserted into the HeaterMeter jacks, can be pushed in too far and lose their connection. This can be avoided by cutting a 1/16" length of tubing and putting it on the connector, or using a small rubber grommet or washer.

These are also available in both "smoker probe" and "food probe"

  • High Heat 6ft length, rated to 716F maximum temperature. 3ft low heat probes are no longer available.

These probes are also subject to more mysterious errors than the ET-72/73 probes, making the temperatures fluctuate for no apparent reason. The only solution is to ground the probe's braided shielding.

The response curve of these probes bottoms out around room temperature when used with the standard 10kohm HeaterMeter resistance, which causes them to read as "off". To get more usable range, replace the standard 10k resistors (R5, R16, R17, R18) with 22k or even 47k 1% tolerance resistors.

If you already have ET-732 probes, you can use them, but because of the problems listed above, they are not recommended for new purchases.

Maverick ET-735

Acceptable

These probes come with the ET-735 Bluetooth Thermometer. Like the ET-723 probes, the 2.5mm connectors on these probes are slightly longer than the ET-72/73 and when inserted into the HeaterMeter jacks, can be pushed in too far and lose their connection. This can be avoided by cutting a 1/16" length of tubing and putting it on the connector, or using a small rubber grommet or washer.

  • High Heat 3ft length, rated to 716F maximum temperature.

The response curve of these probes bottoms out around room temperature when used with the standard 10kohm HeaterMeter resistance. To get more usable range, replace the standard 10k 1% resistors with 22k or even 47k 1% tolerance resistors.

If you already have ET-735 probes, you can use them, but because of the problems listed above, they are not recommended for new purchases.

IKEA Fantast

Not recommended

These probes can be used, but they are wired backwards so the PCB mount probe jacks can not be used. They are also subject to the same floating mysterious errors as the ET-732 probes. Their measurement device actually uses the probe resistance to charge and discharge a capacitor and measure how long it takes, then switches polarity and measures again.

  • Rated to 428F maximum temperature

Thermocouples

Thermocouples have a higher maximum temperature rating, but their low voltage change per degree requires special amplifier circuitry or special purpose thermocouple-reading chips. HeaterMeter hardware v4.2 and above has optional support for a thermocouple pit probe. Any K-type thermocouple is supported, and the HeaterMeter jack is a standard miniature type connector. Thermocouples with spade connectors can be used by clipping them to fit into the jack, although this isn't the most secure of connections.

Thermocouple Advantages

  • Higher maximum temperature - over 1000C possible (HeaterMeter only reads up to 500C). Best thermistor probe we know of only does 570F (300C).
  • Generally more durable than the thermistor probes we use, which are cheap consumer-grade parts.
  • More resistant to moisture (although you should not dunk your exposed thermocouple in water).
  • Available in a variety of shapes, wire sheaths, and attachment mechanisms. Clips, through-hole screw in, washer mount, bare, penetration probe, pick what works best for your grill.
  • Probes can be cheap, as low as a couple of dollars.

Thermistor Advantages

  • HeaterMeter hardware is cheaper to implement, no amplifier or special connector required.
  • Can use a food probe as a pit probe if the pit probe becomes damaged.

Isolated vs Non-Isolated Probes

Most probes, both thermistor and thermocouples, are designed for use with battery powered measurement devices and as such can save money in their construction by not having their wiring isolated from their wire sheaths (the metal tubes and stainless wire braid). This can cause problems with HeaterMeter because if the metal braid becomes grounded to Earth, some current will choose to flow through that connection rather than back to the HeaterMeter and throws off the temperature readings in the form of noise. Non-isolated probes can cause wildly inaccurate readings, off by even 50 degrees or more, that change randomly. For this reason probes that are isolated probes are preferred, however non-isolated probes can be used if great care is taken to keep them from becoming grounded.

To test a probe for isolation, use a multimeter on the continuity setting and test if any of the connectors in the jack have continuity to any part of the probe itself, either the metal braid or the metal probe tip. If the multimeter beeps, the probe is not isolated. This is true for thermistor probes (test both the tip and the ring conductor to all parts of the probe) and thermocouple probes (test both + and - pins/wires to all parts of the probe). "Bare thermocouple" probes are always non-isolated.

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