A bimetal thermometer reads temperature with a coil of two bonded metals that twists as it warms, turning a dial pointer with no battery and no display to fail. For compost piles and windrows, choose a long-stem unit with a 3 in. sealed dial, such as the REOTEMP Heavy Duty or Fast Response Type "A" probes ERE lists, and match the stem length to the depth of your pile. This guide covers stem length, scale, accuracy, calibration and the compost temperature rules the thermometer has to prove.
In this guide
- How does a bimetal thermometer work?
- What temperatures does compost have to reach in Canada?
- Heavy Duty or Fast Response: which probe do you need?
- How long should the stem be?
- How accurate is a bimetal thermometer?
- How do you check the calibration?
- What do the probe guard and handle do?
- How do you choose, step by step?
- Frequently Asked Questions
How Does a Bimetal Thermometer Work?
Two strips of different metals are bonded together and wound into a helix, and because the metals expand at different rates, the helix winds or unwinds as the temperature changes and moves the pointer. The ERE product records describe the dial pointer as directly driven by the sensitive bi-metal helix in the bottom of the stem.
That design suits field work: it needs no power, the dial is hermetically sealed so it will not fog, and the plastic crystal is unbreakable. The trade-off is that you get a spot reading, not a record. If you need a continuous log of temperature over weeks, pair the dial thermometer with a logger; our data logger buyer's guide shows how that kind of instrument is specified.
What Temperatures Does Compost Have to Reach in Canada?
The national benchmark is 55 °C. The CCME Guidelines for Compost Quality say that in-vessel and aerated static pile compost must be held at 55 °C or greater for three days, and windrow compost must reach 55 °C or greater for at least 15 days, with the windrow turned at least five times during the high-temperature period. Provinces and territories may recognize equivalent processes, so confirm the rule that applies to your permit.
The same 55 °C benchmark appears in the U.S. biosolids rule, 40 CFR Part 503, which matters to operators who also report to U.S. customers. In practice this means your thermometer has to be read at several points in the pile, on a schedule you can document, and it has to be accurate near 55 °C. A dial with 55 °C in its middle third, not at the edge, is easier to read.
Heavy Duty or Fast Response: Which Probe Do You Need?
Choose Fast Response when you take many readings a day or need a tougher stem; choose Heavy Duty for routine pile checks. Both use a 3 in. dial and are listed at ±1% of scale. The records compare as follows.
| Item | Heavy Duty Type "A" | Fast Response Type "A" |
|---|---|---|
| Stem | 5/16 in. stainless steel | 3/8 in. stainless steel, 1/4 in. fast-response tip |
| Response | Baseline | Listed as twice as fast as Heavy Duty |
| Dial | 3 in., hermetically sealed | 3 in., hermetically sealed |
| Accuracy | ±1% of scale | ±1% of scale |
| Stem lengths | 24, 36, 48, 60, 72 in. | 24, 36, 48, 60, 72 in. |
| Scales | 0 to 200 °F or 0 to 100 °C | 0 to 200 °F or 0 to 100 °C |
| Other | Pointed stem for easy insertion | Made in USA, 1-year limited warranty |
Each model is a separate listing by scale: Heavy Duty in °F and in °C, Fast Response in °F and in °C. All sit in the thermometers collection.
How Long Should the Stem Be?
Pick a stem long enough that the tip reaches the core of the pile while the dial stays readable above the surface. The records list five stem lengths, 24, 36, 48, 60 and 72 in. (2 to 6 ft), for each probe.
- 24 to 36 in.: low windrows, bins and the outer zone of a pile.
- 48 to 60 in.: typical windrows where the hot core sits well below the surface.
- 72 in.: tall piles and static piles that you cannot walk onto.
These ranges are a starting guide, not a rule from the maker, so size the stem to your own pile height. Because the sensing helix is in the bottom of the stem, insert the probe so the tip sits at the depth you want to measure and allow time for the pointer to settle.
How Accurate Is a Bimetal Thermometer?
The product records list ±1% of scale, which is a percentage of the full dial range, not of the reading. On a 0 to 200 °F dial that is about ±2 °F; on a 0 to 100 °C dial it is about ±1 °C. Against a 55 °C requirement, a reading of 54 °C on a Celsius dial could be a true 55 °C, so build a margin into your own acceptance rule.
A bimetal dial gives you a fast, battery-free check, but it is not a laboratory reference. For permit reporting, confirm with your authority whether a documented dial reading is acceptable or whether a logged digital record is expected.
How Do You Check the Calibration?
Compare the dial against a trusted reference thermometer in a stirred water bath at one or two temperatures near your working range, and record the difference. The maker publishes a calibration instruction sheet, "Calibration Instructions: Dial Compost Thermometers", linked from the Fast Response product page; follow it for any adjustment, since the procedure depends on the instrument.
Check a probe after any drop, bent stem or impact, and on a fixed schedule in a documented program. Keep the check date with your pile temperature records.
What Do the Probe Guard and Handle Do?
They protect and help you push the stem into dense material. The REOTEMP Probe Guard and Probe Handle record lists a thick-walled stainless steel sheath in lengths of 2 to 6 ft (parts FM24-16 to FM72-16), and a probe handle in orange painted steel (FM-0) or stainless steel (FM-0S). The record states they fit the Heavy Duty 5/16 in. stem; if you plan to use them with the Fast Response probe, ask us to confirm fit before you order.
How Do You Choose a Compost Thermometer, Step by Step?
- Find your rule. Check the permit or provincial requirement against the CCME 55 °C benchmark.
- Pick the scale. Choose °F or °C to match the units on your records.
- Pick the probe. Fast Response for frequent readings, Heavy Duty for routine checks.
- Size the stem. Match 24 to 72 in. to the depth of the hot core.
- Add accessories. Probe guard or handle if you insert into dense material.
- Plan the check. Set a calibration schedule and a reading log.
Need a compost thermometer?
Send us the pile depth and the scale you record in and we will confirm the probe, stem length and accessories, with delivery across Canada.
Frequently Asked Questions
What is a bimetal thermometer used for?
It gives a direct dial reading of temperature with no power. The REOTEMP probes ERE lists are described for monitoring the interior of compost piles and windrows.
What temperature must compost reach in Canada?
The CCME guideline asks windrow compost to reach 55 °C or greater for at least 15 days with five turnings, and in-vessel or aerated static pile compost to hold 55 °C for three days.
How accurate is a bimetal dial thermometer?
The ERE records list ±1% of scale, about ±2 °F on a 0 to 200 °F dial or ±1 °C on a 0 to 100 °C dial.
What stem length do I need?
The probes come in 24, 36, 48, 60 and 72 in. stems. Choose the one that puts the tip in the hot core while the dial stays above the pile.
Do you ship compost thermometers across Canada?
Yes. Send the scale, stem length and quantity through the quote form and we confirm availability.
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Lire en français : Thermomètre bimétallique pour le compost : longueur de tige, échelle et étalonnage au Canada