Brew control
The controller is what turns a 104-degree seedling mat into a jar held at 78. Its specifications are unusually well published, which makes it easy to reason about.
By Scooter M.·Published September 1, 2026
The short answer
A dual-stage controller with a stated accuracy and a sensor-fault alarm. Inkbird publishes ±1 C accuracy, 10 A relay capacity per channel and 1100 W maximum load at 110 V for the ITC-308 — far more capacity than an 18 W heat mat needs, which is exactly the margin you want on something running unattended.
It sits between the wall and the heater, reads a probe, and switches the heater on and off to hold a setpoint. That is the entire function, and it is the difference between a heat source and a temperature.
It matters here because seedling heat mats are designed for seedlings. VIVOSUN publishes that its mat reaches around 104 F from a 68 to 77 F ambient — right for germination, roughly twenty degrees above kombucha's band. The mat is not wrong; it is uncontrolled.
| Specification | Published value | What it means for kombucha |
|---|---|---|
| Accuracy | ±1 C (±2 F) | Tight enough that a batch stays inside a two-degree window all week. |
| Control range | -50 to 120 C | Enormously more than needed. The extra range buys nothing here. |
| Display resolution | 0.1 C below 100 C | More precision than the process requires, and harmless. |
| Relay capacity | 10 A at 100-240 VAC per channel | An 18 W mat draws well under an amp. Vast margin. |
| Maximum load | 1100 W at 110 V | Enough for a mat, a belt or a small fridge if you ever cool instead. |
| Outputs | Dual relay, separate heat and cool setpoints | One unit covers winter heating and summer cooling. |
| Alarms | High/low temperature and sensor fault | The sensor-fault alarm is the one that matters. See below. |
Not the accuracy. The sensor fault alarm.
A controller with a probe that has fallen off the jar is a controller reading the room. It sees 70 F, concludes it needs heat, and runs the mat continuously against a jar that is climbing past 95. Nothing in the accuracy figure protects you from that; an alarm does.
The same logic explains why a strip on the opposite side of the jar is worth its couple of dollars even when you own a controller. Two instruments disagreeing is information. One instrument only ever confirms itself.
Controller, mat and an independent check. Buying the mat without the controller is the mistake; buying the controller without a second instrument is the smaller one.
Inkbird
A dual-stage thermostat with published relay ratings and a stated plus or minus 1 C accuracy, which is what turns a 104 F heat mat into a jar held at 78 F.
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VIVOSUN
Eighteen watts, IP67, MET-certified, and published to reach about 104 F from a 68-77 F ambient — which is more heat than kombucha wants, so it belongs under a controller.
#ad No live price feed yet, so no number is shown rather than a stale one. How this is funded.
Craft a Brew
A four-inch liquid crystal strip that sticks to the glass and reads the liquid rather than the air, which is the distinction that matters.
#ad No live price feed yet, so no number is shown rather than a stale one. How this is funded.
Two independent relays: one for heating, one for cooling, with separate setpoints. In winter it drives a heat mat. In summer, plugged into a small fridge or a fan, it can hold a batch down instead.
Most home brewers only ever use the heating side, and that is fine — the second relay costs almost nothing extra and is there if a hot summer makes it relevant. The compressor delay protection Inkbird publishes exists for the cooling case: it prevents a fridge compressor being restarted immediately after shutdown, which is how compressors die.
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Ranked on the published rubric. Every row jumps to that product's full write-up below; the button goes straight to Amazon.
| # | Product | Best for | Score | Price |
|---|---|---|---|---|
| 1 | Score8.4 Inkbird ITC-308 Dual Stage Temperature Controller A dual-stage thermostat with published relay ratings and a stated plus or minus 1 C accuracy, which is what turns a 104 F heat mat into a jar held at 78 F. | Making a heat mat safe to leave alone | 8.4out of 10 | |
| 2 | Score8.6 VIVOSUN Waterproof Seedling Heat Mat, 10 x 20.75 in Eighteen watts, IP67, MET-certified, and published to reach about 104 F from a 68-77 F ambient — which is more heat than kombucha wants, so it belongs under a controller. | Lifting a cold kitchen into the brewing band | 8.6out of 10 | |
| 3 | Score7.4 Craft a Brew Fermometer Adhesive Strip Thermometer A four-inch liquid crystal strip that sticks to the glass and reads the liquid rather than the air, which is the distinction that matters. | Reading the jar, not the room | 7.4out of 10 | |
| 4 | Score8.8 Micro Essential Lab Hydrion 9200 Spectral pH Strips, 0.0-6.0 A 0.0 to 6.0 range in 0.5 steps, which puts four separate color matches inside the 2.5 to 3.5 window kombucha lives in — a 1-to-14 strip gives you one. | Reading kombucha’s actual pH band | 8.8out of 10 |
#1 · Making a heat mat safe to leave alone
A dual-stage thermostat with published relay ratings and a stated plus or minus 1 C accuracy, which is what turns a 104 F heat mat into a jar held at 78 F.
Who it is for. Anyone who has bought a heat mat. The mat without the controller is the mistake; this is the other half.
Source: Inkbird published ITC-308 specification (control range, accuracy, resolution, relay capacity, maximum load, alarms), inkbird.com/products/temperature-controller-itc-308, retrieved 2026-09-01
Skip it if. You are not running a heater or a cooler. A controller with nothing to switch is an expensive thermometer.
#ad No live price feed yet, so no number is shown rather than a stale one. How this is funded.
#2 · Lifting a cold kitchen into the brewing band
Eighteen watts, IP67, MET-certified, and published to reach about 104 F from a 68-77 F ambient — which is more heat than kombucha wants, so it belongs under a controller.
Who it is for. Anyone brewing in a room that drops below 70 F, paired with a controller rather than plugged straight into the wall.
Source: VIVOSUN published heat mat specification (dimensions, 18 W, UL and MET certification, IP67, temperature band and stated maximum against ambient), vivosun.com heat mat product page, retrieved 2026-09-01
Skip it if. Your house sits at 75 F already. You do not have a temperature problem, and adding heat without a controller creates one.
#ad No live price feed yet, so no number is shown rather than a stale one. How this is funded.
#3 · Reading the jar, not the room
A four-inch liquid crystal strip that sticks to the glass and reads the liquid rather than the air, which is the distinction that matters.
Who it is for. Every brewer. This is the cheapest way to stop guessing what temperature the batch is at.
Source: Craft a Brew published Fermometer specification (size, display units, compatible surfaces, operating principle), craftabrew.com/products/fermometer-adhesive-strip-thermometer, fetched 2026-09-01. Comparable-strip ranges from retailer listings, cited as comparisons only.
Skip it if. You want a logged, numeric record. A strip is a glance, not a measurement you can write down with confidence.
#ad No live price feed yet, so no number is shown rather than a stale one. How this is funded.
#4 · Reading kombucha’s actual pH band
A 0.0 to 6.0 range in 0.5 steps, which puts four separate color matches inside the 2.5 to 3.5 window kombucha lives in — a 1-to-14 strip gives you one.
Who it is for. Every kombucha brewer. This is the cheapest instrument on the site and the one that answers the safety question.
Source: Micro Essential Lab published product specification (range, 0.5 graduations, 100 strips per vial, packaging), microessentiallab.com/product/hydrion-9200-spectral-0-0-6-0-plastic-ph-strip, fetched 2026-09-01. Strip length from retailer listings.
Skip it if. You need a number to one decimal place for a written record. Buy a calibrated pH meter; strips are for reading a band, not a value.
#ad No live price feed yet, so no number is shown rather than a stale one. How this is funded.
Every specification cited on this page came from one of these documents, retrieved on the date shown against the product it belongs to.
Temperature and pH — the two readings that decide a batch, and the strips, mats, thermometers and controllers that give them to you.
Why a seedling mat published at 104 F is too hot for kombucha alone, which controller to pair it with, and how to set the two up.
The 75-85 F band, what happens on either side of it, why the jar is not the room, and how to fix a kitchen that sits too cold.
Adhesive strips against probes: what each actually measures, what neither publishes, and which one a home brewer needs.
A day-by-day account of a first batch — what you should see, what alarms people and is fine, and when to start tasting.