An example Slow Thermometer puzzle: digits never decrease from the bulb - they may repeat, here 3, 3, 6, 8.
Slow Thermometer Sudoku is a gentler thermometer. On a normal thermometer the digits must strictly increase from bulb to tip; on a slow thermometer they only have to be non-decreasing - each cell is greater than or equal to the one before it. The values may hold steady, so long as the ordinary Sudoku rules still allow it.
The rule: never go down (but you may stay level)
Read a slow thermometer from bulb to tip: every step is "≥" rather than ">". A strict thermometer must climb (2 < 4 < 5); a slow one may repeat a value where the thermometer bends into a different row, column or box, so a step like 4 ≤ 4 is legal.
Every step is ≤: the values rise or stay level from the bulb (left) to the tip.
A worked example, step by step
Where a slow thermometer runs inside one row (so no repeats are possible there), it still climbs strictly - but its cells can reach a touch further than a strict one. Take a four-cell slow thermometer whose bulb sits in a row that already contains a 1.
Step 1 - the bulb loses its smallest option. On a strict four-cell thermometer the bulb's window is 1-6. Here the row's 1 is taken, so the bulb climbs to 2-6.
Step 1. With the 1 gone from the row, the bulb is 2-6; each later cell is ≥ the one before.
Step 2 - read up the tube. Each cell is at least as large as the last, so the tip carries the biggest value. Because equal steps are allowed only across a box or line border, a slow thermometer that stays inside one unit behaves like a strict one - the "slow" freedom pays off exactly where the thermometer bends.
Where the slow rule matters
The difference between slow and strict shows up whenever a thermometer crosses from one box (or line) into another. There, two touching cells may hold the same digit, because they no longer share a unit - something a strict thermometer forbids everywhere. That extra option makes long slow thermometers a little roomier than their strict cousins, but the "never decrease" backbone still drives every deduction.
Solving strategy
- Still push from both ends. The bulb takes the smallest value, the tip the largest - just as with a strict thermometer.
- Allow equals across borders. Where the tube crosses a box or line, neighbouring cells may match.
- Inside one unit, treat it as strict. Cells sharing a row, column or box can't repeat, so there it climbs step by step.
- Feed the grid. Each digit a thermometer fixes behaves like a given for its row, column and box.
Common mistakes
- Allowing a decrease. Slow means non-decreasing - values may stay level but never fall.
- Repeating inside one unit. Equal steps are only legal where the thermometer crosses into a different row, column or box.
- Treating it exactly like a strict thermometer. The extra "equal" option loosens long tubes that bend between boxes.
- Forgetting ordinary Sudoku. Thermometer cells still obey their rows, columns and boxes.
Can I play it here?
Not yet - Slow Thermometer Sudoku isn't one of the puzzle types SudokuStreak generates for now. But you can learn its rules above, and there's plenty here to sink your teeth into meanwhile:
- Browse all the Sudoku variants we do offer.
- Or print free puzzles from the printable PDF page.
Where to play it
Slow Thermometer Sudoku belongs to the constraint family and runs on 9×9. It is not one SudokuStreak generates yet - but the deduction it trains is the deduction every Sudoku runs on.
Difficulty and type are two separate dials here: the type sets the extra rule, the level sets how much of the grid you start with. Expert Sudoku leaves 25 of the 81 cells filled, where the beginner tools dry up almost entirely; Evil Sudoku leaves 23, with no easy moves at all. Take it at the gentler end while the pattern is still new, and move up once it stops being a fight.