Deze behandeling vereist geen thermometer.

Because behandeling is a de-word: de behandeling.

In Dutch, the singular demonstratives are:

• deze for de-words
• dit for het-words

So:

• deze behandeling = this treatment
• dit boek = this book

A useful shortcut:

• singular de noun → deze
• singular het noun → dit

Geen is used to negate a noun phrase with an indefinite meaning.

Here, thermometer is a singular countable noun, and the positive version would be:

• Deze behandeling vereist een thermometer.
  = This treatment requires a thermometer.

To negate een thermometer, Dutch normally uses geen:

• Deze behandeling vereist geen thermometer.
  = This treatment requires no thermometer / doesn’t require a thermometer.

Very roughly:

• geen = no / not a / not any
• niet = not for verbs, adjectives, adverbs, and definite phrases

So niet thermometer would be wrong here.

Because geen replaces the indefinite article een.

Compare:

• een thermometer = a thermometer
• geen thermometer = no thermometer / not a thermometer

So the sentence is not missing an article. Geen is doing that job.

Vereist comes from the infinitive vereisen, which means to require.

In this sentence, vereist is the present tense, third-person singular form, because the subject is singular:

• deze behandeling = singular
• therefore: vereist

A quick pattern:

• ik vereis = I require
• jij vereist = you require
• hij/zij/het vereist = he/she/it requires

So:

• Deze behandeling vereist ...
  = This treatment requires ...

Because Dutch main clauses normally follow the V2 rule: the finite verb comes in the second position.

In this sentence:

1. Deze behandeling = first element
2. vereist = finite verb in second position
3. geen thermometer = rest of the clause

So the structure is:

• Deze behandeling | vereist | geen thermometer

This is very normal Dutch word order.

It is a de-word: de thermometer.

You cannot see that directly in the sentence, because geen is used instead of an article, but if you put the noun on its own, it is:

• de thermometer

So:

• een thermometer
• de thermometer
• geen thermometer

In this sentence, it means no thermometer is needed.

That is because of the full context of the sentence:

• Deze behandeling vereist geen thermometer.

The verb vereist (requires) makes it clear that the sentence is about necessity, not identity.

So the meaning is:

• This treatment does not require a thermometer.

It does not mean that the treatment somehow isn’t a thermometer.

Yes, vereisen often sounds a bit more formal or technical than everyday nodig hebben (to need).

So this sentence sounds natural in instructions, medical language, manuals, or formal descriptions:

• Deze behandeling vereist geen thermometer.

A more everyday-style alternative could be:

• Voor deze behandeling heb je geen thermometer nodig.
  = You don’t need a thermometer for this treatment.

Both are correct, but vereist is a little more formal.

Yes, the literal translation is:

• This treatment requires no thermometer.

That is understandable English, but in everyday English people often say:

• This treatment doesn’t require a thermometer.

So the Dutch sentence is perfectly normal, but the most natural English translation may be slightly less literal.

The positive version is:

• Deze behandeling vereist een thermometer.

That means:

• This treatment requires a thermometer.

This is a useful pair to remember:

• vereist een thermometer = requires a thermometer
• vereist geen thermometer = doesn’t require a thermometer