What are NFC tags with a built in sensor?
Tags carrying a sensor, most often temperature, which the phone powers along with the chip. Without a battery there is no logging: a passive sensor tag reports the moment of the tap and nothing before it. Battery assisted versions record over time and cost accordingly.
Passive sensor tags
The tag has no battery, so the phone's field powers the chip and the sensor together for the duration of the tap.
Temperature is the common measurement, because it is cheap to integrate and widely useful. Other quantities exist in more specialised products.
The reading is taken during the exchange and returned with the tag's other data, so a single tap gives you a value.
Cost sits above a plain tag and well below anything with a battery, which is what makes this family attractive for spot checks.
The limitation that decides everything
No power means no measurement between taps. The tag is inert until a reader arrives, so it cannot record what happened overnight.
This rules out the application people usually have in mind. Verifying that a shipment stayed cold for three days is a logging problem, and a passive sensor tag cannot answer it.
What it answers well is the question at the moment of arrival: is this cold now. For a spot check at receipt, that is often what actually matters and it costs a fraction of a logger.
Being clear about which question you have is the whole decision, and it is the point at which most sensor tag projects go wrong.
Battery assisted tags
A small battery lets the tag measure and store on a schedule, so a tap retrieves a history rather than an instant.
That buys the logging capability and brings a set of costs: unit price well above a passive tag, a finite life set by the battery, disposal considerations, and a larger body.
It also introduces a failure mode passive tags do not have. A flat battery means a gap in the record, and the gap is discovered when the data is read rather than when it happens.
Choosing between them
Write down the question first. "Is it cold now" and "was it cold throughout" are different products at different prices.
For spot checks at handover, receipt or inspection, a passive tag is the right size of answer.
For a chain of custody where the record matters afterwards, you need logging, and the honest comparison is against a dedicated data logger rather than against a plain tag.
Verify the accuracy claim against your actual requirement, since a sensor adequate for detecting a broken cold chain may not be adequate for a regulated measurement.
Frequently asked questions
Can a passive sensor tag record overnight?
No. Without a battery it has no power between taps, so it can only report the moment it is read. Logging requires a battery assisted tag or a dedicated logger.
How accurate are they?
Accuracy varies by product and is stated on the datasheet. Check it against your requirement, since detecting a failure and measuring for a record are different standards.
Do they work with any phone?
Reading the tag works normally, and interpreting the sensor value usually needs software that knows the product. Ask what is required before deploying.
What else can they measure?
Temperature dominates, and other quantities exist in specialised products. Availability is much narrower than for plain tags.