Product2 publishers3 min readPublished
Apple rebuilds the Watch Ultra's heart sensors under an unchanged case
Apple left the Ultra's case, display and price where they were and spent this year's upgrade underneath, on the sensor array and the silicon. What the watch can measure now depends on hardware a software update cannot supply.
The Product Desk · Product desk
What happened
- Apple Watch Ultra 4 runs a brand-new S11 chip with 4GB of memory, double the 2GB in the Ultra 3, which used the same S10 silicon as the Series 10 and Ultra 2.
- Gizmodo reports Apple redesigned the entire sensor stack, with a larger electrode surface for ECG contact, photodiodes arranged in a ring, and more power-efficient green LEDs for heart rate.
- Apple says HRV is now measured up to 24 times more often, and the watch adds a Recovery HRV reading the company describes as best for identifying daily signals of stress and recovery.
- A 15-minute charge now yields 18 hours of runtime on the Ultra 4, against 12 hours on the Ultra 3.
- The Ultra 4 is on pre-order at $799, the same starting price as last year's model, and it ships Friday, September 18.
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Why it matters
- constraint An Ultra 3 fleet cannot update its way into Audio Intelligence: Gizmodo reports Apple Intelligence on the watch runs on the S11, and the Ultra 3 had none of it.
- cost Every wrist that needs the new sensor array costs another $799, and that lands on whoever owns the health program budget, not the software line.
- exposure Handing someone a watch that listens continuously puts the consent conversation on the person doing the handing, and Apple's off switch is a setting somebody has to explain.
The watch that arrives on Friday looks exactly like the one it replaces. The 49mm titanium case and the always-on display at 3000 nits carry over from the Ultra 3, along with water resistance to 100 meters [2]. 9to5Mac wrote that the new Health Sensing System "might be the biggest change ever to the Watch's internal sensors" [8].
Apple's own description of the system is that it "unlocks deeper health and fitness insights" [7]. For the chip there is no benchmark on offer, only the line that S11 is the company's "most powerful wearable chip" [4].
What the hardware produces, according to Apple, starts with high-frequency background heart rate tracking from a second-generation electrical sensor and an always-on optical sensor [10], and includes step tracking Apple calls "the most accurate step tracking of any smartwatch" [30]. Each of those numbers lands on an Apple surface: a new section of the Heart Rate app, a heart rate watch face complication, a Steps complication [11][10][13]. Neither write-up describes any third-party access to the higher-frequency samples [27]. A team building a training app against Watch data has to go find that in the developer documentation, because the launch material does not contain it.
The battery figures are the easiest thing to check. Up to 50 hours against the Ultra 3's 42 is eight more hours, about 19 percent [25]. In Low Power Mode, 84 against 72 is 12 hours, about 17 percent [26]. Gizmodo's own endurance number is a different measurement again: up to 18 hours of heart rate and GPS tracking on the Ultra 4, where the Ultra 3 reached 20 hours of GPS only in Low Power Mode [17].
Audio Intelligence is the feature that will generate questions in an office. Gizmodo reports that most of the Apple Intelligence features here work because the Ultra 4 is always listening to the user and their surroundings [21]. Apple says the features do not store recorded conversations, do not automatically identify speakers, are end-to-end encrypted and not accessible by Apple, and can be turned off entirely [22]. Part of the suite arrives in a software update later this year [20].
For a rollout, the line worth drawing is between a metric that needs the new sensor hardware and a metric that needs only new code. The first needs a new watch on every wrist. Readiness, for instance, is described as analyzing a user's recent activity, training load, vitals and sleep score to produce one 0-10 score with a recommendation of Recover, Pace Yourself, Ready, or Go For It [19]. 9to5Mac files it under the Ultra 4's exclusive software features, next to Audio Intelligence [28]. Neither source says which part of that needs the new photodiodes [27].
What to watch
- Whether HealthKit exposes Recovery HRV and background heart rate samples to third-party apps, or keeps them inside Apple's Heart Rate app.
- Which Audio Intelligence features are missing on September 18 and which arrive in the update Apple says is coming later this year.
- A watchOS release that brings Readiness to the Ultra 3 would show how much of the exclusivity is silicon.