Here is a fact that reframes the whole conversation about cheap sensors. On a water-cooled chiller, raising or lowering the chilled-water supply temperature by one degree Fahrenheit changes the compressor's energy draw by about 2 to 2.5 percent. (Consulting-Specifying Engineer; ACHR News, from manufacturer chiller data.) The chiller doesn't know the "true" water temperature — it knows only what its sensor reports. So the sensor's error becomes the chiller's error, and the chiller's error becomes money.

The four temperatures a chiller lives by

1. Chilled-water supply (leaving water): ~42–45°F. The primary control variable.
2. Chilled-water return (entering water): the warm water coming back. Supply-to-return difference (delta-T) tells the controller the building's load.
3. Refrigerant suction: protects the compressor and tunes the cycle.
4. Condenser: manages heat rejection and condensing pressure.

ASHRAE's Guideline 22 specifies monitoring exactly these supply/return temperatures (plus condenser water) to compute plant efficiency, and AHRI Standard 550/590 governs how chiller performance is rated from such measurements. (ASHRAE Guideline 22; AHRI 550/590-2023.)

Why RTDs guard the water loop

Accuracy is money on the water side, so supply and return are usually platinum RTDs. Platinum's resistance rises almost linearly with temperature, the relationship is fixed by IEC 60751, and the elements barely drift across years — a Class A PT100 holds about ±0.15°C at 0°C. PT1000 (1000 Ω at 0°C) is increasingly chosen over PT100 (100 Ω) for one practical reason: with ten times the base resistance, lead-wire resistance matters ten times less, which simplifies two-wire and longer runs. The wiring trade-offs are in our 2-, 3- and 4-wire RTD guide.

The PT1000 RTD table technicians actually look up

Because this is the reference people search for by name, here it is for the range a chiller works in (IEC 60751, α = 0.00385):

This table targets the "pt1000 rtd table" query that earns 15,670 impressions and currently zero clicks for the site — answering it on a page that also sells the relevant sensor is the capture play.

Why NTC thermistors take the refrigerant side

On refrigerant lines and pipe surfaces, speed and cost win, so NTC thermistors do the work. A standard HVAC-R NTC is 10 kΩ at 25°C with a β of 3435 or 3977 K over −40 to +105°C — the exact window a refrigeration cycle occupies. Where the sensor must clamp to a pipe instead of sitting in a well, an overmoulded part like the MFE1 pipe-clamp NTC gives a fast, sealed reading. The full NTC/PTC/RTD logic is in Article 5; suction-line specifics are in Article 6.

The reading is only as good as the mounting

None of this accuracy survives a bad install. As our analysis of where BAS energy savings vanish argues, the loss usually happens at the sensor, not the chiller. Firm contact, insulation over the tip, sensing on flowing pipe — the mechanics are in our mounting guide and Article 7. The system-level view is in the pillar.

FAQ

Is a chiller sensor an RTD or a thermistor?

Both. Chilled-water supply and return are usually platinum RTDs for accuracy; refrigerant and pipe-surface points are usually NTC thermistors for speed and cost.

What temperature does a chiller produce?

Comfort-cooling chillers typically deliver chilled water at about 42–45°F (6–7°C), reset upward at part load to save energy.

Why does 1°F matter on a chiller?

Compressor energy moves roughly 2–2.5% per 1°F of chilled-water supply temperature, so a sensor error of even a degree or two changes running cost measurably over time.