---
description: Read the sensors of a server through its BMC — how the SDR repository is walked, selecting sensors, and the Sensor object with its reading, thresholds, status and states.
date_published: 2026-10-14
date_modified: 2026-10-14
canonical_url: https://metricshub.org/ipmi-java/sensors.html
---

# Sensors

On this Page

- [How the sensors are read](#how-the-sensors-are-read) 
  
    - [Selecting sensors](#selecting-sensors)
- [The Sensor object](#the-sensor-object) 
  
    - [Readings](#readings)
    - [States](#states)

The BMC describes its sensors in the **Sensor Data Record (SDR) repository**: one record per sensor, with its name, what it measures (the *entity*: a processor, a power supply, the system board, …), its unit, the formula that converts its raw reading, and its thresholds. The client walks this repository and reads the sensors that have a reading.

```java
try (IpmiClient bmc = IpmiClient.builder(host).credentials(user, password).build()) {
	for (Sensor sensor : bmc.sensors().execute()) {
		System.out.println(sensor);
	}
}
```

## How the sensors are read

[sensors().execute()](apidocs/org/metricshub/ipmi/client/SensorsRequest.html):

1. walks the SDR repository from the first record to the last with **Get SDR**, reading large records in chunks when the BMC cannot return them at once, and reserving the repository (**Reserve SDR Repository**) when the BMC requires it or cancels the reservation; the walk is done once per client and shared with `frus()` ([SDR cache](configuration.html#sdr-cache));
2. sends **Get Sensor Reading** for every selected **Full Sensor** and **Compact Sensor** record.

Event-Only records (sensors without a reading), the locator and association records, and the OEM records are part of the walk but are not returned. A record the library cannot decode is skipped and listed in `skippedRecords()`; the walk goes on ([Supported Commands](supported-commands.html#sdr-records)).

The walk reads only the sensors of the **BMC's** SDR repository, through the BMC itself: sensors owned by satellite controllers that the BMC does not bridge, and the Device SDRs of other controllers, are not read ([#84](https://github.com/metricshub/ipmi-java/issues/84), [#105](https://github.com/metricshub/ipmi-java/issues/105)).

### Selecting sensors

The selection is applied **before** the readings are requested, so that an unselected sensor costs no round trip:

```java
// Only the temperature and fan sensors: one Get Sensor Reading per selected sensor
SensorList sensors = bmc.sensors().types(SensorType.Temperature, SensorType.Fan).execute();

// By name, case-insensitively: * matches any characters, ? one character
SensorList cpuTemperatures = bmc.sensors().names("CPU? Temp", "*DTS*").execute();

// With a deadline of its own
SensorList all = bmc.sensors().timeout(Duration.ofSeconds(30)).execute();
```

## The `Sensor` object

Each [Sensor](apidocs/org/metricshub/ipmi/client/Sensor.html) is immutable and holds:

| Method | Content |
| --- | --- |
| `name()` | The sensor name of the SDR record (`CPU1 Temp`, `PSU2 Present`, …) |
| `number()` | The sensor number |
| `type()` | What it measures or monitors: [SensorType](apidocs/org/metricshub/ipmi/core/coding/commands/sdr/record/SensorType.html) (`Temperature`, `Voltage`, `Fan`, `PowerSupply`, …) |
| `entity()`, `entityInstance()` | The entity the sensor belongs to: its type (`EntityId.Processor`, `EntityId.PowerSupply`, …) and instance number |
| `unit()` | The [SensorUnit](apidocs/org/metricshub/ipmi/core/coding/commands/sdr/record/SensorUnit.html) of the reading and thresholds |
| `reading()` | The converted reading of a threshold sensor (`OptionalDouble`), empty for a discrete sensor or when the reading is unavailable |
| `rawReading()` | The reading byte as the BMC sent it, before conversion |
| `lowerNonCritical()` … `upperNonRecoverable()` | The six thresholds, converted like the reading; empty when the record does not define them, so that a threshold of `0` is a threshold |
| `status()` | [SensorStatus](apidocs/org/metricshub/ipmi/client/SensorStatus.html): `OK`, `NON_CRITICAL`, `CRITICAL` or `NON_RECOVERABLE` for a threshold sensor, against its thresholds; `OK` for a discrete sensor that answers; `UNAVAILABLE` when the reading is unavailable |
| `states()` | The asserted states of a discrete sensor, in the wording of `ipmiutil` (of `ipmitool` for the few states `ipmiutil` does not name), each once |
| `rawStates()` | The same, as [ReadingType](apidocs/org/metricshub/ipmi/core/coding/commands/sdr/record/ReadingType.html)s |
| `problem()` | Why the sensor has no reading and no states: the BMC refused the reading (with its completion code), the reading is not valid yet, or scanning is disabled; empty otherwise |
| `record()`, `readingData()` | The decoded record ([FullSensorRecord](apidocs/org/metricshub/ipmi/core/coding/commands/sdr/record/FullSensorRecord.html) or [CompactSensorRecord](apidocs/org/metricshub/ipmi/core/coding/commands/sdr/record/CompactSensorRecord.html)) and the [GetSensorReadingResponseData](apidocs/org/metricshub/ipmi/core/coding/commands/sdr/GetSensorReadingResponseData.html), for everything else |

[SensorList](apidocs/org/metricshub/ipmi/client/SensorList.html), the result of `execute()`, is an unmodifiable `List<Sensor>` that also gives the SDR records the walk skipped (`skippedRecords()`).

### Readings

For a Full Sensor record, the reading is converted to the sensor's unit with the record's linear formula (`M`, `B` and the exponents of IPMI 2.0, section 36.3), linearization included:

```java
for (Sensor sensor : bmc.sensors().execute()) {
	sensor.reading().ifPresent(value -> System.out.println(sensor.name() + " = " + value + " " + sensor.unit()
			+ " (upper critical: " + sensor.upperCritical() + ", " + sensor.status() + ")"));
}
```

```text
Ambient Temp = 17.0 DegreesC (upper critical: OptionalDouble[39.0], OK)
System Power = 92.0 Watts (upper critical: OptionalDouble.empty, OK)
Fan 1 = 6600.0 Rpm (upper critical: OptionalDouble.empty, OK)
System 3.3V = 3.38 Volts (upper critical: OptionalDouble[3.56], OK)
```

`reading()` is empty when the BMC flags the reading it does not vouch for (unavailable, scanning disabled), when the record has no analog reading, or when it refused the reading: `problem()` says which.

> **Note**
>
> A Compact record that describes several shared sensors is reported as a single sensor ([#100](https://github.com/metricshub/ipmi-java/issues/100)).

### States

Discrete sensors (presence, redundancy, power supply status, processor status, …) report a set of asserted **states** instead of a value. `states()` returns the asserted states that have a description in the wording of `ipmiutil`:

```text
PSU Redundancy: [Fully Redundant]
PSU1 Present: [Presence detected]
CPU0_Status: [Presence detected]
```

The states of a threshold sensor are the thresholds its reading is at or beyond. For OEM sensors (event/reading types `0x70` to `0x7F`), whose states the specification does not define, the single state is the raw value of the state bytes: `0xHHLL` (`0xLL` when the BMC returns a single state byte). An OEM sensor with no state asserted reports no state, like any discrete sensor.