SNMP Humidity & Airflow OIDs (ENTITY-SENSOR Reference)

By the SNMP Monitoring team · Reviewed July 2026

Humidity and airflow are two more physical quantities the ENTITY-SENSOR-MIB types for you: relative humidity as percentRH, airflow as cmm (cubic metres per minute). This page is the paste-ready reference for both, plus the reality that most real-world humidity comes from a room probe on a vendor environmental gateway rather than the generic table. As with every physical sensor, the raw integer means nothing until you apply its scale.

The "how do I set humidity alerts for the server room" side is the twin: monitor temp & humidity. Hubs: sensors, all OIDs.

ENTITY-SENSOR humidity & airflow types

Both are entries in the sensor table at 1.3.6.1.2.1.99.1.1.1, distinguished by entPhySensorType:

They read exactly like temperature or voltage: the value at entPhySensorValue (1.3.6.1.2.1.99.1.1.1.4), the multiplier at entPhySensorScale (.2), decimals at entPhySensorPrecision (.3), and status at entPhySensorOperStatus (.5). Humidity is a percentage 0–100; airflow in cmm is a volumetric rate, most relevant on gear that reports internal or aisle airflow.

Environmental gateway MIBs

Here's the practical caveat: while ENTITY-SENSOR can type humidity as percentRH, most server-room humidity readings actually come from a dedicated environmental gateway — an appliance from vendors like APC NetBotz, HWg, or Sensaphone — that publishes its probes through its own enterprise MIB under 1.3.6.1.4.1. You poll the gateway, not the probe. So on a lot of real deployments the humidity OID you end up using is a vendor leaf, not the generic ENTITY-SENSOR object.

The exact probe leaf differs per vendor, so this reference doesn't invent one — learn the pattern here and pull the precise objects from the device pages for your gateway. The generic ENTITY-SENSOR path still matters as the fallback and as the way network gear with a built-in humidity sensor reports it.

There's a subtlety in how those gateways structure humidity that's worth knowing before you walk one. A single environmental gateway typically hosts several probes, and each probe can carry more than one measurement — a combined temp/humidity probe reports both from one physical unit. So the vendor MIB usually presents a sensor table indexed per probe (and sometimes per measurement within a probe), not a single scalar humidity OID. That means the practical first step on any new gateway is to walk its whole sensor subtree, read the descriptions, and map which index is humidity on which probe in which location — rack inlet, room ambient, under-floor — before you graph anything. Skipping that mapping is how you end up alerting on the wrong probe, or trending "humidity" that's actually the reading from a sensor in a different part of the room. Do the mapping once at setup and label the indexes, exactly as you would for the temperature sensors that usually share the same gateway.

Applying scale

Same decode as any physical sensor:

1. Read entPhySensorType (.1) — confirm percentRH or cmm.
2. Read entPhySensorScale (.2) — the SI multiplier.
3. Read entPhySensorPrecision (.3) — decimal places.
4. Read entPhySensorValue (.4) — the raw integer.
5. Apply: real value = entPhySensorValue × 10^(−precision), in the scale's unit.

A humidity value of 472 at precision 1 decodes to 47.2 %RH; at precision 0 it would be a nonsensical 472% — again the classic skip-the-decode bug. Room probes commonly report tenths, so watch the precision.

Worked snmpwalk example

Walk the sensor table (or the gateway's vendor subtree) for humidity:

snmpwalk -v2c -c <RO_string> <host> 1.3.6.1.2.1.99.1.1.1

ENTITY-SENSOR-MIB::entPhySensorType.30 = INTEGER: percentRH(9)
ENTITY-SENSOR-MIB::entPhySensorScale.30 = INTEGER: units(9)
ENTITY-SENSOR-MIB::entPhySensorPrecision.30 = INTEGER: 1
ENTITY-SENSOR-MIB::entPhySensorValue.30 = INTEGER: 472

Sensor .30 is humidity: 472 × 10^(−1) = 47.2 %RH. On a vendor gateway you'd instead walk its enterprise arc, where a humidity object might read 472 and mean the same thing once you check the MIB's scale. <RO_string> is a placeholder; keep live communities out of committed scripts.

Airflow specifics

Airflow typed as cmm (cubic metres per minute) is less common than humidity but shows up on gear that measures internal or aisle airflow — some chassis, some environmental probes. It matters for hot-aisle/cold-aisle containment: a drop in airflow through a rack means restricted cooling, even if the temperature hasn't climbed yet. Read it the same way (value × scale), and treat a falling airflow reading as an early warning that pairs naturally with rising temperature and fan speed. Because airflow support is patchy across gear, confirm the sensor exists and its unit before building a graph on it.

How to monitor humidity (how-to)

Reference stops here. The alerting logic — the ASHRAE-style bands, the two-sided floor/ceiling for condensation and static, sustained-vs-spike — is the twin: monitor temperature & humidity via SNMP. Related: temperature OIDs, device pages, the OID catalog, and glossary.

Frequently asked questions

What OID is humidity in SNMP?

The portable one is entPhySensorValue (1.3.6.1.2.1.99.1.1.1.4) from the ENTITY-SENSOR-MIB, for any sensor whose entPhySensorType is percentRH. In practice, most server-room humidity comes from a vendor environmental gateway's enterprise MIB under 1.3.6.1.4.1, which you poll instead of the probe directly.

What unit is airflow reported in?

The ENTITY-SENSOR type for airflow is cmm — cubic metres per minute, a volumetric flow rate. It's read like any other sensor (value with scale applied) and is most useful for hot-aisle/cold-aisle context, where falling airflow signals restricted cooling before temperature rises.

What MIB has humidity sensors?

The standard ENTITY-SENSOR-MIB (1.3.6.1.2.1.99) types humidity as percentRH. But most room humidity is delivered through a vendor environmental gateway — APC NetBotz, HWg, Sensaphone — via its own enterprise MIB under 1.3.6.1.4.1, so the OID you use is often vendor-specific.

How do I read a room probe?

Point your manager at the environmental gateway the probe plugs into, walk its vendor subtree (or the ENTITY-SENSOR table if it uses the standard objects), and apply the sensor's scale and precision to the raw value. Load the vendor MIB so names and units resolve rather than reading bare integers.

Key takeaways

• Humidity = percentRH, airflow = cmm — both ENTITY-SENSOR types read from entPhySensorValue (1.3.6.1.2.1.99.1.1.1.4).
• Apply scale/precision — a 472 at precision 1 is 47.2 %RH, not 472%.
• Most real room humidity comes from a vendor environmental gateway MIB under 1.3.6.1.4.1, not the generic table.
• Airflow (cmm) is patchy across gear — confirm it exists; falling airflow is an early cooling warning.
• Vendor probe leaves are device-specific — read the vendor MIB, don't invent.
• Procedure: temp & humidity how-to; catalog: sensors/all.