EKS-HUM-023 / Condensation, measurement and control

Humidity Setpoint

湿度设定值

A humidity setpoint is the desired value of the humidity variable that a controller is instructed to maintain.

Abbreviations: SP
Aliases: humidity control set point, humidity target

QUANTITY, SYMBOL AND UNIT

Quantity / objectDesired value of a humidity controlled variable

Symbolx_sp

UnitNot applicable

Definition

A humidity setpoint is the desired value of the humidity variable that a controller is instructed to maintain.

The setpoint must identify the controlled quantity—for example RH, dew point or humidity ratio—and the sensor location. Controller action, dead band, process lag and disturbances determine how closely the actual condition follows it.

Relations and distinctions

Setpoint and achieved condition

The setpoint is a command or target; the measured condition can differ because of load, control performance and sensor uncertainty.

Setpoint and alarm limit

A control target, switching threshold and alarm threshold can be different values with different functions.

Engineering meaning

State the humidity quantity, unit, sensor location, schedule and acceptable operating band with every setpoint.
Check whether the sensor controls room air, return air, process air or a protected enclosure.

Common misconceptions

MisconceptionSetting 50% RH guarantees that the whole space remains at exactly 50% RH.

Accurate explanationThe controller acts on one measured point and a dynamic process; spatial variation, dead band, lag and uncertainty remain.

Application boundaries

A setpoint is not an equipment-capacity statement or a guaranteed uniform condition.
Do not transfer an RH setpoint to dew-point control without converting the complete air state and control intent.

Supporting sources

ASHRAE / SRC-ASHRAE-F17-CH07

ASHRAE Handbook - Fundamentals, Chapter 7: Fundamentals of Control ↗

2017 IP edition / Chapter 7, control terminology, lines 13–27
Supports: A set point is the desired value of the controlled variable; The controller compares the measured controlled variable with the set point; Humidity can be the controlled variable
National Institute of Standards and Technology / SRC-NIST-HUMIDITY-CAL

Humidity Calibration Services at NIST ↗

Updated 2025-08-14 / Description and calibration-uncertainty sections
Supports: Humidity measurands include RH and dew/frost point; Measurement uncertainty belongs to the interpretation of a humidity reading

Maintenance record