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Cisco Meraki

MT Psychrometric Chart


MT Psychrometric Readings Charts introduces a new way for a user to understand the current temperature/humidity status on an aggregate view for across all sensors in a particular network.

There are 2 places where a user can see an Psychrometric Readings Chart.

  1. Environmental Overview Page - Shows collected data of the current metrics across all MT10 and MT11

  2. Sensor Details Page - Plots the data point of the latest metric on an individual basis

Basics of Psychrometric Readings Chart

The Psychrometric Readings Chart is based off of a psychrometric chart. Psychrometric charts plot wet bulb and dry bulb data for air-water vapor mixtures at atmospheric pressure.

Here’s an example of a simple psychrometric chart with the assumption of standard air pressure at sea level.

Explaining the Chart

  • The x-axis is dry bulb temperature(in °C or °F ).

  • The y-axis is the humidity ratio(in gram moisture/kg dry air or grain* moisture/lb dry air)

  • The curved line is a relationship between dry bulb temperature and humidity ratio is plotted for air at various relative humidity levels: 20%, 40%, 60%, 80%, and 100%.

  • For the purpose of the Psychrometric Readings chart we will be excluding the Wet Bulb temperature as it doesn’t affect the energy envelope. 

*Grain is an imperial unit and 1 pound = 7000 grain. This unit is chosen for better scalability of the chart. 


  • Dry Bulb temperature - The dry-bulb temperature is the temperature of air measured by a thermometer freely exposed to the air, but shielded from radiation and moisture. MT10 records dry bulb temperature

  • Wet Bulb Temperature - The wet-bulb temperature is defined as the temperature of a parcel of air cooled to saturation (100% relative humidity) by the evaporation of water into it, with the latent heat supplied by the parcel. For the purpose of this feature, wet-bulb temperature is ignored for the sake of simplicity.

  • Humidity Ratio - Humidity ratio is the ratio of weight of moisture to the weight of dry air in the air–vapor mixture. Its unit is gram vapor per KG dry air.

  • Psychrometric chart - The most basic definition of psychrometric chart is that it plots wet bulb and dry bulb data for air-water vapor mixtures at a constant atmospheric pressure.

Reading the Chart

With the MT10 Temperature/Humidity Sensor, the dashboard collects a data point for Dry Bulb temperature and the relative humidity of the area. Based on that, the air’s mixture ratio can be determined.

Example: Suppose an MT10 is reporting a temperature of 70° F or 21° C, and 40% humidity. Find the 40% relative humidity line, then follow it along to 70° F on the x-axis, and plot a point there. The y-value of that point is the mixture ratio (about 6.29 gm vapor/kg dry air).

Understanding the Envelopes

Along with the data points from all sensors across the network, the Psychrometric Readings chart also has 4 envelopes defined. These envelopes are based on the recommendation of ASHRAE standards for maintaining the temperature and humidity conditions of different types of Data Centers. Below are the 4 recommended ranges:

  • A1[15-32°C, 20-80% RH] - Typically a data center with tightly controlled environmental parameters

  • A2[10-35°C, 20-80% RH] - Typically an information technology space with some control of environmental parameters

  • A3[5-40°C, 8-85% RH] - Typically a storage room with backup drives

  • A4[4-45°C, 8-90% RH] - Least environmentally sensitive areas.

Customizing the Chart and Envelopes

The Psychrometric Readings Chart has a default setting to show all MT10 sensors across the network and the Envelope defaults to A2 type. The dashboard also allows users to customize the chart using the Chart settings button under the 3 dot menu of the chart


The following settings can be customized by the user

  1. Visible Tags - The user has the ability to only show certain sets of sensors using the Tags option. For example, in a DC all racks have an MT10 near the Inlet of the rack. Scoping the Visible Tag to only those sensors via Tags can give a user a good idea of how warm/dry the air is in the hot aisle of the racks.

  2. Psychrometric Readings Envelope - Here the user can change the envelope as explained in the section above. Selecting the appropriate envelope can allow users to get an allowable range for controlling the environmental condition of the area.


Now once a datapoint from an MT sensor is plotted on the Psychrometric Readings chart, the dashboard also determines a Conformance score which is essentially a percentage of sensors adhering to the range set by the envelope.


For example, out of 100 sensors deployed in an IDF, if 2 of the sensors are recording unusually warm or cold temperatures, the Dashboard will mark those 3 sensors are Non Conforming and the Conformance Score will drop to 98%

Clicking on the Non Conformance button will navigate to a predefined filter of the Sensors that are non conforming. Using this data, a user can determine exactly which sensors are out of the required variance at a glance.

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