Humidity is underestimated as a threat to infrastructure. Everyone knows about temperature and its risks, but many are unaware that humidity is an equally significant one.

To address it we designed a reliable, calibratable relative humidity and temperature sensor.

The humidity component inside the sensor is made in Switzerland and is tested against several national standards.

Most digital relative humidity sensors on the market are not calibratable. When they lose accuracy you are forced to buy a replacement — without knowing whether the replacement is accurate either.

• Recalibration instead of replacement: our in-house lab issues calibration certificates for both new and already deployed sensors.


• NIST traceable: humidity certificates comply with NIST traceability requirements. ISO 17025 certificates are available for temperature.


• Trade-in programme: each year we ship a newly calibrated sensor, so the deployed measurement stays both accurate and precise.

Although our sensors are factory calibrated, any rH sensor does drift over time.

• Why it drifts: a humidity probe has to breathe the air it measures, which brings it into contact with contaminants and pollutants that gradually affect its accuracy.


• Accuracy, not precision: a drifted sensor still returns consistent readings — they are simply consistently wrong. Nothing in the data itself reveals the problem.


• Once a year: indoor sensors should be calibrated at least annually.

There are many humidity sensors on the market and understanding the differences matters when choosing one for your application. The selection criteria are precision, accuracy, resolution, calibratability and dew point measurement.

Precision

Precision is fundamentally different from accuracy: it is the degree to which repeated measurements under unchanged conditions produce the same result — reproducibility and repeatability. Very few vendors publish anything about it, although it matters a great deal. Testing precision requires scientific instrumentation. We built a lab with certified climate chambers that reproduce specific conditions, which lets us test a sensor's precision and issue a certificate for it. Almost no other vendor has that capability; at best they refer you to a third party lab.

Accuracy

Accuracy is the proximity of a measurement to the true value. The two are independent: in the illustration the readings are tightly grouped, so precision is good, but they sit well away from the centre — accuracy is poor. A precise instrument is not necessarily an accurate one.

The true value is maintained by international standards organisations. In the US that is the National Institute of Standards and Technology, whose Thermodynamic Metrology Group holds the national standards for humidity. We verify sensors in our lab against those standards, following an unbroken chain of documentation and incorporating the uncertainties. Using ISO certified and NIST traceable instrumentation means our tests cover both precision and accuracy — how close the readings sit to the true value, uncertainty included.

Resolution

Resolution is the smallest change in a measured value the sensor can detect — neither precision nor accuracy, but granularity. It is also called sensitivity. Our temperature and humidity sensors resolve to 0.1% relative humidity, so any change greater than 0.1% is detected and reported.

Calibratability

Unlike ours, most digital relative humidity sensors cannot be calibrated. Once accuracy is lost, the only option is a new sensor — with no way of knowing whether that one is accurate. If accurate humidity measurement is central to the project, insist on a calibrated sensor with a certificate, and choose a vendor able to calibrate its own sensors so replacement is not the only path.

Dew Point

Allowable humidity ranges are expressed in relative humidity, but for high availability configurations the dew point is what should be monitored rather than rH alone. See the ASHRAE tips for the recommended envelope.

ASHRAE defines three humidity and temperature ranges for a data center: recommended, allowable and long exposure. For high availability the recommended range is the one to follow.

Too much humidity

Most servers tolerate non-operating humidity around 90%, but operating tolerances are considerably lower. Moisture in the air is not in itself a problem — condensing humidity is. Water vapor turning into droplets can short out a server as quickly as spraying it with a hose.

Too little humidity

Low relative humidity allows static electricity to build up, and a single ESD discharge from touching equipment can cause permanent damage. The best practice for mitigating this is ESD rated floor tiles. Research by the University of Missouri with ASHRAE showed that with those in place the humidity envelope can go as low as 8%, rather than the current 40% lower limit, depending on equipment class — a revision ASHRAE is expected to make official, and one that would open the door to open air cooling. The common practice of wearing ESD shoes was shown to have little to no effect.

Dew point & why continuous monitoring matters

Humidity conditions are directly tied to room temperature. Water vapor begins to condense once the ambient temperature reaches the dew point, and because temperatures vary widely between day and night, a computer room's dew point changes regularly. Normal operation can turn into a condensation event in minutes, which is the argument for 24/7 monitoring of relative humidity rather than periodic checks.

Any humidity probe loses accuracy over time while retaining its precision, because it breathes the air it measures and meets contaminants and pollutants along the way. Calibrate at least once a year for indoor use — our lab issues certificates meeting NIST traceability requirements.

Need to connect multiple temperature & humidity sensors? A base unit supports up to 8 sensors with the optional Sensor Hub. Beyond 8, the daisy chained version connects a whole string of temperature & humidity sensors to a single base unit port.

More on daisy chained sensors

The effect of humidity on equipment is well known. Its effect on the transmission of viruses is less so.

Ventilation is key to bringing fresh air in, but outdoor humidity levels have to be taken into account with it — particularly during a pandemic or flu season.

Relative humidity below 40% or above 60% increases the infectivity rate of viruses. In other words, the humidity level in the air makes it easier for viruses to infect people.

• Rack level: the intake temperature and humidity at the equipment is what determines its operating conditions, and it differs from the room average.


• Ambient: room level measurement establishes the baseline the cooling system is working against.


• Multiple heights: conditions at the top of a loaded rack differ substantially from the bottom.


• Away from direct discharge: mounting immediately in a CRAC discharge reports the supply air rather than the conditions the equipment experiences.

Application Guide

The ENV-THUM connects to the base unit, which makes it an Ethernet sensor. The base unit is a standalone device with its own IP address in your network. On its own it monitors the sensor and triggers alerts via email, SNMP or optional SMS when needed.

There is no software or PC required for the sensor to run.

With our optional Monitoring Software you can centrally monitor sensors with trending and more. This is optional and not required.