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Intrinsically safe humidity and temperature transmitter HMT370EX
Suitable up to zone 0/20
If you work in hazardous areas, it is important that your devices are "intrinsically safe". This designation confirms that devices and cables cannot emit enough electrical or thermal energy to cause ignition. For decades, people working in hazardous areas have relied on the Vaisala HMT360 to safely and reliably measure humidity and temperature even in Zone 0 areas. Now the intrinsically safe Vaisala HMT370EX can provide the same safe and reliable measurement data with several important new benefits.
Safe measurement in hazardous and potentially explosive areas
The intrinsically safe HMT370EX is the ideal solution for use in hazardous areas up to zones 0 and 20. In addition to measuring relative humidity and temperature, the transmitter also outputs other humidity parameters: Dew point temperature, wet bulb temperature, absolute humidity, mixing ratio, water concentration, water mass fraction, water vapor pressure and enthalpy. Special models are available for measuring moisture in oil and kerosene.

Outstanding properties
- Intrinsically safe (Ex-i)
- Replaceable probes
- Removable probe module
- Model with graphical LC display and model without display
- Vaisala HUMICAP sensor
- Compatible with Vaisala Insight PC software
- Traceable calibration (certificate included)
In addition to relative humidity and temperature, other humidity parameters are also output: Dew point and wet bulb temperature, absolute humidity, mixing ratio, water concentration, water mass fraction, water vapor pressure and enthalpy. Special models are available for measuring moisture in oil and kerosene.
Intrinsically safe
The entire HMT370EX transmitter can be installed directly in potentially explosive atmospheres. It can withstand continuous exposure to potentially explosive environments containing flammable liquids, gases or dust. No additional protective housing is required for operation in any of these environments.
Easily replace probes
The probes can be easily replaced and removed for calibration outside the hazardous area without having to remove the entire transmitter. Attaching new probes requires minimal redesign as the current settings can be restored from the transmitter.
Easy to configure
Output configuration, measurement, calibration and adjustment can be done directly via the optional local display interface. Alternatively, you can connect the transmitter to the Vaisala Insight PC software and configure both the transmitter and the probe together or separately.

Typical applications
Typical applications include fuel storage tanks, chemical, pharmaceutical and food manufacturing, biogas production, paint booths and other industries or processes where intrinsically safe measurement of moisture or moisture in oil is required.
The following current certificates are available for the HMT370EX:
- Europe (ATEX)
- Global (IECEx)
- Japan (CML)
- China (NEPSI)
- Korea (KC Ex-Certificate)
- USA (FM Ex-Certificate)
- USA & Canada (MET-Certificate)
- USA & Canada (MET-Certificate HazLoc)
- Canada (CSA EX-Certificate)
- UK (UKEX)
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Learn more about application examples
Ensuring a safe and high-quality finish
The HMT370EX transmitter can be fully installed in all hazardous or potentially explosive areas including Zone 0. One such environment is paint booths in the automotive industry. Here it is important to measure both relative humidity and temperature to ensure an even and consistent paint finish. However, the solvents used in automotive paints are highly flammable. The HMT370EX is Ex-approved to be intrinsically safe in this environment. The aim is to maintain relative humidity and temperature at a level that ensures a high quality finish.
Another typical application for the HMT370EX is the tablet coating process in pharmaceutical manufacturing. In this process, a fine powder is sprayed to coat tablets with a uniform surface. This type of fine powder can be explosive, so an intrinsically safe device is essential. The HMT370EX monitors humidity and temperature to ensure that the powder does not clump and that the coating is applied smoothly. It also ensures that the quality of the product or its shelf life is not jeopardized by powder that is too moist. In bakeries and flour mills, moisture and temperature measurements are also necessary to ensure the quality of their end products as well as protection against the flammability of flour dust. Similar problems with flammable fine grinding dust also exist in many other industries, for example in furniture making.
From hydrogen cooling to oil and gas applications
Hydrogen-cooled generators must keep the hydrogen very dry to avoid corrosion. Therefore, constant reliable measurement of relative humidity and temperature is just as important as Ex-approved devices, as there is a risk of explosion. Oil and gas rigs also often require intrinsically safe devices, either for specific hazardous areas or to simplify the offshore specification. For example, the HMT370EX is ideal for monitoring flight conditions in offshore helideck operations. For fuel tanks and chemical storage, the HMT370EX offers a special sensor for measuring moisture in oil to prevent losses and maintain product quality.
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What does intrinsically safe mean? What are the advantages of an intrinsically safe product?
Intrinsically safe or Ex-i describes a device that is operated at such a low voltage and low current that it cannot ignite any flammable material. This type of device cannot cause a hazardous event because the energy required is below the level needed for ignition. Intrinsic safety is a globally recognized protection concept.Intrinsically safe devices provide explosion protection in a "natural" way, as they are not inherently capable of storing enough energy to create a spark. This means that they do not require heavy fireproof enclosures, continuous pressurization with compressed air or other cumbersome alternatives that are available to enable the use of inherently unsafe designs in areas where there is a risk of explosion. As a result, they are typically no larger or more complex than general purpose non-Ex devices. - +
Can I mount an intrinsically safe device directly in hazardous areas, or do I also need to take other precautions?
An intrinsically safe device is only completely safe if it is mounted in combination with a device that forms a barrier between the hazardous area and a non-classified (safe) area. For example, power and signal lines entering the danger zone must be protected with an intrinsically safe galvanically insulated barrier or a Zener diode barrier in the safe area. This barrier prevents dangerously high currents or voltages from being transmitted into hazardous areas where they can cause electrical failure of the intrinsically safe device and create a source of ignition. - +
Why do I need safety barriers if the device itself is considered safe?
The design of Ex-i devices only considers the operation of the device itself in certain foreseeable fault situations. The device is designed so that the ignition energy value is not exceeded as long as the characteristic values for voltage, current, capacitance and inductance are not exceeded. The device is of course part of an electrical circuit. This means that the safety of the entire circuit - including the devices located in the safe area - must be taken into account, as dangerously high amounts of electrical energy can accumulate elsewhere in the circuit due to a failure. For this reason, the Ex i device must be separated from the rest of the circuit by a barrier in the safe area. - +
How can I recognize an intrinsically safe product?
The Ex mark on a product always contains the type of protection offered. The "i" in Ex i indicates that a product is intrinsically safe. Usually, the symbol also includes another letter representing the severity of the hazardous area for which it is suitable. For example, the Vaisala HMT360 humidity and temperature transmitter is marked "ia". This means that it can be used in environments where flammable gas or dust mixtures are constantly present (zone 0/20). Alternatively, "ib" would indicate zone 1/21, in which explosion hazards can only occur periodically and for a short time; "ic" indicates the lowest hazard zone 2/22, in which explosion hazards can only occur under abnormal conditions. - +
What improvements are there compared to the predecessor model HMT360?
Improvements to the proven design
Unlike the HMT360, the HMT370EX is equipped with a graphical display with data and trend display functions, making it much easier to monitor temperature and relative humidity over time. It also has a new anodized surface that offers better corrosion resistance - a particular advantage for offshore applications. For applications with dust, the housing of the HMT370EX is designed so that no dust cover is required. Laser markings have replaced adhesive labels, providing an important safety improvement as safety and identification information can no longer become illegible. The HMT370EX is also compatible with the free Vaisala Insight PC software. This facilitates configuration and calibration. The HMT370EX features the latest materials and components, including the state-of-the-art HUMICAP® R2 sensor for long-term stable measurements and corrosion resistance. It also offers improved moisture measurement performance. The robust design has been extensively tested to ensure a long product life.
Wide range of probe options for different applications
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HMP371 - for wall mounting
HMP371 probe with stainless steel mesh filter
Temperature range: from -40 °C to +60 °C Probe diameter: 12 mm - +
HMP373
- für beengte Bereiche
Thanks to its small design, the HMP373 probe is suitable for areas that are difficult to access: The picture shows the version with Teflon-insulated cable.
Temperature application range with Teflon cable: from -40 °C to +60 °C Temperature range with rubber cable: from -40 °C to +80 °C Probe diameter: 12 mm Length of the probe cable: 2, 5 or 10 m - +
HMP374
- für Hochdruck-Rohrleitungen
The HMP374 probe can be installed directly in high-pressure pipelines or vacuum chambers.
Temperature application range: from -70 °C to +180 °C Pressure application range: from 0 to 10 MPa Length of the probe cable: 2, 5 or 10 m Probe diameter: 12 mm
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HMP375
- für hohe Temperaturen
The HMP375 is designed for use at high process temperatures.
Temperature range: from -70 °C to +180 °C Length of the probe cable: 2, 5 or 10 m Probe diameter: 13.5 mm - +
HMP377
- für hohe Feuchte
The HMP377 is designed for use in high process humidity.
Temperaturbereich: von –70 °C bis +180 °C Länge des Sondenkabels: 2, 5 oder 10 m Sondendurchmesser: 12 mm - +
HMP378 -
für Druckleitungen
The HMP378 can be flexibly installed in pressure lines thanks to the ball valve installation kit.
Temperaturbereich: von –70 °C bis +180 °C Druckeinsatzbereich: 0 bis 4 MPa Länge des Sondenkabels: 2, 5 oder 10 m Vefügbare Fühlerlängen: 226 mm/448 mm Sondendurchmesser: 12 mm / 13,5 mm
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