Specific conditions of use - HMT370EX

HMT370EX User Guide

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User guide
CAUTION With the installation of the equipment in Zone 0 Group II area it has to be ensured that sparks due to impact or friction do not occur.

Wiring requirements

  • The cable glands and cables used for wiring the device must not impair the Ex protection.
    • Unused lead-throughs must be sealed using Ex compliant plugs.
  • Select a strain relief option that suits the application (either use cable glands that include strain relief or install separate clamps: see IEC 60079-14).
CAUTION Connect only de-energized wires. Never switch on the power supply input before completing the wiring and closing the transmitter body.

Intrinsic safety

The overvoltage category of HMT370EX transmitters is I (non-mains equipment), and ambient pollution degree is 4, as specified in IEC 60664-1. For intrinsically safe input parameters, see Table 1 .

Table 1. Intrinsically safe input parameters
Parameter Value Associated apparatus entity parameters
Ui 28 VDC Uo ≤ Ui
Ii 100 mA Io ≤ Ii
Pi 700 mW Po ≤ Pi
Ci 12.1 nF Co ≥ Ci + Ccable
Li 16 µH Lo ≥ Li+ Lcable

Using analog output test points

There are test points for measuring the voltages and currents of the analog outputs, located above each screw terminal block as shown in Figure 1. Accessing the test points on the component board requires opening the transmitter enclosure.
Figure 1. Location of test points and service port
Multimeter test points for analog output channels 1 and 2
Service port for PC connection

The transmitter body enclosure must not be opened in an explosion hazardous area, unless a safe work permit has been issued in accordance with the standard IEC 60079-14. Either remove the transmitter from the hazardous area before opening the enclosure, or ensure that an IEC 60079-14 compliant safe work procedure has been implemented in the hazardous area.

Use an intrinsically safe multimeter that will not cause the intrinsically safe input parameters listed in Table 1 to be exceeded when it is connected in series (current measurement) or parallel (voltage measurement) to the associated apparatus.

Using the service port

The service port (see Figure 1) must only be used in a safe area. Either remove the transmitter from the hazardous area or ensure that a safe work procedure has been implemented in the hazardous area. Only use the Vaisala accessory PC connection cable with the service port.


The probe (includes probe body and head) can be detached and replaced by the user. The probe head filter (see Figure 1) is also user-replaceable. For other maintenance requirements, contact Vaisala.

CAUTION The probe can be detached and changed when HMT370EX is powered. Any other live maintenance, including changing the probe head filter, is not allowed.
CAUTION To maintain the required Ex protection, inspect the equipment for signs of deterioration at an interval of 6 months. If maintenance requirements are found, contact Vaisala.

Installation restrictions

CAUTION To maintain the required Ex protection, do not install the transmitter model with an LCD display in a location where it is exposed to direct sunlight.
The content in this chapter is maintained in the following separately tracked document:
Document ID: M212506EN Revision: E (9 May 2023)