1. Physical measurement – Probe head 

Both the design concepts of the HMT330 and Indigo smart probes are built on Vaisala’s proven HUMICAP® capacitive thin-film polymer sensor technology. The probe head structures, filters, and installation accessories are fully compatible, which means that, for example, the HMP5 humidity probe fits the same process connection as the HMT335 probe.

2. Signal conditioning 

Vaisala HUMICAP® is a capacitive thin-film polymer sensor and it is always accompanied by resistive temperature measurement. These electrical quantities must be properly conditioned in order to obtain a high-quality measurement signal. In the HMT330 platform this conditioning is performed inside the transmitter housing. This means that the probe is a permanent part of the transmitter and cannot be removed without compromising measurement reliability. 

Signal conditioning in Indigo smart probes happens in the probe body, and therefore it is not tied to the transmitter.

3. Analog-digital converter 

Conditioned analog signals are converted into digital format. To reveal the physical quantities being measured, the measurement signals must be further processed by adding various factors, such as linearization, pressure model, and calibration factors, etc. These physical quantities can be, for example, relative humidity and temperature or a calculated humidity parameter, such as dew point temperature. 

Analog-digital conversion in Indigo smart probes takes place in the probe body, and therefore these probes can be used independently without the need for a separate transmitter. Measurement readings from the standalone probes are available in digital Modbus RTU format.

4. HMI – Human-Machine interface 

The need for a local display and interactivity with the instrument depends on the specific application. The user interface can often be a valuable tool, for example, in the case of a process failure or if there is a need for local troubleshooting. 

The HMT330-series transmitters can be ordered with or without a local user interface. The Indigo platform offers the choice of either a standalone smart probe without a local user interface or connection to an Indigo transmitter.

5. M2M – Machine-to-machine communication 

Often these measurements are used for process control. The system interface can be either an analog signal, for example 4 ... 20 mA, 0 ... 10 V, or digital, for example Modbus RTU. 

The output of a standalone Indigo probe is limited to Modbus RTU only, but the interface selection can be extended by connecting it to an Indigo transmitter. For example, the Indigo520 transmitter offers the same system interfaces as the HMT330 in addition to new interface options.