Although analog probes and data collection technology have a lot to offer, they also present several risks. Modbus is a digital communication protocol that overcomes many of these issues. In this first blog post in a series of three about Modbus, we introduce this established fieldbus protocol, discuss its benefits, and explore how it is implemented in Vaisala devices.

What is Modbus?
Modbus was developed by Modicon in 1979 and is one of the oldest fieldbus protocols available. The initial intention was to create an open protocol; thanks to the success of this strategy Modbus is now seen as an industry-standard protocol and supported by most programmable logic controller (PLC) brands because no licenses or proprietary hardware are needed and it’s free to implement a control system. Vaisala products use either Modbus RTU, which is built on the RS-232 or RS-485 protocol or Modbus TCP/IP, which uses an Ethernet network. One potential downside of Modbus especially in serial line communication (RS-485) is that if you have a multi-vendor configuration that uses instruments from different manufacturers in the same network, there may be some compatibility issues between the different brands. 

The Vaisala Indigo201 analog transmitter
The Vaisala Indigo platform is used extensively in a wide range of challenging industrial applications. The product consists of an analog data transmitter attached to an intelligent, interchangeable probe that measures variables such as relative humidity and temperature, dew point temperature, carbon dioxide or hydrogen peroxide concentration. All measurements and calculations are performed digitally by the Indigo compatible probe, which is also capable of operating as a standalone unit without a transmitter. The probe passes the data on to the transmitter, which then sends it to a PLC in analog form.

The issue with analog signal transmission
Typically, a digital humidity probe, in this case, a Vaisala HMP3, has two sensors – one that measures temperature, such as a PT100 sensor, and one for relative humidity, for example, a Vaisala HUMICAP^® sensor; in reality, these sensors are actually measuring changes in resistance and capacitance respectively. The probe converts the analog signals into digital data, makes the necessary calculations for temperature and relative humidity, and sends the accurate digital data to the Indigo201 analog transmitter. The transmitter then converts this signal back into analog form to be transmitted via analog cabling to the PLC, where it is once again converted to digital data for processing and storage.

As might be expected, converting the digital data into an analog form in the data transmitter and then back to digital in the PLC or other control system will introduce additional error sources. Ensuring accurate measurements is important from both a quality and regulatory perspective, and one way to minimize errors is to use digital communication. This is where Modbus comes in.

The advantages of Modbus 
The same probes that can be used with the Indigo201 are also compatible with the Modbus RTU digital communication protocol. Because Modbus uses digital communication, in addition to relative humidity and temperature the probe can also be used to measure many other parameters including dew point/frost point temperature, absolute humidity, mixing ratio, wet-bulb temperature, and enthalpy, as well as additional information such as device status, serial number, and security hash. Allowing the Modbus master to check the security hash status allows you to ensure that the sensor hasn’t been tampered with and that the probe configuration has not been altered. It’s also possible to connect multiple probes to the same network; with an analog system, this would require a substantial amount of additional cabling and (I/O) modules to be installed in the PLC system. Modbus also reduces the risk of data errors by minimizing conversions between analog and digital, making it a great option for digital data transmission.

Transmitter is not just an analog to digital protocol converter 
Although the Indigo compatible probes can be used as standalone measurement instruments, the best usability is often achieved together with a transmitter that offers a local user interface, display, different power supply options, and some of them with the advantages of digital communication. The Indigo202 transmitter provides the same Modbus RTU as the probe and the Indigo500 series transmitters communicate with Modbus TCP/IP-protocol. 

To learn more, watch the Modbus 101 webinar or or Contact us.
 

From the automotive industry to food production and from pharmaceuticals to steel or chemicals production, humidity measurement is a key part of many industrial processes. With potentially hundreds or even thousands of measurement points needed, ensuring stable, precise, and reliable measurement is extremely important.

This becomes more difficult in environments with high relative humidity (over 90% RH). In high relative humidity, all objects, including humidity sensors, are likely to be at a temperature close to saturation. In such an environment the vapor pressure gradient is not sufficient for effective evaporation of liquid water from the sensor surface. The sensor may stay wet for a several minutes or more, causing a prolonged measurement outage even after the environment is no longer saturated.

Modular probes and transmitters

In order to ensure stable and reliable measurement for tough industrial environments, Vaisala created the Indigo500 series of top-of-the-line, modular transmitters that pair with our industry-leading measurement probes. Solutions are available for all industrial and even outdoor measurement applications, including humidity probes with Vaisala warmed probe technology for reliable and accurate measurement even in environments with 100% relative humidity. The Indigo500 series also supports probes for temperature, dew point, moisture in oil, carbon dioxide (CO₂), and vaporized hydrogen peroxide (H₂O₂) measurement, offering users a great deal of measurement flexibility. 

Whatever the application, all Indigo probes are extremely accurate and reliable with excellent long-term stability, allowing you to detect even the weakest signals to make better and more timely decisions based on accurate data. 

Introducing the Indigo510 

The first product in the Indigo500 range was the Indigo520, which offers the widest range of customization options and features, including support for two probes. It was designed for use in tough industrial environments, with a highly robust metal casing that has been proven in reference cases worldwide. For customers that don’t require the full range of functionality offered by the Indigo520 and who only need one probe, we now offer an equally robust alternative. 

The new Indigo510 builds on the same platform and includes the same metal casing, software, display, and modularity as our flagship Indigo520 product. The features include modular support for a single Vaisala probe (meaning the probe type can be changed if needed), DC input, and two analog outputs. While the build quality and industry-leading measurement capability remain the same, the reduced feature set translates into a more streamlined price point. This makes the Indigo510 ideal for applications where precise and reliable measurement is needed for a single variable (like humidity) even under harsh conditions. Indigo510 customers will also benefit from regular platform updates and software improvements for their transmitter. 

Combined with Vaisala Insight software, the Indigo series of transmitters and probes provides a solid ecosystem to ensure energy efficiency, safety, and end-product quality in your operations.  

Read more about the Indigo series or contact us for more information.