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Dew Point Transmitter (608 Series, RS485)
A digital dew point transmitter that reports moisture in gas over RS485 Modbus and logs it on board, so several units can sit on one bus and keep their data through a power loss. Stainless steel probe, four readings from one sensor, ultra-low power.
- Output: RS485 / Modbus-RTU (digital, multi-drop)
- Logging: 65,000 records on board, battery-backed
- Reads: dew point, temperature, humidity, wet bulb
- Dew point range: −60 to +60°C (−76 to +140°F)
- Power: 4.5–12 VDC + onboard battery; IP65
Overview
A dew point transmitter measures the moisture in a gas and sends it on to a control system. The 608 series is the digital member of the line: instead of a 4–20 mA loop it speaks RS485 Modbus-RTU, so a string of units can share one cable, each with its own address, and a PLC or SCADA can poll every dew point on the bus. It reports dew point, temperature, relative humidity, and wet bulb from one capacitive polymer sensor.
What sets it apart is that it does not need the host to be watching. An onboard flash chip logs up to 65,000 records, a button battery keeps the unit and its data alive through a power failure, and the average draw on battery is only tens of microamperes. If you want an analog 4–20 mA loop and a pressure rating to 16 bar instead, use the online dew point meter in the same series; the comparison below shows which output to pick.
Features
Several transmitters on one cable, each polled by a PLC or SCADA.
Onboard flash stores history; Smart Logger software for download.
An onboard button battery keeps the unit and its log through a power loss.
Dew point, temperature, humidity, and wet bulb together.
4.5 to 12 VDC wide input with reverse-polarity protection; microamps on battery.
Sintered filter protects the sensor; RS485 line is over-voltage protected.
How a dew point transmitter works
The probe holds a capacitive polymer humidity sensor and a temperature sensor. The polymer film takes up water vapor, which changes its capacitance and gives relative humidity; the temperature sensor reads air temperature at the same point. From those two values the transmitter computes the dew point, the temperature at which the gas would saturate, and the wet-bulb temperature. The 608 then puts all four readings on the RS485 bus as Modbus registers, rather than scaling one value onto a current loop, so the host reads exact digital numbers and can also configure and diagnose the unit over the same line.
Technical specifications
| Parameter | Specification |
|---|---|
| Sensor | Capacitive polymer humidity and temperature module |
| Measured values | Dew point, temperature, relative humidity, wet bulb |
| Range | −30 to +80°C, 0 to 99 %RH; dew point −60 to +60°C |
| Accuracy | Temperature ±0.1°C (20–60°C); humidity ±1.5 %RH (0–80 %RH, 25°C) |
| Long-term stability | Humidity < 1 %RH/yr; temperature < 0.1°C/yr |
| Output | RS485 / Modbus-RTU; baud 1200–115200 (default 9600); over-voltage and over-current protected |
| Logging | 65,000 records on board; Smart Logger software for download |
| Response | 10 s (at 1 m/s air) |
| Power | 4.5–12 VDC + onboard CR2450 button battery; < 0.1 W; reverse-polarity protected |
| Protection | IP65; stainless steel |
Representative specifications; confirm the exact range and probe per datasheet for your order.
RS485 Modbus or 4-20 mA: which output
The output is the real choice between this transmitter and the analog one in the series. Pick by how the reading has to travel and what the host expects.
| Your need | Use |
|---|---|
| Several points on one cable, digital values, on-board logging, configuration and diagnostics over the bus | This 608 transmitter (RS485 Modbus) |
| A simple analog loop into an existing 4–20 mA input, or a pressure rating to 16 bar | Online dew point meter (602, 4–20 mA) |
RS485 Modbus carries many transmitters over a long run on one pair, with the dew point as an exact number rather than a scaled current, and lets the host read alarms and set the unit up remotely. The trade is a digital network to set up and address, where a 4–20 mA loop is simpler to wire into older equipment.
Applications
The 608 suits networked, online moisture monitoring: dew point on compressed air after a dryer, plastic-resin drying hoppers, freeze dryers, heat-treatment and electroplating furnace atmospheres, and the purity check on nitrogen and oxygen gas generators. Wherever moisture has to be held low and logged across several points, the Modbus bus and the on-board logger do the job with one cable.
Application example
Compressed air, networked dryer monitoring. A plant with several dryers wanted the dew point on each one in the control room without running an analog pair back from every outlet. The 608 transmitters were addressed and multi-dropped on a single RS485 line, and the SCADA polls each one for its dew point over Modbus. Because every unit logs on board and the button battery holds the data, a power blip on the bus does not lose the record, and the stored history can be pulled back with the Smart Logger software for a moisture report. The number of points and the polling are set in the SCADA.
Related products
Online Dew Point Meter602 series with 4-20 mA and RS485, rated to 16 bar, dew point to -100 C.
Portable Dew Point MeterHandheld temperature, humidity and dew point meter for spot checks.
See all gas analysis instruments, or tell us the gas and the network and we will size the right transmitter.
FAQ
What is the purpose of a dew point transmitter?
It measures the moisture in a gas, reported as dew point, and sends it to a control system to keep the gas dry enough, for example confirming that a compressed air dryer or a gas generator is holding its moisture limit. The 608 reports over RS485 Modbus and logs the data on board.
What is the difference between a dew point transmitter and a dew point meter?
They measure the same thing; the difference here is the output and the use. The 608 transmitter is digital, RS485 Modbus, multi-dropped and self-logging for networked monitoring. The online dew point meter in the series adds a 4–20 mA analog loop and a 16-bar rating for a single process point.
Should I choose RS485 Modbus or a 4-20 mA output?
Choose RS485 Modbus when several points share one cable, you want exact digital values, on-board logging, and remote configuration. Choose 4–20 mA when you need a simple analog loop into existing equipment or a higher pressure rating. The two are different members of the same series.
What is the dew point range of the 608?
The dew point reads from −60 to +60°C (−76 to +140°F), derived from the humidity and temperature measurement, with temperature accuracy of ±0.1°C and humidity accuracy of ±1.5 %RH. For a lower dew point on a pressurized line, use the 602 online meter.
About this product page
Specifications drawn from the dew point transmitter 608 series datasheet and reviewed by the Instranova engineering team — last reviewed 2026-06-30. RS485 Modbus and data-logging behavior follow established practice. Questions? Reach our application engineers.
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