Vaisala HMP Series
Humidity and temperature probes for demanding processes, with dew point, sensor purge and condensation prevention.
The Vaisala HMP Series are humidity and temperature probes for demanding processes such as compressed air lines, ovens, dryers and spaces with constant high humidity. This Modbus RTU template reads relative humidity, temperature, dew point, the other humidity values and the probe status, and lets you set pressure compensation, condensation prevention, sensor purge and filtering from ModbusCloud.
- Protocol
- Modbus RTU
- Connection
- RS485, 19200 baud
- Registers
- 95, read and write
- Brand
- Vaisala
- Key readings
- Relative humidity · Temperature · Dew/frost point · Absolute humidity · Mixing ratio · Water vapor pressure · Enthalpy · Device status
- Available commands
- Pressure compensation · Condensation prevention · Sensor purge · Measurement filtering · Modbus address and bit rate · Humidity and temperature adjustment
- Compatible models
- HMP1 · HMP3 · HMP4 · HMP5 · HMP7 · HMP8 · HMP9 · MMP8 · TMP1
- Last updated
Registers (95)
92 read, 42 write
| Name | Unit | Access | Address (Hidden until you enter your email) | Function (hidden) | Type (hidden) |
|---|---|---|---|---|---|
| Relative Humidity | %RH | Read | |||
| Temperature | °C | Read | |||
| Dew Point Temperature | °C | Read | |||
| Dew/Frost Point Temperature | °C | Read | |||
| Dew/Frost Point Temperature at 1 atm | °C | Read | |||
| Dew Point Temperature at 1 atm | °C | Read | |||
| Absolute Humidity | g/m³ | Read | |||
| Mixing Ratio | g/kg | Read | |||
| Wet-Bulb Temperature | °C | Read | |||
| Water Concentration | ppm | Read | |||
| Water Vapor Pressure | hPa | Read | |||
| Water Vapor Saturation Pressure | hPa | Read | |||
| Enthalpy | Read | ||||
| Water Activity | Read | ||||
| Dew/Frost Point Depression | °C | Read | |||
| Absolute Humidity at NTP | g/m³ | Read | |||
| Water Concentration in Oil | ppm | Read | |||
| Relative Saturation | % | Read | |||
| Water Concentration (Wet Basis) | % | Read | |||
| Relative Humidity (Dew/Frost) | %RH | Read | |||
| Water Mass Fraction | ppm | Read | |||
| Relative Humidity (Integer) | %RH | Read | |||
| Temperature (Integer) | °C | Read | |||
| Dew Point Temperature (Integer) | °C | Read | |||
| Dew/Frost Point Temperature (Integer) | °C | Read | |||
| Dew/Frost Point at 1 atm (Integer) | °C | Read | |||
| Dew Point at 1 atm (Integer) | °C | Read | |||
| Absolute Humidity (Integer) | g/m³ | Read | |||
| Mixing Ratio (Integer) | g/kg | Read | |||
| Wet-Bulb Temperature (Integer) | °C | Read | |||
| Water Concentration (Integer) | ppm | Read | |||
| Water Vapor Pressure (Integer) | hPa | Read | |||
| Water Vapor Saturation Pressure (Integer) | hPa | Read | |||
| Enthalpy (Integer) | Read | ||||
| Water Activity (Integer) | Read | ||||
| Dew/Frost Point Depression (Integer) | °C | Read | |||
| Absolute Humidity at NTP (Integer) | g/m³ | Read | |||
| Water Concentration in Oil (Integer) | ppm | Read | |||
| Relative Saturation (Integer) | % | Read | |||
| Water Concentration (Wet Basis) (Integer) | % | Read | |||
| Relative Humidity (Dew/Frost) (Integer) | %RH | Read | |||
| Water Vapor Mass Fraction (Integer) | ppm | Read | |||
| Error Status | Read | ||||
| Online Status | Read | ||||
| Error Code | Read | ||||
| Security Hash | Read | ||||
| RH Measurement Status | Read | ||||
| T Measurement Status | Read | ||||
| Td/f Measurement Status | Read | ||||
| Device Status | Read | ||||
| Pressure Compensation Setpoint | hPa | Read and write | |||
| Temperature Compensation Setpoint | °C | Read and write | |||
| Condensation Prevention | Read and write | ||||
| Start Sensor Purge | Read and write | ||||
| Sensor Purge Interval | min | Read and write | |||
| Interval Purge | Read and write | ||||
| Startup Purge | Read and write | ||||
| Measurement Filtering Factor | Read and write | ||||
| Measurement Filtering | Read and write | ||||
| Modbus Address | Read and write | ||||
| Bit Rate | Read and write | ||||
| Parity, Data and Stop Bits | Read and write | ||||
| Response Delay | ms | Read and write | |||
| Restart Device | Write | ||||
| Calibration Mode Passcode | Read and write | ||||
| Calibration Year | Read and write | ||||
| Calibration Month | Read and write | ||||
| Calibration Day | Read and write | ||||
| Calibration Text | Read and write | ||||
| RH Adjustment Reference Point 1 | %RH | Read and write | |||
| RH Adjustment Measured RH Point 1 | %RH | Read and write | |||
| RH Adjustment Measured T Point 1 | °C | Read and write | |||
| RH Adjustment Reference Point 2 | %RH | Read and write | |||
| RH Adjustment Measured RH Point 2 | %RH | Read and write | |||
| RH Adjustment Measured T Point 2 | °C | Read and write | |||
| RH Adjustment Reference Point 3 | %RH | Read and write | |||
| RH Adjustment Measured RH Point 3 | %RH | Read and write | |||
| RH Adjustment Measured T Point 3 | °C | Read and write | |||
| RH Adjustment Reference Point 4 | %RH | Read and write | |||
| RH Adjustment Measured RH Point 4 | %RH | Read and write | |||
| RH Adjustment Measured T Point 4 | °C | Read and write | |||
| RH Adjustment Reference Point 5 | %RH | Read and write | |||
| RH Adjustment Measured RH Point 5 | %RH | Read and write | |||
| RH Adjustment Measured T Point 5 | °C | Read and write | |||
| Activate RH Adjustment | Read and write | ||||
| Restore Factory RH Adjustment | Write | ||||
| T Adjustment Reference Point 1 | °C | Read and write | |||
| T Adjustment Measured T Point 1 | °C | Read and write | |||
| T Adjustment Reference Point 2 | °C | Read and write | |||
| T Adjustment Measured T Point 2 | °C | Read and write | |||
| Activate T Adjustment | Read and write | ||||
| Restore Factory T Adjustment | Write | ||||
| Signed Integer Test | Read | ||||
| Floating Point Test | Read | ||||
| Text String Test | Read |
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If a device is not in the library yet, we build the template for free as soon as we have the register data.
Use cases
- Show humidity, temperature and dew point per greenhouse compartment, drying chamber or air line on a dashboard and keep 365 days of history.
- Get an email, in-app or webhook alert when the dew point in a compressed air line rises above the limit you set.
- Raise an alert when Error Code is not 0 or Device Status reports an error, for example a humidity measurement error from a wet sensor.
- Track Online Status to see when sensor purge or condensation prevention locks the readings, and leave those moments out of your analysis.
- Update the pressure compensation from the portal when an HMP4 or HMP8 sits in a line above atmospheric pressure.
- Start a sensor purge or switch the automatic purge on from the portal, for example after cleaning the probe or before a calibration.
Installation
Connect pin 4 (RS-485+, black) and pin 2 (RS-485-, white) of the M12 probe cable to the RS485 port of the ModbusCloud Gateway, and pin 3 (blue) to ground. Keep the RS-485 line within 30 m.
Power the probe with 15 to 30 V DC on pin 1 (brown). The HMP7 needs 18 to 30 V DC, and its probe heating stays off below 18 V.
The probe answers at address 240 with 19200 baud, no parity, 8 data bits and 2 stop bits. With more probes on one line, up to ten, give each its own address, for example with Vaisala Insight.
In ModbusCloud, add a Modbus device on the gateway with these settings and import the Vaisala HMP Series template.
Check that Floating Point Test reads -123.45 and that Relative Humidity and Temperature show plausible values.
Things to watch for
The probe does not answer on the RS485 bus. What should I check?
Check 19200 baud, no parity, 8 data bits and 2 stop bits at address 240. Many masters default to 1 stop bit. Check that RS-485+ is on pin 4 and RS-485- on pin 2, and swap them if nothing answers. Every probe on the line needs its own address. Check the supply: at least 15 V DC, and 18 V DC for the HMP7. A red LED on the probe body means the probe reports an error.
Relative humidity shows no value on my HMP7. Why?
While condensation prevention warms the sensor or probe head, the probe returns values that depend on its own temperature, such as relative humidity, as not available. Dew point stays available. Write the ambient temperature from another sensor to Temperature Compensation Setpoint, or switch Condensation Prevention off. During a sensor purge all values freeze for a few minutes and Online Status reads 0.
The values look plausible but do not match my reference. How do I check the readout?
Read Floating Point Test. It must show -123.45, and Signed Integer Test -12345. The probe sends 32-bit values low word first, and a wrong word order can still give values that look right. The template already uses the order of the probe. In a pressurized line, set Pressure Compensation Setpoint to the process pressure, otherwise the mixing ratio and other pressure-dependent values are off.
Writing a setting returns an error or has no effect. Why?
The probe only accepts writes as write multiple (function code 16). Pressure compensation, filtering factor and adjustment values are 32-bit and always go that way. If a single 16-bit setting such as Condensation Prevention returns an error, change it with Vaisala Insight. The probe confirms a write even when it rejects the value, so read the register back. Writing registers in ModbusCloud is off by default and is enabled per gateway on request.
Can I calibrate the probe from ModbusCloud?
The template contains the adjustment registers, but Vaisala recommends its calibration service, Vaisala Insight or a compatible handheld meter. An adjustment needs a stable reference, a sensor purge first, the passcode 1300 to open calibration mode, and the calibration date and text each written in one go. The probe rejects corrections larger than 10 %RH or 0.5 °C. You can follow the values in ModbusCloud while the probe stabilizes.
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