The iSentrol Mod X0 pulse counter

- tsp
Last update 21 May 2025
Reading time 5 mins

Introduction

In this article, we explore the iSentrol Mod X0, a versatile and compact flow sensor module designed to read and interpret pulses from standard flow transducers. Acting as a digital water flow meter, the Mod X0 supports ModBus communication, allowing for seamless integration into a variety of data logging and automation setups.

I’ve tested the module in two real-world scenarios:

First, with the YF-B5 Hall-based water flow sensor as its primary use case, where the module uses the sensor’s K factor to convert pulses into meaningful flow rates.
Second, in a more experimental setup, with the WH-SP-WS01 Anemometer, utilizing only its raw pulse frequency output to determine wind speed. In this configuration, I bypass cumulative counting and instead apply a simple K factor conversion to estimate current air velocity in meters per second.

This post includes:

A breakdown of the default settings
Documentation of the ModBus registers (including corrections where the original documentation was incorrect)
Detailed formulas for converting raw register values into physical quantities like liters per minute and meters per second.

Default settings


Baud rate	9600
ModBus address	1
Flowmeter constant	500

ModBus Registers

Supported commands are:

Read holding registers
Write single register
Write multiple registers

Unfortunately the documentation supplied by many merchants is not correct. I’ve found one in a YouTube video by the manufacturer though that seems to fit when applied to my sensor:

Address	Name	Description	Unit	Scaling / Notes
`0x0000`	Baud Rate	Communication baud rate setting	-	Value is 0: 2400, 1: 4800, 2: 9600, 3: 19200. Reverts to 9600 on invalid values.
`0x0001`	Device Address	ModBus device ID	-	Range: 1–247 (`0x0001`–`0x00F7`).
`0x0002`	Decimal places	Number of decimal places, defaults to 2	-	Not sure of effect, default 2
`0x0003`	Sensor Pulse K Factor	Number of pulses per liter	pulses/L	Stored as 100x the actual value (50000 is 500.00 pulses/L)
`0x0004`	Permanent Cumulative Flow – High Register	High 16 bits of permanently stored accumulated volume	Liters	Combine with `0x0004`: `Total_Liters = High * 65536 + Low`.
`0x0005`	Permanent Cumulative Flow – Low Register	Low 16 bits of permanently stored accumulated volume	Liters	See above. Rolls into `0x0003` when reaching 65535.
`0x0006`	Permanent Cumulative Flow – Decimal	Decimal portion of cumulative volume	Liters	Stored as 100× actual decimal value. E.g., `25` → `0.25 L`.
`0x0007`	Permanent Accumulated Pulse Counter	Pulse counter until 1L is reached	pulses	Resets to 0 after 1L worth of pulses is accumulated (based on K factor).
`0x0008`	?	?	-	Reserved
`0x0009`	?	?	L/min	Reserved (default 100)
`0x000A`	?	?	L/min	Reserved (default 1500)
`0x000B`	?	?	-	Reserved (default 4000)
`0x000C`	?	?	-	Reserved (default 50)
`0x000D`	Temporary Cumulative Flow – High Register	High 16 bits of temporarily stored accumulated volume (cleared on power loss)	Liters	Same format as `0x0003/0x0004`, resets on power loss.
`0x000E`	Temporary Cumulative Flow – Low Register	Low 16 bits of temporarily stored accumulated volume	Liters	See above.
`0x000F`	Temporary Cumulative Flow – Decimal	Decimal portion of temporary cumulative volume	Liters	Stored as 100× actual decimal value. Resets on power loss.
`0x0010`	Temporary Accumulated Pulse Counter	Pulse counter toward 1L for temporary total	pulses	Same as `0x0006`, but for temporary flow.
`0x0011`	Real-Time Flow Rate	Instantaneous flow rate	L/min	Stored as 100× actual value. E.g., `315` → `3.15 L/min`.
`0x0012`	Real-Time Pulse Rate	Current sensor pulse frequency	Hz	Useful for debugging and calibration.
`0x0013`	?	?	-	Reserved
`0x0014`	?	?	-	Reserved

Encoding and decoding registers

K factor

The K factor represents the number of pulses generated per liter of flow. Since the Mod X0 stores only integer values, the K factor is scaled by a factor of 100 to retain two decimal places of precision, enabling a resolution down to 0.01 pulses per liter. The stored value in the register can thus be calculated as:

$ \begin{aligned} K_{reg} &= 100 * \frac{f}{Q}
&= 100 * \frac{\frac{pulses}{s}}{\frac{l}{s}} = 100 * \frac{pulses}{liters} \end{aligned} $

Permanent cumulative flow

The total accumulated flow volume (in liters) is stored across three 16-bit registers:

0x0004: high word of the integer part ($F_{high}$)
0x0005: low word of the integer part ($F_{low}$)
0x0006: fractional part scaled by 100 ($F_{frac}$)

The total value can be reconstructed as:

$ \begin{aligned} F_{total} = F_{high} * 65536 + F_{low} + \frac{F_{frac}}{100} \end{aligned} $

Temporary cumulative flow

This register group behaves identically to the permanent cumulative flow, representing a resettable volume counter (in liters). It is stored in:

0x000D: high word of integer part ($F_{high}$)
0x000E: low word of integer part ($F_{low}$)
0x000F: fractional component again scaled by 100 ($F_{frac}$)

The value is calculated using the same formula:

$ \begin{aligned} F_{total} = F_{high} * 65536 + F_{low} + \frac{F_{frac}}{100} \end{aligned} $

Pulse counters

Registers 0x0007 and 0x0010 store the raw pulse counts for the permanent and temporary flow measurements, respectively. These counters increment with each detected pulse and can be used for low-level diagnostics or debugging. Once the count reaches the stored $K$ value, it indicates that one liter of fluid has passed and they wrap to zero again.

Real-Time flow rate

The real-time flow rate is stored in register 0x0011, expressed in liters per minute, scaled by a factor of 100 for precision. The actual flow rate is obtained as:

$ \begin{aligned} Q &= \frac{F_{flow}}{100} \end{aligned} $