I have now "seen the light" about how this has been designed, amended the outbound mqtt message from the controller to request ACK:
and this is working and sending back an echo message:

mygateway1-in/100/2/1/1/3 100
mygateway1-out/100/2/1/1/3 100

So I can now build a rule round this in openhab to handle errors. I suppose I should have thought this through before pestering the forum but then again that is what it's for! Thanks again.

@mfalkvidd Hi. Thank you. Most useful. Just tried and it reports:

I2C Scanner
Scanning...
I2C device found at address 0x40 !
done

So seems to be the same as si7021 and correct. Will respond to @Yveaux with more details as still not working with si7021 library.

@mfalkvidd thank you for this helpful response. I will respond once I get a chance to have a look at this properly again.

Hi. I have used the divider recommended in https://www.mysensors.org/build/battery and with a 0.1uF electrolytic cap. There is still a ripple on this when i attach the scope. Maybe that's the issue but I get the same value (325 as it happens) from A0 even if I vary the supply voltage.

Just to confirm you have had this working after the pro mini onboard regulator is removed?

Hi
Using a clone pro mini + NRF24L01 and HTU21DF sensor.
I have built a battery powered sensor with the regulator removed to reduce power consumption. When I test the level using the analog read method (split resistors) I always get the same value no matter what supply voltage i use (between 2.8v and 3,2v). Is this because analog read is no longer able to get a fixed reference point? I have also tried using the internal cpu voltage method and I get unpredictable results as well.

@4994james
Hi. I have been reading https://forum.mysensors.org/topic/476/htu21d-humidity-temperature-sensor/8 and have now got this working using the adafruit htu21d library. Not sure what the difference is but there obviously is something. I also confirm that the board works without external pull up resistors. Thanks for you help.

This is my new (working) code:

/**
 * The MySensors Arduino library handles the wireless radio link and protocol
 * between your home built sensors/actuators and HA controller of choice.
 * The sensors forms a self healing radio network with optional repeaters. Each
 * repeater and gateway builds a routing tables in EEPROM which keeps track of the
 * network topology allowing messages to be routed to nodes.
 *
 * Created by Henrik Ekblad <henrik.ekblad@mysensors.org>
 * Copyright (C) 2013-2015 Sensnology AB
 * Full contributor list: https://github.com/mysensors/Arduino/graphs/contributors
 *
 * Documentation: http://www.mysensors.org
 * Support Forum: http://forum.mysensors.org
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * version 2 as published by the Free Software Foundation.
 *
 *******************************
 *
 * REVISION HISTORY
 * Version 1.0: Andrew Sutcliffe
 *  
 * DESCRIPTION
 * This sketch provides an example of how to implement a humidity/temperature
 * sensor using a HTU21DF Sensor.
 *  
 * For more information, please visit:
 * http://www.mysensors.org/build/humidity
 * 
 * Connect Vin to 3-5VDC
 * Connect GND to ground
 * Connect SCL to I2C clock pin (A5 on UNO)
 * Connect SDA to I2C data pin (A4 on UNO)
 * 
 */

// Enable debug prints
#define MY_DEBUG

// Enable and select radio type attached 
//#define MY_RADIO_NRF24
//#define MY_RADIO_RFM69
#define MY_RS485

// Define this to enables DE-pin management on defined pin
#define MY_RS485_DE_PIN 2

// Set RS485 baud rate to use
#define MY_RS485_BAUD_RATE 9600

#include <MySensors.h>  
#include <Wire.h>
#include "Adafruit_HTU21DF.h"

Adafruit_HTU21DF htu = Adafruit_HTU21DF();

// Set this offset if the sensor has a permanent small offset to the real temperatures
#define SENSOR_TEMP_OFFSET 0

// Sleep time between sensor updates (in milliseconds)

static const uint64_t UPDATE_INTERVAL = 60000;

// Force sending an update of the temperature after n sensor reads, so a controller showing the
// timestamp of the last update doesn't show something like 3 hours in the unlikely case, that
// the value didn't change since;
// i.e. the sensor would force sending an update every UPDATE_INTERVAL*FORCE_UPDATE_N_READS [ms]
static const uint8_t FORCE_UPDATE_N_READS = 10;

#define CHILD_ID_HUM 0
#define CHILD_ID_TEMP 1

#define MY_NODE_ID 104

float lastTemp;
float lastHum;
uint8_t nNoUpdatesTemp;
uint8_t nNoUpdatesHum;
bool metric = true;

MyMessage msgHum(CHILD_ID_HUM, V_HUM);
MyMessage msgTemp(CHILD_ID_TEMP, V_TEMP);

void presentation()  
{ 
  // Send the sketch version information to the gateway
  sendSketchInfo("HTU21DF-TemperatureAndHumidity", "1.0");

  // Register all sensors to gw (they will be created as child devices)
  present(CHILD_ID_HUM, S_HUM);
  present(CHILD_ID_TEMP, S_TEMP);
}

void setup()
{
  Serial.begin(115200);
  Serial.println("HTU21D-F test");

  if (!htu.begin()) {
    Serial.println("Couldn't find sensor!");
    while (1);
    }
}

void loop(){
  // Get temperature from HTU21DF library
  float temperature = (htu.readTemperature());
 if (temperature != lastTemp || nNoUpdatesTemp == FORCE_UPDATE_N_READS) {
    // Only send temperature if it changed since the last measurement or if we didn't send an update for n times
    lastTemp = temperature;
    
    // Reset no updates counter
    nNoUpdatesTemp = 0;
    temperature += SENSOR_TEMP_OFFSET;
    send(msgTemp.set(temperature, 1));

    #ifdef MY_DEBUG
    Serial.print("T: ");
    Serial.println(temperature);
    #endif
  } 
  else {
    // Increase no update counter if the temperature stayed the same
    nNoUpdatesTemp++;
  }

  // Get humidity from HTU21DF library
  float humidity = (htu.readHumidity());
  if (humidity != lastHum || nNoUpdatesHum == FORCE_UPDATE_N_READS) {
    // Only send humidity if it changed since the last measurement or if we didn't send an update for n times
    lastHum = humidity;
    // Reset no updates counter
    nNoUpdatesHum = 0;
    send(msgHum.set(humidity, 1));

    #ifdef MY_DEBUG
    Serial.print("H: ");
    Serial.println(humidity);
    #endif
}
  else {
    // Increase no update counter if the humidity stayed the same
    nNoUpdatesHum++;
  }

  // Sleep for a while to save energy
  sleep(UPDATE_INTERVAL); 
}

@Yveaux
This is what I see on trigger. this is identical when HTU21 and Si7021 boards are connected.

@Yveaux
Many thanks for your response.
I think the board has onboard pull up resistors, But to eliminate doubt I added 2 1k pull ups to sda and scl. This is the debug output when i use a sparkfun si7021 board (also with same pull ups - though not needed for this board for sure):

setup!10446 MCO:BGN:INIT OK,TSP=1
Temp 18.83C	Hum 50.94

and this is a screenshot of the trace on sda and scl:

E3 is temp command.

followed by

Temp reading and another (not shown) set of commands and humidity reading.

However with the HTU21D the sketch hangs here:

setup!10397 MCO:BGN:INIT OK,TSP=1

i get the same first command trace as you would expect (same library) but no response from the board.

@mfalkvidd Hi. Thank you. Most useful. Just tried and it reports:

I2C Scanner
Scanning...
I2C device found at address 0x40 !
done

So seems to be the same as si7021 and correct. Will respond to @Yveaux with more details as still not working with si7021 library.

I have had plenty of success with s17021 sensors and the mysensors library in the past however I have recently purchased some more boards for what I thought was a reasonable deal from Amazon. This is exactly what I have.

The issue I have is that none of them work. When it gets to Begin() it just stops. It doesn't return at all so doesn't issue a "sensor not detected" which is what I would expect if not connected. I hooked up my logic analyser trace and can see the initial command but nothing comes back.
I can see the basic chip is different to the ones to the boards I have used previously so assume there is some software incompatibility. However I have looked at the data sheet and it is a I2C component and seems to have identical addresses and commands as the si7021. Have I just got a load of defective items or should I be using different software?

I have now "seen the light" about how this has been designed, amended the outbound mqtt message from the controller to request ACK:
and this is working and sending back an echo message:

mygateway1-in/100/2/1/1/3 100
mygateway1-out/100/2/1/1/3 100

So I can now build a rule round this in openhab to handle errors. I suppose I should have thought this through before pestering the forum but then again that is what it's for! Thanks again.

@electrik hi. The is fine for a node where the code is part of the node sketch. However, because the gateway code is built into the libraries, it makes it difficult to implement in the gateway. As on the other thread one answer may be to change the controller code to request a confirmation from the gateway of delivery. In my case, I am using generic mqtt thing in openhab so not sure how to to be honest but will have a think.

@electrik thanks. Good thought. Will report back once I have looked at this.