Template sensor - different resultsfrom sensor and in template preview

Configuration
1

I have an MQTT sensor that is returning temperature from ESP8266 device, defined like this:

  - platform: mqtt
    state_topic: "Temp1/Salon/Temperature"
    name: "salon_temperature"
    unit_of_measurement: "°C"
    expire_after: 180

This sensor is then used in template sensor defined as below:

      temperatura_salon:
        friendly_name: "Salon"
        unit_of_measurement: '°C'
        value_template: >-
          {{ states.sensor.salon_temperature.state | float - 2.5 }}

I use a template to apply offset. In the above case, I need to subtract 2.5 Celsius degrees.
I’m getting weird results because the value looks like this:
image

The weirdest part is that when I use template editor I get two results:

Temp7_Salon
Temp7_Salon1239×285 26.4 KB

In the preview, you can see ORG - the original value (MQTT sensor), MOD is the value of my template sensor and NEW is the value template taken from my template sensor.

First of all, I get weird rounding. All I need is one decimal place, second is the wrong value.
In my template, I’m subtracting 2.5, but the MOD has 2.4 subtracted.

2

That’s a long way to go about it when the mqtt sensor supports a value template.

  - platform: mqtt
    state_topic: "Temp1/Salon/Temperature"
    value_template: "{{ value - 2.5 }}"
    name: "salon_temperature"
    unit_of_measurement: "°C"
    expire_after: 180

No template sensor required.

1 Like
3

Doh :man_facepalming:
I’ll try that when I get back home.

But the thing that troubles me so much is why there is a difference in the first place and why I get that weird rounding. Any ideas on that?
Maybe it’s a bug or it’s just the lack of knowledge.

4

Epsilon error. Wrong link. Try https://en.wikipedia.org/wiki/Machine_epsilon

5

So why there are differences in evaluating the same template.
Especially those two give bad results:

MOD: {{ states.sensor.temperatura_salon.state }}
NEW: {{ states.sensor.salon_temperature.state | float - 2.5 }}

image

MOD is the state of the template sensor and NEW is just the value_template of that template sensor.
In theory, they should have the same value.
The value_template is returning the correct value, but the state of the template sensor is wrong.
Not sure how they are evaluated, I don’t know the insides of HA.

6

What does this give:

MOD: {{ states.sensor.temperatura_salon.state }}
NEW: {{ states.sensor.salon_temperature.state }}
7

MOD: 24.0
NEW: 26.4

I didn’t apply the value_template for the MQTT sensor.

8

So why do you think they should be the same?

9

The problem is with sensors naming.
I’ll describe this with English names, it will be easier for all of us.

I have an MQTT sensor:

  - platform: mqtt
    state_topic: "Temp1/Salon/Temperature"
    name: "original_sensor"
    unit_of_measurement: "°C"
    expire_after: 180

and a template sensor:

fixed_sensor:
  friendly_name: "Salon"
  unit_of_measurement: '°C'
  value_template: >-
    {{ states.sensor.original_sensor.state | float - 2.5 }}

Now inside the template editor, I have 3 lines:

ORG: {{ states.sensor.original_sensor.state }}
MOD: {{ states.sensor.fixed_sensor.state }}
NEW: {{ states.sensor.original_sensor.state | float - 2.5 }}

Assuming ORG is returning 26.4, then MOD is returning 24 and NEW 23.9 (this is correct).

My point is that I have the same template, for value_template of fixed_sensor and as the third line in the template editor. So both values (second and third) should be the same.

I hope the description is more clear now.