How can I access a precedent data in the Payload Parser.

How can I access a precedent data in the Payload Parser.

Hi,
I need to access, when a data enter the Payload Parser, the data who  was sent just before. It is for something akin to a derivation and I need 2 data for the calcul. Do you have an idea how to do that? Is there another solution?

Here is more detail on my situation:
I have rain gauge data who enter through one of my device. The rain gauge is a really basic model. It has a small container and each time the container is filled with water, the container is emptied and a pulse is send. I ultimatly receive the pulse in Tago. I am currently using the Payload Parser and my data take this form:
  1. [{
      "variable": "pulse_precipitation",
      "value": ??,
      "time": ??,
      "location": {
          "lat": ??,
          "lag": ??
      }
      "metadata": {
          "volume": ??
          "battery": ??
      }
    }]
I want to change the way my data are presented and use another unit. My first version used the rain gauge pulse as the main value. (The value was a sequential) Now, I want to calcul the precipitation for my main value. For my calcul, I need to use the precedant data that was send. Is there a way to get that data in the Payload parser?
  1. [{
      "variable": "precipitation",
      "value": ??,
      "unit": "mm³/h",
      "time": ??,
      "location": {
          "lat": ??,
          "lag": ??
      }
      "metadata": {
          "pulse_value": ??,
          "volume": ??
          "battery": ??
      }
    }]