No data alarm for PowerShell

nodata_alarm.ps1

# Check data is being written
# Tim Brooks Sept 2023 <brooks@skoorb.net>
#
# Specify a hashmap of Contacts with WhatsApp numbers and APIKeys
# E.g. > $Contacts=@{'+271234567890'=123456}; .\nodata_alarm.ps1
#
# Complete the enrolment described below to get an APIKey for each number:
# https://www.callmebot.com/blog/free-api-whatsapp-messages/

$FileName = "foo.txt"           #File that should grow in size
$PollTime = 5                   #Check interval in seconds
$ReminderTime = 1800            #Send a reminder message after alerting for interval in seconds

if ( $Contacts -eq $Null ) {
    Write-Host "No Contacts provided!"
    Write-Host "Enroll on callmebot.com and provide WhatsApp and APIKey e.g.:"
    Write-Host "> `$Contacts=@{'+271234567890'=123456}; .\nodata_alarm.ps1"
    exit
}

function Send-Message {
    param (
        $message
    )

    $message = [uri]::EscapeDataString($message)
    foreach ( $number in $Contacts.Keys ) {
        $APIKey=$Contacts[$number]
        Invoke-WebRequest "https://api.callmebot.com/whatsapp.php?phone=$number&text=$message&apikey=$APIKey"
    }
}

function Set-Alert {
    $message = "Warning, no increase in data size of $FileName in past $PollTime seconds."
    Write-Host "$message"
    Send-Message $message
}

function Clear-Alert {
    $message = "All clear, $FileName now increasing in size."
    Write-Host "$message"
    Send-Message $message
}

function Remind-Alert {
    param (
        $StartTime
    )

    $message = "Warning, still no increase in data size of $FileName since $StartTime."
    Write-Host "$message"
    Send-Message $message
}

$LastLength = -1
$AlertSet = $false
while ( $true ) {
    $NewLength = ( Get-Item $FileName ).length
    if ( $NewLength -eq $LastLength ) {
        if ( $AlertSet -eq $false ) {
            $AlertSet = $true
            Set-Alert
            $AlertTime = ( Get-Date )
        } else {
            $delta = ( Get-Date ) - $AlertTime
            Write-Host $delta.Seconds
            if ( $delta.TotalSeconds % $ReminderTime -lt $PollTime ) {
                Remind-Alert $AlertTime
            }
        }
    } else {
        if ($AlertSet -eq $true ) {
            $AlertSet = $false
            Clear-Alert
        }
    }

    $LastLength = $NewLength
    Start-Sleep -Seconds $PollTime
}