Gas should be around 20-30B per kilo (or is it per pound?). 10kg should be enough to completely empty and refill a normal air-con, so it sounds like you were done. Even topping-up is rarely necessary.

I'm dont think they weigh the gas ,I've never seen them place the gas canisters on a set of scales !

When they say "pounds" I think they mean Pounds per square inch or PSI
a measure of pressure not weight.

Topping up shouldn't be necessary just like its not necessary to top up your fridge...except aircons are more prone to leaks due to bad installation and or damage while being cleaned there are also sometimes manufactured defects that allow the gas to escape very very slowly over months and those leaks can be very hard to find and fix
without ripping the whole aircon out
taken back to the workshop for fixing then reinstall....most companies either dont want to or cant do it so they just refill the gas.

1 hour ago, johng said:

I'm dont think they weigh the gas ,I've never seen them place the gas canisters on a set of scales !

When they say "pounds" I think they mean Pounds per square inch or PSI
a measure of pressure not weight.

 

This is something that so far I have not found a clear answer to. My air-con unit has a label on the side that appears to show the total weight of gas in it, in kilos. Some research I did online also gave average weights for the amount of gas in air-con units, in kilos and, in the US, in pounds. For gasses there is of course a direct correlation between weight and pressure at the same volume and temperature but as you say the measurement used when topping up will be one of pressure. In the end it makes no real difference as the cost of topping up should be the amount of gas added and not the total amount in the unit, unless of course all the gas has leaked out.

 

When I asked my current air-con people they quoted me 20-30B per unit of gas depending on the type of gas. They also told me that my unit was reading 9 and should be reading 10. What they could not say was how many units of gas would be needed to put the 9 up to the desired 10. I even asked the owner over the phone and he couldnt say either. He just didnt seem to understand the question and kept repeating the price per unit. So I had no idea whether they would be charging me for 1 unit or 10 units or 100 units. In the end I left it as the air-con works fine on 9 and the owner said there was no hurry for it anyway.

My receipt says 10 psi / unit price 30 / total 300 baht for gas, There are also as description something thai and R 410 A 

18 minutes ago, thaitero said:

My receipt says 10 psi / unit price 30 / total 300 baht for gas, There are also as description something thai and R 410 A 

 

The R410 bit is the type of gas, and with my people one type is indeed 30B instead of the 20B charged for the other type.

 

Your 10psi seems to correspond with the "10" that I was told my unit should be at, though I fail to see why you should be charged for the whole 10 as presumably there must have been some gas already in it. If not it would not have been cooling at all. In my case I would expect the charge to be for 1 unit only (ie from 9 to 10) and when topping-up I would expect most air-cons to need a similarly small amount.

 

The only exception would be if they drained the entire unit then refilled it completely. But why do that unless the air-con is being moved or some piping or joint replaced? As long as there is still some positive pressure in the unit the gas inside cant be contaminated with air and so should only need topping up and not completely replacing.