Ask HN: How to measure if a person is sitting on a chair?

I have restaurant biz startup. A big problem I have is how to measure the impact of my solution, i.e. measure the occupancy at the restaurants. Neither I, nor the restaurants have a good solution for this. So I thought this could be a business I could provide.

So my question is: which technology would be a better aproach to record if there is a person sitting on a chair?

Requirements: - Must be very cheap - Must record somehow how much time is a chair empty or with someone sitting on it - Must be reliable

I imagine something that could be applied on a sticker. Maybe measuring pressure or something and sending a signal to a hardware when it is occupied. Then this central hardware keep track of the time it is recieving a signal or not.

Any ideas?