OK slightly more scientific than looking for black smoke, but basicaly correct
Had my MOT in the last couple of weeks, and just checked it.
A metered smoke test is performed, and a pass is an avergae of 1.5 1/m (mine was 0.83 1/m).
Ripped the following from a pdf leaflet
[font=Arial][size=3][font=Arial][size=3][b][font=Arial][size=5][color=#FF6600][font=Arial][size=5][color=#FF6600][font=Arial][size=5][color=#FF6600]Metered Smoke Test (Diesel)[/color][/size][/font][/color][/size][/font][/color][/size][/font][/b][/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]The metered smoke test has been conducted in[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]EU countries for many years. During this test, a[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]calibrated smoke meter is used to assess the[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]density of the smoke from compression ignition[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]([/size][/font][/size][/font]
[b][font=Arial][size=3][font=Arial][size=3]diesel[/size][/font][/size][/font][/b][font=Arial][size=3][font=Arial][size=3]) engined vehicles.[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]The engine will be accelerated up to governed[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]speed and the smoke density measured.[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]Engines that emit very little smoke and achieve[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]a meter reading of [/size][/font][/size][/font]
[b][font=Arial][size=3][font=Arial][size=3]1[/size][/font][/size][/font][/b][font=Arial][size=3][font=Arial][size=3].[/size][/font][/size][/font][b][font=Arial][size=3][font=Arial][size=3]5m[/size][/font][/size][/font][font=Arial][size=1][font=Arial][size=1]-1 [/size][/font][/size][/font][/b][font=Arial][size=3][font=Arial][size=3]or less will pass the[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]test after the first acceleration. However, if the[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]test is not passed on the first acceleration a[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]further two accelerations will be carried out.[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]The average of the three acceleration readings[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]will be calculated and if the reading is at or[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]below [/size][/font][/size][/font]
[b][font=Arial][size=3][font=Arial][size=3]2.5m[/size][/font][/size][/font][font=Arial][size=1][font=Arial][size=1]-1 [/size][/font][/size][/font][font=Arial][size=3][font=Arial][size=3]for non-turbocharged [/size][/font][/size][/font][/b][font=Arial][size=3][font=Arial][size=3]engines[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]or [/size][/font][/size][/font]
[b][font=Arial][size=3][font=Arial][size=3]3.0m[/size][/font][/size][/font][font=Arial][size=1][font=Arial][size=1]-1 [/size][/font][/size][/font][font=Arial][size=3][font=Arial][size=3]for turbocharged [/size][/font][/size][/font][/b][font=Arial][size=3][font=Arial][size=3]engines, the[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]vehicle will pass this element of the test.[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]However, if the average is higher, a further[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]acceleration will be carried out, and the average[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]of the last three readings will be calculated. This[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]will continue up to a maximum of six[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]accelerations. If the average of the fourth, fifth[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]and sixth accelerations is higher than the[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]appropriate level, the vehicle will not pass the[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]test.[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]In addition, vehicles may be refused a[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]certificate if the exhaust emits excessive smoke[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]or vapour, to an extent likely to obscure vision.[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]A metered smoke test will not be carried out if:[/size][/font][/size][/font]
[color=#008000][font=Symbol][size=3][font=Symbol][size=3][font=Symbol][size=3] [/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]The exhaust tail pipe is damaged or an[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]accessory is fitted which prevents[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]insertion of the smoke meter probe.[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Symbol][size=3][font=Symbol][size=3][font=Symbol][size=3] [/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]The exhaust system is extensively[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]fractured or holed.[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Symbol][size=3][font=Symbol][size=3][font=Symbol][size=3] [/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]There are obvious signs of an engine[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]defect such as an unusual noise or[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]emission of excessive smoke.[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Symbol][size=3][font=Symbol][size=3][font=Symbol][size=3] [/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]There is an insufficient or excessive[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]amount of oil in the engine or, low[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]engine oil pressure, which could cause[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]engine damage if the engine is[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]accelerated.[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Symbol][size=3][font=Symbol][size=3][font=Symbol][size=3] [/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]There are obvious signs that the[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]governors have been tampered with or[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]are not operating.[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Symbol][size=3][font=Symbol][size=3][font=Symbol][size=3] [/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]The vehicle owner / presenter cannot[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]confirm that the vehicle has been[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]properly maintained, the camshaft drive[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]belt has been replaced at the[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]recommended intervals and the engine[/size][/font][/size][/font][/size][/font][/color]
[color=#008000][font=Arial][size=3][font=Arial][size=3][font=Arial][size=3]is in a suitable condition for testing.[/size][/font][/size][/font][/size][/font][/color]
[font=Arial][size=3][font=Arial][size=3]In the above circumstances where a metered[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]smoke test is not carried out, the vehicle will fail[/size][/font][/size][/font]
[font=Arial][size=3][font=Arial][size=3]the test.[/size][/font][/size][/font]
[b][font=Arial][size=3][font=Arial][size=3]Note: It is important that vehicles are[/size][/font][/size][/font][/b]
[b][font=Arial][size=3][font=Arial][size=3]properly maintained prior to testing[/size][/font][/size][/font][/b]
[b][font=Arial][size=3][font=Arial][size=3](including changing of the camshaft drive[/size][/font][/size][/font][/b]
[b][font=Arial][size=3][font=Arial][size=3]belts in accordance with the manufacture[/size][/font][/size][/font][/b]
[b][font=Arial,Bold][size=3][font=Arial,Bold][size=3]r’s[/size][/font][/size][/font][/b]
[b][font=Arial][size=3][font=Arial][size=3]recommendation) and presented for test at[/size][/font][/size][/font][/b]
[b][font=Arial][size=3][font=Arial][size=3]normal working temperature. This may[/size][/font][/size][/font][/b]
[b][font=Arial][size=3][font=Arial][size=3]require the engine to be kept running prior[/size][/font][/size][/font][/b]
[b][font=Arial][size=3][font=Arial][size=3]to the start of the test.[/size][/font][/size][/font][/b]